Stair Calculator — You Do Not Choose the Riser Height, You Choose How Many There Are
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Calculate
This selector governs every field, label, result and export here and takes priority over the site header switch. Switching converts what you entered rather than reinterpreting it, so 109 in becomes 2,769 mm and returns to exactly 109.
The Code Basis
Answer this before any dimension. The two codes differ on the riser maximum, on the tread minimum, and on whether a minimum riser exists at all, and that last difference decides whether the count list has an upper bound.
The residential riser maximum moved from 7 3/4 inches to 8.25 in the 2024 edition and the tread minimum from 10 to 9. No verdict is issued until the edition is named, because the same rise gives a different lowest count under each.
The Total Rise
In inches, measured vertically from the finished surface of the lower floor to the finished surface of the upper one. This is the one figure the building fixes and the whole page divides.
The uniformity tolerance is spent before any cutting begins, so how the rise was arrived at decides how much of it is already gone. A subfloor measurement produces a preliminary result rather than a blocked one.
In inches, and optional. Flooring still to be laid on the upper floor raises it and increases the rise. A three quarter inch hardwood floor moves the top riser by twice the entire uniformity tolerance.
In inches, and optional. Flooring still to be laid on the lower floor raises it and reduces the rise, which is the opposite direction to the top figure. A tile bed with its setting material is around an inch and a quarter.
In inches, and optional. Leave it empty where the stair is not constrained by a space, and the tread is taken at the code minimum. Enter it and the tread comes from the run instead and is tested against that minimum.
The Count You Choose
Leave this empty on the first run and the page returns the counts that comply, with the riser height each produces. Then enter one of them. The page will not choose for you, because the counts differ in comfort, run and fit.
The Other Tests
In inches, and optional. Named explicitly because the code sets a different clear width at and below handrail height, where handrails intrude. Measure above the permitted handrail height and below the required headroom height.
The reduced clear widths at and below handrail height are allowances that exist only where handrails do. With none installed the full 36 inches governs at every height, and the answer decides which figure is reported.
In inches, and optional. Measured vertically from the sloped line adjoining the tread nosings, or from a landing surface, to the lowest point above. Not from a tread and not perpendicular to the slope.
Overview
The riser count in a flight is a whole number, so nobody chooses a riser height.
The total rise is fixed by the building, finished floor to finished floor, and the height is what falls out of dividing it by a count. On 109 inches of total rise under the 2021 residential code, fourteen risers is 7.786 inches and fails the maximum, fifteen is 7.267 and passes, sixteen is 6.813 and also passes.
Nothing in between exists. There is no 7.5 inch riser available on that rise, because 109 divided by 7.5 is 14.53 and there is no such thing as 14.53 risers.
Most stair calculators ask for a riser height and hand it back. This one takes the rise and returns the counts, with the code test applied to each.
The second rule is where the arithmetic becomes a safety question. The greatest riser in a flight may not exceed the smallest by more than three eighths of an inch, which means a remainder cannot be dumped into the last step. And that tolerance is smaller than it looks: a half inch error in the total rise is divided across every riser before a single cut is made.
What to Look at First
Read the count list before the riser height. The riser height is the figure every other stair calculator returns and this one returns the same arithmetic, but it is a consequence rather than a choice: the total rise is fixed by the building and the count is a whole number, so the only decision on the page is which count.
Then read the bound on the list. A range labelled as bounded by code has a published minimum riser behind it and a range labelled as bounded by practicality does not, and the difference decides whether counts above the ones shown are still compliant.
Then read the uniformity block, which is separate from every dimension above it. It is the only rule here that a correctly computed stair can still fail in the building, because the arithmetic produces a uniform stair and the risk lives between the calculation and the cut.
How to Use This Calculator
Name the code basis and the adopted edition. Both change the answer. The residential and commercial codes differ on the riser maximum and the tread minimum, and the 2024 residential edition moved both again.
Enter the total rise, finished floor to finished floor, and say how you measured it. A measurement taken to a subfloor with the finish still to come produces a preliminary result rather than a wrong one, and entering the finish thicknesses adjusts it.
Read the count options. Each count comes with the riser height it produces and whether it passes, including the counts either side that fail, because seeing the failures is what shows that the choice was between whole numbers.
Choose a count. The page will not choose for you. Where several comply they differ in comfort, in the run they need and in how they fit the opening, and none of that is knowable from a rise.
