Fence Calculator — A Closed Yard Does Not Need the Extra Post
On this page
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 a 200 ft run becomes 60.96 m and returns to exactly 200.
The Layout
This is the one answer that changes the post count by a whole post, and there is no default that is right across the four. Two of them close and two do not, and what separates them is whether the run has ends rather than whether it is straight.
The Fence
In feet, above ground. This is not the post length: on a six foot fence the post is eight or nine feet depending on which reading of the depth rule you take.
Eight feet is the published residential standard for wood and chain link and matches prebuilt six by eight and four by eight panels. Horizontal slat fences take four to six, because horizontal boards sag between posts. High wind areas reduce to six or less.
The Depth
Published sources give two readings of the same rule and their worked examples follow neither consistently, so there is no default here that is not a choice made for you. Both readings are reported whichever one you pick.
Over most of the northern half of the country the frost line sets the depth and the fence height drops out of the calculation entirely. Not known is an answer and is offered as one.
The Gates
None is an answer and is offered as one. A gate is not fence: the opening comes off the length before the sections are counted, and it splits one run into two.
The Waste Allowance
Published guidance gives ten percent for bad cuts, warped boards and mistakes, which are lumber problems. It runs on the rails and the pickets and on nothing else: a post, a hole and a bag are counted rather than cut.
The Hole and the Concrete
Optional. Nothing here changes the post count above. A four by four is a trade designation rather than a measurement, and the actual dressed size is what the hole rule and the concrete volume run on.
In inches. Leave empty to take the published rule of about three times the post width, which is 10.5 inches for a four by four and lands inside the published 10 to 12 inch band.
In inches, at the bottom of the hole. Published guidance gives it as improving drainage and reducing the concrete fill depth. The post sits on it.
In pounds. Published yields are 0.375 cubic feet for a fifty pound bag, 0.45 for a sixty and 0.6 for an eighty, and the page uses them only where you do not enter a yield of your own.
In cubic feet, printed on the bag. It beats the published figure outright, because the three published bag counts per post differ by the hole rather than by the concrete.
The Rails and the Pickets
Optional whole number. Leave empty to take the published height rule, which gives two rails for most privacy fences and three above six feet. A second published source instead gives the height in inches divided by twenty four, rounded up, and the page reports both.
In inches, the actual dressed width rather than the nominal name. Enter the gap as well, or the page reports no picket count.
In inches. A gap of nought is a valid answer and gives a tight board fence, so leaving the field empty is not the same as entering nought.
Prices
Optional, in your own currency. Any cost here is arithmetic on a price you entered.
Optional, in your own currency. Applied to the order quantity rather than the counted one.
Optional, in your own currency. Applied to the order quantity rather than the counted one.
Optional, in your own currency. Applied to the bag count computed from your hole.
Overview
Two hundred feet of fence at eight foot spacing is twenty five sections. On a straight run that is twenty six posts. Around a yard it is twenty five.
Every fence calculator adds the extra post, and one publishes it as a law: a fence post count follows a single rule across every style, material and code jurisdiction in the United States, which is to divide the fence length by the post spacing and add one for the end of the run.
A closed loop has no end of the run. The last section arrives back at the post the first section started from, so the count is the section count and nothing is added.
That is one post, one hole, one set of concrete, and it is in the direction of over-ordering.
The depth rule is the second thing to get wrong. Published manufacturer guidance is clear: bury one third of the total post length, so a six foot fence takes a nine foot post with thirty six inches in the ground. Four calculator sites state that wording and then work an example burying two feet of an eight foot post, which is a quarter rather than a third.
And in the northern half of the country none of it matters, because the hole has to reach below the frost line. In Minnesota a six foot fence can need seventy eight inches of hole, which is deeper than the fence is tall.
What to Look at First
Read the layout line before you read the post count. It is the only input on this page that moves the total by a whole post, and the four answers give four different counts from the same length. A closed loop and a closed segmented perimeter return the section count with nothing added. A straight run and an open segmented run return one more, because each has two terminal ends.