Add the available run if the stair has to fit a space. The tread depth then comes from the run rather than from the code minimum, and it is tested against the minimum rather than assumed to meet it.
Add the width, the headroom and the nosing where you have them. Anything not entered is reported as not evaluated rather than passed.
Inputs & Outputs
Inputs
Outputs
Stair Formula
Every riser height on this page comes from a whole count divided into a fixed rise. All lengths are canonical in inches internally.
Riser height from a count
riser_in = rise_in / riser_count
Adjusted rise for finishes
rise_adj_in = rise_in + top_fin_in - bot_fin_in
Lowest compliant count
count_min = rise_in / riser_max_in
Highest compliant count
count_max = rise_in / riser_min_in
Tread count in a straight flight
tread_count = riser_count - 1
Total run
run_in = tread_count * tread_in
Tread depth from an available run
tread_in = run_avail_in / tread_count
Comfort check
comfort_in = 2 * riser_in + tread_in
Uniformity spread
spread_in = riser_high_in - riser_low_in
Error shared per riser
err_each_in = rise_error_in / riser_count
The mode to equation map. A total rise with no run constraint uses the riser height from a count equation, and the tread depth is taken as the code minimum for the selected basis. A total rise with an available run uses the riser height from a count equation and the tread depth from an available run equation together, and both are tested. A rise measured to a subfloor uses the adjusted rise equation before anything else, and every count is computed from the adjusted figure.
The lowest and highest count equations produce fractions and are rounded outward: the lowest rounds up and the highest rounds down, because a count between them is a whole number and a count outside them is not compliant.
The highest compliant count exists only where the code publishes a minimum riser height. The commercial code does, at 4 inches, which bounds the range at both ends: on a 109 inch rise that is sixteen to twenty seven counts. The residential code publishes a maximum and no minimum, so the upper end of a residential list is a display choice rather than a code limit, and the page says which kind of bound it is showing.
There is no equation that takes a riser height and works backwards. A page whose argument is that the height is a consequence of the count cannot offer a route treating it as an input.
The stair angle is the arctangent of the riser over the tread. It is not in this block because a function call is not an arithmetic expression, and it is reported in the result instead.
The uniformity spread is nought on a stair built to a computed uniform height. It is non zero only where a builder sets a nominal height and lets a remainder fall into one step, which is what the tolerance exists to prevent, and this page demonstrates that with a fixed example rather than computing it from an input it deliberately does not have.
1 inch is 25.4 mm and 1 foot is 0.3048 metres. The 3/8 inch tolerance is 9.5 mm.
Riser Height Is Not an Input
Every other stair calculator asks for a riser height. That question cannot be answered, and asking it is what makes those pages agree with whatever you type.
Three things are fixed before anyone thinks about a stair.
The total rise is fixed by the building. It is the vertical distance from the finished surface of the lower floor to the finished surface of the upper one, and nobody framing the stair can change it.
The riser count is a whole number. Half a step does not exist, so the count is an integer and not a continuous variable.
And the riser height is what those two produce. It is the rise divided by the count, and there is no third thing to choose.
On a 109 inch rise, fourteen risers gives 7.786 inches, fifteen gives 7.267, sixteen gives 6.813 and seventeen gives 6.412. Those are the only riser heights available on that building. A 7.5 inch riser would need 14.53 risers, and 14.53 risers do not exist.
So a calculator that asks for a riser height is asking the reader to supply the answer, and then confirming it back to them. If the number they typed happens to be unreachable, nothing on the page says so.
This page takes the rise and returns the counts. Each one comes with the height it produces and whether it passes, and the counts either side that fail are shown as well, because the failures are what demonstrate that the choice was between whole numbers all along.
It does not select for you either. Where several counts comply they differ in comfort, in the total run they need, and in how they fit the opening they pass through. None of that is knowable from a rise, and a page that picked one would be making a design decision on your behalf with nothing behind it.
Stair Tread Depth and Total Run
The tread is the second decision and it is looser than the first, because code sets a floor rather than a value.
The tread count is one less than the riser count on a straight flight that ends on the upper floor. Fifteen risers gives fourteen treads, because the fifteenth riser lands on the floor rather than on a tread. That off by one is the commonest arithmetic slip on the subject.