Then read the depth line and check which figure is named as governing. On a six foot fence the two published readings of the one third rule differ by a foot of post, and in a cold state neither of them is the answer: at a thirty inch frost line the requirement is thirty six inches, at forty two it is forty eight, and the fence height has dropped out of the calculation entirely.
If the frost line reads as not evaluated, that is not the same as frost not binding. It means the page was not given a figure and will not guess one, because the same fence is right in Georgia and wrong in Minnesota.
And read the gate post row as a split rather than an addition. The two posts a gate needs are the end posts of the runs either side of the opening, so they are already positions on the line. Adding two on top of a continuous count is the same error as adding one to a closed yard, in a different place.
How to Use This Calculator
Say what shape the fence is before anything else. A straight run, a closed loop and two kinds of segmented layout give four different post counts from the same length, and the difference is a whole post.
Where the fence is segmented, say whether it closes. A perimeter around a yard closes and a run along two sides of a plot does not, and nothing in the segment lengths tells the page which you have. Enter each straight run between corners as its own segment, because a segment rounds up on its own.
Enter your local frost line if you know it. Over most of the northern half of the country it sets the depth and the fence height drops out of the calculation entirely. If you do not know it, say so: the page will report the height based depth and state plainly that frost was not evaluated.
Choose which reading of the one third rule you want. Manufacturer guidance uses one third of the total post length, and the page reports the other reading beside it so you can see where an eight foot post came from.
Subtract nothing for gates yourself. Enter the gate widths and the page takes them off the fence length before the sections are counted. On a segmented layout it also asks which segment each gate is in, because the opening has to come off the segment that holds it.
Read the picket count as a module. The last gap almost never lands on the gap you typed, and the page reports the remainder rather than spreading it. A gap of nought is a valid answer and gives a tight board fence, so leaving the field empty is not the same as entering nought.
Inputs & Outputs
Inputs
Outputs
Fence Formula
The run, the sections and the posts.
Fence length less the gates
net_len_ft = fence_len_ft - gate_total_ft
Sections
sections = net_len_ft / spacing_ft
Sections across segments
sections_all = sec_1 + sec_2 + sec_3
Runs created by the gates, open layout
runs = gates + 1
Runs created by the gates, closed layout
runs_loop = gates
Posts on an open run
posts_open = sections + runs
Posts on a closed loop
posts_loop = sections + gates
Posts on an open segmented run
posts_seg_open = sections_all + gates + 1
Posts on a closed segmented perimeter
posts_seg_loop = sections_all + gates
The posts by kind.
Gate posts, already inside the count
gate_posts = gates * 2
Terminal end posts
end_posts = runs * 2 - gate_posts
Line posts
line_posts = posts_total - corners - gate_posts - end_posts
The depth and the post length.
Depth, one third of the total
depth_total_ft = height_ft / 2
Post length, one third of the total
post_len_total_ft = height_ft * 1.5
Depth, one third of the height
depth_height_ft = height_ft / 3
Post length, one third of the height
post_len_height_ft = height_ft + depth_height_ft
Depth from frost
depth_frost_in = frost_in + 6
The hole.
Hole volume
hole_cf = 3.1416 * hole_r_ft ^ 2 * depth_ft
Concrete depth in the hole
conc_depth_ft = depth_ft - gravel_ft
Post volume in the concrete
post_cf = post_w_ft * post_w_ft * conc_depth_ft
Gravel per hole
gravel_cf = 3.1416 * hole_r_ft ^ 2 * gravel_ft
Concrete per hole
conc_cf = hole_cf - post_cf - gravel_cf
Bags per hole
bags = conc_cf / bag_yield_cf
The rails, the pickets and the order.