Where the stair has no run constraint, the tread depth is a choice above the code minimum: 10 inches under the 2018 and 2021 residential editions, 9 inches under 2024 per published guidance, and 11 inches under the commercial code. Deeper treads give a shallower, more comfortable stair and cost floor space.
Where the run is constrained, the arithmetic reverses. The tread depth becomes the available run divided by the tread count, and it is then tested against the code minimum rather than assumed to clear it. A run that is too short does not produce a shallower stair, it produces a stair that cannot be built.
Worked on a fifteen riser stair, which has fourteen treads. At the 10 inch residential minimum the total run is 140 inches, which is 11 feet 8 inches or 3.56 metres. At an 11 inch tread it is 154 inches, 12 feet 10 inches or 3.91 metres. The extra inch on each tread costs over a foot of floor.
That is why the count and the run are connected. A higher count gives a shorter riser and a more comfortable stair, and it also adds a tread, which lengthens the run. On a tight opening the comfortable option may not fit, and the page reports the run for each count so the trade is visible rather than discovered later.
The nosing does not enter this arithmetic. Tread depth is measured between adjacent treads and excludes the projection, which is covered in its own section below.
Stair Code by Edition
The governing numbers on this page have moved, and a figure quoted without its edition is not checkable.
Under the 2018 and 2021 International Residential Code, R311.7.5.1 sets a maximum riser height of 7 3/4 inches and R311.7.5.2 a minimum tread depth of 10 inches.
Published guidance on the 2024 International Residential Code gives a maximum riser height of 8.25 inches and a minimum tread depth of 9 inches. Both figures moved in the same revision, in the same direction: steeper stairs are permitted.
Under the International Building Code, Section 1011.5.2 sets a maximum riser of 7 inches, a minimum of 4 inches, and a minimum tread depth of 11 inches for stairs in the means of egress.
The effect is not a matter of wording. On a 109 inch total rise the lowest compliant count is fifteen risers under the 2021 residential code, fourteen under the 2024 residential code, and sixteen under the commercial code. Three editions, three different stairs on the same building.
There is a second difference between the two codes that matters for how a count list is presented. The commercial code publishes a minimum riser as well as a maximum, so the range of compliant counts is bounded at both ends: on that rise it runs from sixteen to twenty seven. The residential code publishes a maximum and no minimum, so there is no upper bound in code at all. Fifty risers of 2.18 inches sits inside a 7 3/4 inch maximum and is not a stair anyone would build, so a residential list is bounded by practicality and this page says which kind of bound it is showing.
What governs is the edition your jurisdiction has adopted, which is not always the newest one. Adoption lags publication by years in many places, and some jurisdictions amend the figures further. That is why the code basis and the edition are both required before any verdict is issued here.
Stair Riser Uniformity and the 3/8 Inch Rule
This is the rule where the arithmetic stops being arithmetic.
Published IRC R311.7.5.2.1 and IBC 1011.5.4.1 both require that the greatest riser height within a flight not exceed the smallest by more than 3/8 inch, which is 9.5 mm. The same limit applies to the difference between the greatest and smallest tread depth.
Published guidance calls the riser version the single most commonly violated stair code requirement, and explains why it exists: the body develops a walking rhythm from the first few steps, and an unexpected change in riser height disrupts that rhythm and causes stumbles. The same guidance cites a United States emergency department study estimating about 1,076,558 stair related injuries a year.
The way it is violated is almost always the same. A builder sets a nominal riser height, cuts that many steps, and lets the last one take whatever is left.
On a 109 inch rise, fourteen risers cut at 7.25 inches uses 101.5 inches, leaving a last riser of 7.5. The spread is a quarter of an inch, which happens to sit inside the tolerance. Cut at 7.0 inches and fourteen risers uses 98, leaving a last riser of 11.0 and a spread of four inches, which is not a stair.
The correct method removes the question entirely. Divide the total rise by the count and every riser is identical by construction. The spread is nought rather than within a tolerance.
And the tolerance is smaller than it looks, because it is spent before any cutting begins.
Published guidance states that if the total rise is off by even half an inch, the error is distributed across all risers and the top or bottom may end up outside the tolerance. Half an inch over fifteen risers is 0.033 inches each, nine percent of the allowance. A three quarter inch hardwood floor laid after the stair is built moves the top riser by three quarters of an inch, which is twice the entire tolerance.
The remedy is one sentence: measure from finished floor to finished floor, and account for any flooring not yet installed. That is why this page asks how the rise was measured and marks the result preliminary where the finish is not in the figure.
Stair Headroom Requirements
Headroom is the test most likely to fail on a stair whose every dimension complies, because it depends on the opening above rather than on the flight.
Published IRC R311.7.2 and IBC 1011.3 both require headroom of not less than 6 feet 8 inches, which is 80 inches or 2,032 mm.
How it is measured matters as much as the figure. It is taken vertically from the sloped line adjoining the tread nosings, or from the floor surface of a landing or platform, to the lowest point above: a ceiling, a beam, a duct or a light fitting. The measurement is not from a tread and it is not perpendicular to the slope.
That plane runs the length of the flight, so the clearance has to hold at every point along it and across the full width, not only at the middle.
Three exceptions are published. Where the nosings of treads at the side of a flight extend under the edge of a floor opening the stair passes through, the opening may project horizontally into the required headroom by a maximum of 4 3/4 inches. Spiral stairways are permitted 78 inches. And existing stairs serving unfinished attics or basements being converted to habitable space may be 6 feet 4 inches where the pitch cannot be reduced because of existing construction.
That third exception is worth knowing before an attic conversion, because it is the difference between a stair that can be kept and one that has to be rebuilt.
A worked case. A compliant fifteen riser stair under an opening giving 76 inches of clearance fails headroom by 4 inches while every dimension of the flight complies. Nothing about the riser height or the tread depth will fix that. The opening has to change, or the stair has to move.
Stair Width and Handrail Clearance
Stair width is two requirements at two heights, which is why it is easy to measure the wrong one.
Published IRC R311.7.1 requires stairways to be not less than 36 inches in clear width at all points above the permitted handrail height and below the required headroom height. That is the figure most people know.
At and below handrail height a different requirement applies, because handrails intrude. Handrails may project not more than 4.5 inches on either side, and the minimum clear width at and below handrail height is 31.5 inches where a handrail is installed on one side and 27 inches where handrails are provided on both.
So a stair framed at 36 inches with two handrails is compliant above them and has 27 inches of clear width where the handrails are, which is what a person carrying furniture actually has to pass through.
A handrail is required on at least one side of every stairway with four or more risers. Below that count the requirement does not apply, which is why short entry flights often have none.
This page tests the width you enter and reports the reduced widths that apply at and below handrail height. It does not design a handrail: the graspable cross section, the height, the continuity, the returns and the extensions at the top and bottom are a separate set of requirements and a separate exercise.
Where no handrail answer is given, the full 36 inches governs, because the reduced widths are allowances that exist only where handrails do.
Stair Nosing and How Tread Depth Is Measured
The nosing is the part of a tread that overhangs the riser below, and the commonest error on this subject is counting it twice.
Tread depth is measured horizontally between the vertical planes of the foremost projection of adjacent treads. It does not include the nosing projection. A tread with a 10 inch depth and a 1 inch nosing is a 10 inch tread, not an 11 inch one.
That matters because under the commercial code an 11 inch minimum applies, and a reader who added the nosing would believe a 10 inch tread passes. Under the 2024 residential edition, published guidance describes the measurement as nose to nose on treads with nosings and surface to surface on treads without.
The nosing is required conditionally rather than always. Where the tread depth is less than 11 inches, published code requires a projection of not less than 3/4 inch and not more than 1 1/4 inches beyond the face of the riser below. Where the tread is 11 inches or deeper the requirement does not attach, which is why this page asks about the nosing only after the tread depth is known.
The profile is regulated too. The nosing must have a radius of curvature not greater than 9/16 inch or a bevel not greater than 1/2 inch. A sharp square edge is a common deficiency at final inspection.
What the nosing does is give the foot more landing surface during descent than the tread depth alone provides. It is extra room to put a foot on, not extra tread.
One further point about measurement. Published code states that for the purposes of the tread and riser section, all dimensions and dimensioned surfaces are exclusive of carpets, rugs and runners. A stair measured over its carpet is not measured to the code.
A 3/8 inch uniformity limit on nosing projection appears in one published reproduction. It is not stated here as a general requirement, because a single source is not enough to put a compliance test in front of a reader.
Stair Comfort Rule Versus Code
Two different questions get asked with the same numbers, and confusing them produces a stair that is compliant and unpleasant, or comfortable and non compliant.