Rails
rails = sections * rails_per
Pickets per section
pickets_sec = spacing_in / (pick_w_in + gap_in)
Pickets total
pickets = net_len_in / (pick_w_in + gap_in)
Rails with the waste allowance
rails_order = rails * (1 + waste_pct / 100)
Pickets with the waste allowance
pickets_order = pickets * (1 + waste_pct / 100)
A gate splits a run rather than adding to it, and what it adds depends on whether the layout already had ends.
On an open run each gate turns one run into two, so the post count is the sections plus the number of runs, which is the gate count plus one.
On a closed layout there were no ends to begin with, so the first gate opens the loop and each gate after it splits again. A loop with gates has as many posts as sections plus gates: a closed yard of twenty five sections has twenty five posts with no gate, twenty six with one and twenty seven with two.
The four post equations then say one thing in four forms: the post count is the sections plus the number of places the line ends. That is zero on a closed loop with no gate, one for each gate that opens it, and two for each open run. The two gate posts for each gate are the end posts of the runs either side of it and are already inside that figure, so the gate post row names them rather than adding them.
On a hundred foot straight run at eight foot spacing with one four foot gate, the fence is ninety six feet in two runs of forty eight. That is twelve sections and two runs, so fourteen posts, of which two are gate posts. Counting thirteen posts for a continuous run and adding two gate posts gives fifteen, and there are fourteen positions on the line.
The same run with no gate at all also has fourteen posts. The gate removed a section of fence and left the post count where it was.
There are four post equations and the layout selects one. Two add a post and two do not, and what separates them is whether the run closes rather than whether it is straight. A closed loop and a closed segmented perimeter have as many posts as sections. A straight run and an open segmented run have one more, because each has two terminal ends.
The four kinds of post add back to the total rather than sitting on top of it. That is why the line post equation subtracts the terminal ends as well as the corners and the gate posts: a post is a position, and two roles meeting at one position is still one post.
The segmented pair is asked rather than inferred. A perimeter around a yard closes and a run along two sides of a plot does not, and nothing in the segment lengths distinguishes them. Inferring it would be the same error this page exists to correct, applied to the route where a closed layout is likeliest.
The depth equations look strange because the rule does. Burying one third of the total post length means the buried part is half the exposed part, so a six foot fence takes a three foot burial and a nine foot post. Burying one third of the above ground height means two feet and an eight foot post on the same fence. The page runs whichever was chosen and reports the other beside it.
The frost equation adds six inches to the frost line. Where a frost line is entered the result reports the deeper of the figures and names which governs, and the published twenty four inch minimum sits alongside as a third contender that binds on short fences.
The hole equations subtract the post from the concrete and the gravel from the hole, and the post is subtracted only over the concrete depth, because the post sits on the gravel base rather than passing through it. Leaving the post in over-orders on every hole.
The pickets per section and pickets total equations are separate because they answer different questions. A section count times a per section count is not the total where the last section is short, and a total divided by a module is not a layout.
The waste allowance runs on the rails and the pickets and on nothing else. Published guidance gives ten percent for bad cuts, warped boards and mistakes, which are lumber problems. Posts, holes and bags are counted rather than cut.
Sections round up. Rounding is a function rather than an arithmetic expression, so it is stated here: a hundred feet at eight foot spacing is 12.5 sections, which is 13, and the last bay is four feet.
The three terms in the sections across segments equation are representative. The build sums as many segments as were entered.
What Is a Fence Calculation
A fence calculation is a post count with three things hanging off it, and the post count depends on a question most calculators do not ask.
The count is the length divided by the spacing, rounded up to whole sections. Whether a post is then added depends on whether the run has ends. A straight fence has two and needs one more post than it has sections. A perimeter has none, because the last section returns to where the first one started.
Hanging off the count are the holes, the rails and the pickets. The holes need a depth, and the depth comes from whichever is deeper of a rule about the post and a line on a frost map. The rails follow the height. The pickets follow a module of width plus gap, and the last one lands where it lands.
Gates sit outside all of it. A gate is not fence, so its width comes off the length before the sections are counted, and it brings two posts of its own that are not line posts and are set deeper.