The comfort rules are trade guidance with no legal force. The classic pairing is a 7 inch riser with an 11 inch tread. The equation behind it is 2R plus T, which should land between 24 and 25 inches.
A 7 inch riser with an 11 inch tread gives exactly 25.0. A 7.5 inch riser with a 10 inch tread also gives 25.0, and is a noticeably steeper stair with the same score, which is a limit of the rule rather than a coincidence.
The code rules are limits and they are enforceable. A 7 3/4 inch riser with a 10 inch tread gives a 2R plus T of 25.5. That sits outside the comfort band and is fully compliant under the 2018 and 2021 residential codes.
So the two tests disagree, and neither is wrong. One asks whether the stair may be built and the other asks whether it will be pleasant to climb.
This page reports the comfort figure and labels it. It never returns a comfort result as a failure, because failing a guideline is not failing anything, and a reader who saw a red mark against a compliant stair would reasonably conclude the page had found a code problem.
The practical use of the rule is in choosing between compliant counts. Where three counts all pass, the comfort figure is one of the few things that distinguishes them, alongside the run each needs and how it fits the opening.
What Is a Stair Calculation
A stair calculation turns a fixed vertical distance into a set of steps that a person can climb without thinking about it.
The vertical distance is the only thing truly given. It is the total rise, measured from the finished surface of the lower floor to the finished surface of the upper one, and nobody building the stair can change it.
Everything else follows from a single decision, which is how many steps to divide that distance into. That number is a whole number, and once it is chosen the riser height is fixed by arithmetic rather than by preference.
Where the codes intervene is in capping the riser, flooring the tread, holding the risers in a flight within three eighths of an inch of each other, and requiring headroom, width and a landing at intervals.
Key Facts
- Published IRC R311.7.5.1 sets a maximum riser height of 7 3/4 inches in the 2018 and 2021 editions, measured vertically between the leading edges of adjacent treads.
- Published guidance on the 2024 IRC gives a maximum riser height of 8.25 inches and a minimum tread depth of 9 inches, so the edition changes the answer and not only the wording.
- Published IBC 1011.5.2 sets a maximum riser of 7 inches, a minimum of 4 inches, and a minimum tread depth of 11 inches for stairs in the means of egress.
- Published IRC R311.7.5.2 sets a minimum tread depth of 10 inches in the 2018 and 2021 editions.
- On a 109 inch total rise the lowest compliant count is fifteen risers at 7.267 inches under the 2021 residential code, fourteen at 7.786 under the 2024 residential code, and sixteen at 6.813 under the commercial code.
- Published IRC R311.7.5.2.1 and IBC 1011.5.4.1 require that the greatest riser height within a flight not exceed the smallest by more than 3/8 inch, which is 9.5 mm. The same limit applies to tread depth.
- Published guidance calls the riser uniformity rule the single most commonly violated stair code requirement.
- Half an inch of error in the total rise spread over fifteen risers is 0.033 inches each, nine percent of the tolerance, before any cutting begins.
- Published IRC R311.7.2 and IBC 1011.3 require headroom of not less than 6 feet 8 inches, which is 80 inches or 2,032 mm, measured vertically from the sloped line adjoining the tread nosings or from a landing surface.
- A floor opening whose tread nosings extend under its edge may project 4 3/4 inches into the required headroom. Spiral stairways are permitted 78 inches. Existing stairs serving unfinished attics or basements being converted may be 6 feet 4 inches.
- Published IRC R311.7.1 requires a clear width of not less than 36 inches above handrail height, with handrails projecting not more than 4.5 inches, and a clear width at and below handrail height of not less than 31.5 inches with one handrail and 27 inches with two.
- A handrail is required on at least one side of every stairway with four or more risers.
- Published code requires a nosing projection of 3/4 to 1 1/4 inches where the tread depth is less than 11 inches, with a radius not greater than 9/16 inch or a bevel not greater than 1/2 inch.
- Tread depth does not include the nosing projection. In the 2024 edition it is measured nose to nose on treads with nosings and surface to surface on treads without.
- Published code states that dimensions in the tread and riser section are exclusive of carpets, rugs and runners.
- The vertical rise permitted between floor levels or landings is given as 147 inches by several sources and as 12 feet 7 inches, or 151 inches, by another, while 144 inches circulates widely. This page reports the disagreement rather than choosing.
- The comfort equation 2R plus T of 24 to 25 inches is guidance and not code. A 7 3/4 inch riser with a 10 inch tread gives 25.5 and is compliant under the 2021 residential code.
- A fifteen riser flight has fourteen treads, and at a 10 inch tread the total run is 140 inches, which is 11 feet 8 inches or 3.56 metres.
- Published guidance cites a United States emergency department study estimating about 1,076,558 stair related injuries per year.
Applications
- A builder framing a new stair finds the counts that comply before cutting a stringer, rather than discovering at inspection that the riser is an eighth over.
- A homeowner adding a basement stair sees that the rise measured to a subfloor will change when the floor goes down, and by how much.
- A designer fitting a stair into an opening compares the run each compliant count needs against the space available.
- A remodeller working on an older house finds that the existing stair fails the current riser maximum and learns which counts would pass if it were rebuilt.
- An inspector or plan reviewer checks a submitted riser height against the edition the jurisdiction has adopted rather than against the figure they remember.
- Someone converting an attic finds the existing stair headroom exception at 6 feet 4 inches and whether it reaches their work.
- A contractor pricing a commercial fit out sees that the residential figures they know do not apply and that the count list is bounded differently.
- A homeowner choosing between two compliant counts uses the comfort figure and the run to decide, having been told that neither is a code test.
Worked Examples
Example 1. The counts on a real rise.
Given: a total rise of 109 inches, residential code, 2021 edition, no run constraint.
Result: fourteen risers gives 7.786 inches and fails the 7 3/4 inch maximum. Fifteen gives 7.267 and passes. Sixteen gives 6.813 and passes. Seventeen gives 6.412 and passes.
The stair is fifteen, sixteen or seventeen risers, and the page reports all three with the failing fourteen alongside. It selects none of them.
Example 2. The same building, three different codes.
Given: the same 109 inches under the 2021 residential edition, the 2024 residential edition and the commercial code.
Result: the lowest compliant count is fifteen, fourteen and sixteen respectively. Under 2024 the fourteen riser stair that failed in example 1 now passes, because the maximum moved from 7 3/4 inches to 8.25.
The edition is not a formality. It changes which stair can be built on an unchanged building.
Example 3. The run each count needs.
Given: the same rise, at the 10 inch residential tread minimum.
Fifteen risers has fourteen treads and a run of 140 inches, which is 11 feet 8 inches. Sixteen risers has fifteen treads and 150 inches, 12 feet 6 inches. Seventeen has sixteen treads and 160 inches, 13 feet 4 inches.
Result: each step down in riser height costs ten inches of floor. On an opening that allows 145 inches, only the fifteen riser stair fits, and the more comfortable options are unavailable regardless of code.
Example 4. The remainder that does not fit.
Given: the same 109 inch rise, built by cutting fourteen risers at a nominal 7.25 inches and letting the last take what is left.
Fourteen at 7.25 is 101.5 inches, so the last riser is 7.5 and the spread is 0.25 inches, inside the 3/8 inch tolerance. Cut at 7.0 instead and fourteen risers is 98 inches, leaving a last riser of 11.0 and a spread of 4 inches.
Result: the first is legal by luck and the second is not a stair. Dividing 109 by fifteen gives 7.267 on every riser and a spread of nought.
Example 5. The floor that has not been laid.
Given: a rise measured to a subfloor, with three quarters of an inch of hardwood still to go down at the top, on a fifteen riser stair.
Result: 0.05 inches per riser, thirteen percent of the tolerance, from one flooring decision. A half inch measurement error over the same fifteen risers adds 0.033 inches each, another nine percent.
Together that is almost a quarter of the allowance spent before any saw cut, material variation or layout error.
Example 6. The stair that fails on the ceiling.
Given: a compliant fifteen riser stair under an opening giving 76 inches of clearance from the sloped line adjoining the tread nosings.
Result: fails headroom by 4 inches against the 80 inch requirement, while every dimension of the flight complies. No change to the riser or the tread fixes it, because the constraint is the opening.
Example 7. The tread that is an inch shallower than it looks.
Given: a tread with a 10 inch depth and a 1 inch nosing projection, under the commercial code.