What the arithmetic cannot do is place anything. Where the short bay falls, how the last gap is absorbed, and what happens where the ground slopes are layout decisions, and this page reports quantities rather than drawing them.
The Picket Count Is a Module and the Last Gap Is Where It Fails
The picket arithmetic is simple: the run in inches divided by the picket width plus the gap. Published examples reproduce. An eight foot section with 5.5 inch pickets and 1 inch gaps takes about fifteen, and with 3.5 inch pickets and 2 inch gaps takes seventeen or eighteen. A hundred feet of 5.5 inch pickets butted tight is about 218 boards.
What the arithmetic cannot do is land the last gap.
A hundred feet is 1,200 inches. At a 6.5 inch module that is 184.6 pickets, so 185 boards, and after 184 full modules the run has four inches left. Four inches is less than one 5.5 inch picket, so the last board is ripped down, or every gap is opened slightly, or an odd gap is left at one end.
None of those is arithmetic and this page does none of them. It reports the count, reports the closing remainder, and says that the count is a module rather than a layout.
That matters because the alternative is worse. A page that silently distributed the remainder across every gap would report a picket count that is right and a gap that is not the one you typed, and you would only find out with a tape measure against a finished fence.
The same thing happens one scale up, with the bays rather than the boards. A hundred foot run at eight foot spacing is 12.5 sections, which rounds to thirteen, and the last bay is four feet. The page reports it rather than widening every bay to absorb it, because where a short bay falls is a decision made on the ground: usually at a corner, or behind a shed, or wherever the ground is worst.
Key Facts
- A closed loop has as many posts as sections, because the last section returns to the first post. A straight run has one more, because it has two ends.
- Two hundred feet at eight foot spacing is twenty five sections: twenty six posts on a straight run and twenty five around a yard.
- One published source states the add one rule as a single rule across every style, material and code jurisdiction in the United States, and then states two paragraphs later that one end post is added only at the terminal ends of the complete run.
- A segmented layout can be either. A perimeter around a yard closes and a run along two sides of a plot does not, and the segment lengths do not say which.
- Published manufacturer guidance gives the depth rule as burying one third of the total post length: a six foot fence with a nine foot post is thirty six inches below grade, and an eight foot fence with a twelve foot post is forty eight.
- Four calculator sites state that same wording and then work a six foot fence as an eight foot post buried two feet, which is a quarter of the total rather than a third.
- Two other sources state the rule as one third of the above ground height, which gives the eight foot post directly.
- The bottom of the concrete should sit at least six inches below the local frost line, and the published minimum otherwise is twenty four inches, whichever is deeper.
- Frost lines are published from six to twelve inches in Texas and Florida to forty eight to sixty in Minnesota and Wisconsin.
- In northern states a six foot fence can need a hole fifty four to seventy eight inches deep, and seventy eight inches is deeper than the fence is tall.
- The frost protection section covers foundation walls, piers and other permanent supports of buildings and structures, and its only exception names decks rather than fences, so whether it reaches a fence post is a local determination.
- Where fences under six feet are permit exempt, frost depth compliance is the owner's responsibility without any inspection.
- Gate posts are published as set four to six inches deeper than line posts, with many contractors going eight, because a gate hangs its weight off one post and swings it.
- A gate splits a run rather than adding to it. Its two posts are the end posts of the runs either side, so they are already positions on the line: a hundred foot run with one gate has fourteen posts, the same as without it, and two of them are gate posts.
- The opening comes off the fence length before the sections are counted, so the gate removes a section of fence and leaves the post count where it was.
- Eight foot spacing is the residential standard and matches prebuilt six by eight and four by eight panels; chain link spans ten; horizontal slat fences take four to six because horizontal boards sag; and high wind areas reduce to six or less.
- Vinyl and aluminium panels come in fixed widths that set the spacing rather than allowing a choice, and published guidance requires half inch expansion gaps because vinyl expands about a quarter inch per eighteen feet with temperature.
- The hole diameter is published as about three times the post width, which is ten to twelve inches for a four by four. A four by four is actually 3.5 inches across, so three times it is 10.5.
- Published bag counts per post run from 1.5 to four, and they differ because each names a different hole size rather than a different concrete. Against the published bag yields they all reconcile at around the twenty four inch minimum depth.
- Published rail counts are two for most privacy fences and three above six feet. A second published source gives the height in inches divided by twenty four, rounded up, which returns three on a six foot fence.
- An eight foot section takes about fifteen pickets at 5.5 inch width with 1 inch gaps, and about eighteen at 3.5 inch width with 2 inch gaps.
- A hundred foot run at a 6.5 inch picket module takes 185 boards and leaves four inches at the end, which is less than one picket.
- Published waste is ten percent, and it belongs on rails and pickets rather than on posts and bags, because it covers bad cuts and warped boards.
Applications
- Someone fencing a yard finds out that the extra post every calculator adds is not needed.
- A reader with an L shaped run along two sides of a plot finds the layout that does need it.
- Someone quoted a nine foot post by one supplier and an eight by another sees both readings of the rule and which sentence each came from.
- A reader in a cold state finds that the frost line, not the fence height, sets the hole depth, and by how much.
- Someone planning a gate sees the opening removed from the fence length and two deeper posts counted as positions already on the line.
- A reader ordering concrete gets a bag count from their own hole size rather than a figure from someone else's.
- Someone laying out pickets sees the closing gap before the fence is built rather than at the last board.
- A reader comparing quotes checks whether a contractor counted the loop correctly.
Example Calculations
Example 1. The same fence, two shapes, two counts
Given: two hundred feet at eight foot spacing.
Two hundred divided by eight is twenty five sections either way.
On a straight run the count is twenty five plus one, which is twenty six posts, because the run has two terminal ends and each needs a post.
Around a yard the count is twenty five, because the last section arrives at the post the first section started from.
Result: twenty six or twenty five, from the shape rather than the length. The extra post is a post, a hole, its gravel and its concrete, and every published calculator adds it to both.
Example 2. Segments, and the question the lengths do not answer
Given: three segments of eighty, sixty and sixty feet at eight foot spacing.
Eighty divided by eight is ten sections. Sixty divided by eight is 7.5, which rounds up to eight, twice. That is twenty six sections in total.
Closed as a perimeter: twenty six posts.
Open as a run along two sides: twenty seven.
Result: nothing in eighty, sixty and sixty says which. A perimeter around a yard closes and a run along the back and one side does not, and the page asks rather than guessing, because guessing wrong on the closed case is the error this page exists to correct.
Example 3. Two readings of one rule, on one fence
Given: a six foot fence.
One third of the total post length: the buried part is half the exposed part, so the post is nine feet and thirty six inches go in the ground. Published manufacturer guidance gives exactly that, and gives an eight foot fence as a twelve foot post buried forty eight inches.
One third of the above ground height: two feet buried and an eight foot post.
Result: a nine foot post or an eight foot post on the same fence, and a foot of timber per post between them. On twenty five posts that is twenty five feet of post.
Four calculator sites state the total length wording and then give the eight foot post buried two feet, which is a quarter of the total. The wording and the example do not match in the same article.
Example 4. Where the fence height stops mattering
Given: the same six foot fence, in Minnesota.
The one third rule gives thirty six inches.
Published frost lines for Minnesota run forty eight to sixty inches, and the hole must reach six inches below, so the requirement is fifty four to sixty six. Published guidance gives up to seventy eight inches for a six foot fence in northern states.
Result: seventy eight inches of hole under a seventy two inch fence. The hole is deeper than the fence is tall, the one third rule delivers less than half of what the ground demands, and the fence height has dropped out of the calculation entirely.
Example 5. A gate, and what it does to both counts
Given: a hundred foot straight run at eight foot spacing with one four foot gate in the middle.
The gate comes off first: a hundred less four is ninety six feet of fence.