Result: the tread depth is 10 inches and fails the 11 inch commercial minimum. Adding the nosing would give 11 and suggest a pass. The nosing is extra foot landing surface during descent, not tread depth.
Example 8. Compliant and uncomfortable.
Given: a 7 3/4 inch riser with a 10 inch tread, 2021 residential edition.
Result: 2R plus T is 25.5 inches, outside the 24 to 25 comfort band, and the stair is fully compliant. The page reports the comfort figure and does not mark it as a failure, because failing a guideline is not failing anything.
Standards & References
- International Code Council Digital Codes, 2018 International Residential Code, Section R311.7.2 Headroom The requirement that headroom in stairways be not less than 6 feet 8 inches measured vertically from the sloped line adjoining the tread nosing or from the floor surface of a landing, with the exception permitting a floor opening to project 4 3/4 inches into the required headroom.
- International Code Council Digital Codes, 2021 International Residential Code, Section R311.7 Stairways The stairway section covering width, headroom, vertical rise, riser height, tread depth and the uniformity limits.
- UpCodes, Section 1011 Stairways, International Building Code The commercial requirements including the riser height maximum and minimum, the tread depth minimum, headroom and the uniformity limit.
- Minnesota Administrative Rules 1309.0311, Section R311 Means of Egress A state adoption reproducing the stairway section in full, including the clear width figures of 36 inches above handrail height, 31.5 inches with one handrail and 27 inches with two, and the headroom requirement with its exceptions.
- Occupational Safety and Health Administration, 29 CFR 1910.25 Stairways The workplace standard, including the spiral stair vertical clearance of 6 feet 6 inches, for context on where a different rule set applies.
- National Institute of Standards and Technology, Special Publication 811, Guide for the Use of the International System of Units The authority for the conversion factors used on this page: 1 inch is 25.4 millimetres and 1 foot is 0.3048 metres, both exact.
Units
Rises, runs, riser heights and tread depths are entered and reported in inches with millimetres alongside, at 25.4 mm per inch. A 7 3/4 inch riser is 197 mm, a 7 inch riser is 178 mm, a 10 inch tread is 254 mm and an 11 inch tread is 279 mm.
The uniformity tolerance of 3/8 inch is 9.5 mm, which the code itself states.
Headroom is reported in feet and inches with millimetres alongside. Six feet eight inches is 80 inches and 2,032 mm, and the code states both.
Total rise is reported in inches and millimetres, with feet and metres alongside. A 109 inch rise is 2,769 mm, 9.08 feet and 2.77 metres. A 140 inch run is 3,556 mm, 11.67 feet and 3.56 metres.
The comfort figure, 2R plus T, is a sum of two quantities in the same unit and converts directly: 25 inches is 635 mm and the 24 to 25 band is 610 to 635.
Riser and tread counts are dimensionless and identical in both systems.
The internal unit selector governs the fields, labels, math, on-screen result and any exported result, and it takes priority over any site-wide unit switch. Switching converts the values you entered rather than reinterpreting them.
Limitations
- This page computes stair geometry and tests it against published limits. It does not design a structure, size a stringer, or calculate loads.
- It does not choose a count. Where several comply they differ in comfort, in the run they need and in how they fit the opening, and none of that is knowable from a total rise.
- It accepts no riser height in any form, including a target height or a nominal height for field layout, because the riser height is what it computes.
- It does not cover winders, spiral or circular stairs, which have their own code sections and their own headroom and tread rules.
- It does not design landings, handrails or guards. It names the handrail requirement at four or more risers and reports the clear widths that follow, and stops there.
- It issues no verdict on the vertical rise between landings while three figures are in circulation, and reports the rule with the disagreement named. Above 151 inches, where all three agree, it reports that the flight needs a landing.
- It does not state a nosing uniformity limit, which rests on a single source.
- The code figures come from published restatements as well as from the code text, and the table is to be locked against the text for every edition cited before a compliance verdict is relied upon.
- It calculates one flight at a time. A stair with a landing between two flights is two calculations.
- Every test runs on figures the reader supplied, and the local building department governs over any model code figure.
Common Mistakes to Avoid
- Choosing a riser height. The total rise is fixed and the count is a whole number, so the height is arithmetic rather than a decision.
- Letting the last step absorb the remainder. Cutting to a nominal height and taking what is left on the last riser is what the 3/8 inch tolerance exists to prevent, and it is named as the most commonly violated stair requirement.
- Measuring to a subfloor. A three quarter inch floor laid after the stair moves the top riser by twice the entire tolerance.
- Treating the tolerance as a working margin. It is what remains after the measurement error, and half an inch over fifteen risers already spends nine percent of it.
- Using a riser maximum from the wrong edition. The residential maximum is 7 3/4 inches in the 2018 and 2021 editions and 8.25 in the 2024 edition, and the tread minimum moved from 10 inches to 9 in the same change.
- Applying residential figures to commercial work. The commercial maximum riser is 7 inches and the minimum tread is 11, so a stair that passes in a house fails in an office on both counts.
- Counting the treads as equal to the risers. A flight ending on the upper floor has one fewer tread than risers, and the run is computed from the treads.
- Counting the nosing in the tread depth. The nosing overhangs the riser below and is not part of the measurement, so a 10 inch tread with a 1 inch nosing is 10 inches and not 11.
- Treating the 7 by 11 pairing as a code minimum. It is a comfort guideline, and a 7 3/4 by 10 stair is compliant residentially while sitting outside the band.
- Assuming headroom passes because the flight does. Headroom depends on the opening above, and it is the test most likely to fail where every dimension of the flight complies.
- Measuring headroom from a tread rather than from the nosing line. The code measures vertically from the sloped line adjoining the tread nosings.
- Measuring stair width at the wrong height. The code sets 36 inches clear above handrail height and different figures at and below it, 31.5 inches with one handrail and 27 with two.
- Quoting the landing rule as twelve feet. Three figures are in circulation, 144, 147 and 151 inches, and none should be quoted until the edition is checked.
- Including carpet in the dimensions. Code states that tread and riser dimensions are exclusive of carpets, rugs and runners.
Frequently Asked Questions
What is the maximum stair riser height?
What is the minimum tread depth?
How do I calculate the number of stairs?
Why can I not just pick a riser height?
How much can stair risers vary?
What headroom does a staircase need?
How wide does a staircase have to be?
Is the 7 by 11 rule a code requirement?
How many treads does a staircase have?
How long is a staircase?
When is a handrail required on stairs?
Does the nosing count towards the tread depth?
Related Calculators
Explore similar calculators that might be useful for your project:
Calculate
This selector governs every field, label, result and export here and takes priority over the site header switch. Switching converts what you entered rather than reinterpreting it, so 109 in becomes 2,769 mm and returns to exactly 109.
The Code Basis
Answer this before any dimension. The two codes differ on the riser maximum, on the tread minimum, and on whether a minimum riser exists at all, and that last difference decides whether the count list has an upper bound.
The residential riser maximum moved from 7 3/4 inches to 8.25 in the 2024 edition and the tread minimum from 10 to 9. No verdict is issued until the edition is named, because the same rise gives a different lowest count under each.
The Total Rise
In inches, measured vertically from the finished surface of the lower floor to the finished surface of the upper one. This is the one figure the building fixes and the whole page divides.
The uniformity tolerance is spent before any cutting begins, so how the rise was arrived at decides how much of it is already gone. A subfloor measurement produces a preliminary result rather than a blocked one.
In inches, and optional. Flooring still to be laid on the upper floor raises it and increases the rise. A three quarter inch hardwood floor moves the top riser by twice the entire uniformity tolerance.
In inches, and optional. Flooring still to be laid on the lower floor raises it and reduces the rise, which is the opposite direction to the top figure. A tile bed with its setting material is around an inch and a quarter.
In inches, and optional. Leave it empty where the stair is not constrained by a space, and the tread is taken at the code minimum. Enter it and the tread comes from the run instead and is tested against that minimum.
The Count You Choose
Leave this empty on the first run and the page returns the counts that comply, with the riser height each produces. Then enter one of them. The page will not choose for you, because the counts differ in comfort, run and fit.
The Other Tests
In inches, and optional. Named explicitly because the code sets a different clear width at and below handrail height, where handrails intrude. Measure above the permitted handrail height and below the required headroom height.
The reduced clear widths at and below handrail height are allowances that exist only where handrails do. With none installed the full 36 inches governs at every height, and the answer decides which figure is reported.
In inches, and optional. Measured vertically from the sloped line adjoining the tread nosings, or from a landing surface, to the lowest point above. Not from a tread and not perpendicular to the slope.