The gate then splits that into two runs of forty eight feet. Each run is six sections and seven posts, because each has two ends of its own.
Result: fourteen posts, of which two are gate posts. They are the end post of the first run and the start post of the second, and they are gate posts and end posts at the same position.
Counting a continuous thirteen and adding two for the gate gives fifteen, and there are fourteen positions on the line. That is the same error as adding a post to a closed yard, in a different place: a post is a position, and two roles meeting at one position is still one post.
The same run without a gate also has fourteen posts. The opening removed a section of fence and left the count where it was. What the gate does change is the two posts at its opening, which published practice sets four to six inches deeper than the line posts, and often eight, because a gate swings its weight off one of them.
Standards & References
- International Code Council Digital Codes, 2021 International Residential Code, Chapter 4 Foundations The chapter containing the frost protection provisions for footings supporting buildings and structures, whose only stated exception names decks rather than fences.
- American Fence Association The trade association for fence contractors, publishing installation practice and industry guidance.
- American Wood Council The association publishing design and construction guidance for wood structures, including preservative treated posts in ground contact.
- American Wood Protection Association The standards body behind the ground contact retention levels that govern which treated post may be buried.
- ASTM International, ASTM F567 Standard Practice for Installation of Chain-Link Fence The installation standard covering post setting, spacing and footing practice for chain link.
- United States Department of Agriculture, Natural Resources Conservation Service The agency publishing soil survey and frost depth data that local frost line figures derive from.
- 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.
The frost protection requirement on this page is reported as published practice and as a local determination. The residential code section covers foundation walls, piers and other permanent supports of buildings and structures, and its only stated exception names decks rather than fences, so whether it reaches a fence post is decided locally: one published source attributes it to fence footings and qualifies itself in the same sentence with in jurisdictions that require permits for fence work, and another states that where fences under six feet are permit exempt the compliance is the owner's responsibility without inspection.
The depth rule is published manufacturer guidance stating one third of the total post length, with worked examples that follow the wording; four calculator sites state the same wording and work an example burying a quarter, and two state the rule as one third of the above ground height, so this page asks which reading you want and reports both.
Frost line depths, post spacing figures, hole diameters, bag counts, rail counts and picket counts are published trade guidance rather than code. The bag count is computed from the hole rather than repeated, because the three published bag figures differ by the hole size rather than by the concrete: checked against the published bag yields, 1.5 bags of fifty pound mix fills about 25.6 inches of a four by four in an eight inch hole, two sixty pound bags fill about 23.5 inches of a ten inch hole, and three to four bags of fifty pound mix fill 23.5 to 31.3 inches of a six by six in a twelve inch hole. All three sit at or around the published twenty four inch minimum, which is why they disagree with each other and agree with their own holes.
The local building department publishes the frost line for your area and governs over any figure here.
This page reports quantities and issues no pass or fail: it does not evaluate a property line, a setback or a permit, does not model slope, stepping or racking, does not compute wind load, and does not carry the concrete mix arithmetic, which is the Concrete Calculator's.
Units
Lengths are in feet with metres alongside at 0.3048. Two hundred feet is 60.96 metres, a six foot fence is 1.829 metres and an eight foot spacing is 2.438.
Depths, post dimensions, picket widths and gaps are in inches with millimetres alongside at 25.4. Thirty six inches is 914 mm, seventy eight inches is 1,981 mm and a 5.5 inch picket is 140 mm.
Hole and concrete volumes are in cubic feet with cubic metres alongside at 0.0283168. One cubic foot is 0.0283 cubic metres.
A concrete bag size is in pounds with kilograms alongside at 0.45359237, and a bag yield is in cubic feet with cubic metres alongside, because the yield is printed on the bag in the unit the market sells it in.
Post and picket sizes are nominal lumber designations rather than measurements. A four by four is a trade name, and its actual size is 3.5 by 3.5 inches, which the hole diameter rule of three times the post width uses. Metric markets sell a different product list rather than a conversion of these.
Frost line depths are published in inches by local building departments and this page takes the figure you enter in the unit selected.
The post count, the section count, the rail count, the picket count, the bag count and the waste percentage 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 reports quantities and issues no pass or fail. The published figures are trade guidance and a local frost map rather than limits it can apply.
- It does not evaluate a property line, a setback, a permit or a neighbour rule. All of those are local and none is arithmetic.
- It does not model slope. A stepped or racked fence changes the section geometry and the picket lengths, and the page assumes level ground.
- It does not compute wind load. The published practice of reducing spacing to six feet or less in high wind areas is named and not calculated.
- It does not carry the concrete mix arithmetic. It produces a hole volume and a bag count against the bag yield you enter, and the mix is the Concrete Calculator's.
- It does not place anything. The short bay, the closing picket gap and the position of the gate are layout decisions the page reports rather than resolves.
- It does not decide whether the frost section reaches your fence. That is a local determination and the section names decks as its exception rather than fences.
- It does not pick between the two readings of the depth rule. Manufacturer guidance uses the total length reading, four calculator sites state it and work another, and the page reports both.
- It holds up to three gates and six segments. A larger job runs the arithmetic the same way, but the form does not carry the rows for it.
- Any cost is arithmetic on a price you entered.
Common Mistakes to Avoid
- Adding a post to a fenced yard. A closed loop has as many posts as sections, and the extra post is a post, a hole and its concrete.
- Treating a segmented layout as one case. A perimeter around a yard closes and a run along two sides does not, and the difference is a whole post.
- Taking the add one rule as universal. The source that publishes it as a single rule for every jurisdiction states the exception two paragraphs later.
- Burying two feet of an eight foot post and calling it a third. Two of eight is a quarter, and one third of a total gives a nine foot post buried three feet.
- Using the fence height to set the depth in a cold state. In Minnesota a six foot fence can need seventy eight inches of hole, which is deeper than the fence is tall.
- Assuming the code settles it. The frost section covers supports of buildings and structures and names decks as its only exception, so whether it reaches a fence is decided by your building department.
- Setting gate posts to line post depth. Published practice is four to six inches deeper, and many contractors go eight, because a gate swings its weight off one post.
- Adding two posts for a gate on top of a continuous count. The gate posts are the end posts of the runs either side, so they are already positions on the line, and adding them gives one post too many.
- Forgetting that a gate is not fence. The opening comes off the length before the sections are counted.
- Repeating someone else's bag count. The three published figures differ because the holes differ, so the count has to come from your hole and your bag yield.
- Leaving the post out of the hole volume. The concrete fills the hole less the post and less any gravel base, and a volume that ignores the post over-orders on every hole.
- Putting the waste allowance on posts. Ten percent covers bad cuts and warped boards, which are lumber problems, and a post is counted rather than cut.
- Spreading the closing picket gap silently. The remainder has to land somewhere, and a page that distributed it would report a gap you did not choose.
- Spacing a horizontal slat fence at eight feet. Published guidance gives four to six, because horizontal boards sag between posts.
- Choosing a spacing on a panel fence. Vinyl and aluminium panels come in fixed widths that set the spacing for you.
Frequently Asked Questions
How many fence posts do I need?
Why does my calculator say one post more than yours?
How deep should a fence post go?
Why do I see both nine foot and eight foot posts for a six foot fence?
Does the frost line really matter that much?
How many posts does a gate need?
How many bags of concrete per post?
How many pickets do I need?
Frequently Used Together
Engineers often use these calculators in combination for complete project workflows:
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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 a 200 ft run becomes 60.96 m and returns to exactly 200.
The Layout
This is the one answer that changes the post count by a whole post, and there is no default that is right across the four. Two of them close and two do not, and what separates them is whether the run has ends rather than whether it is straight.
The Fence
In feet, above ground. This is not the post length: on a six foot fence the post is eight or nine feet depending on which reading of the depth rule you take.
Eight feet is the published residential standard for wood and chain link and matches prebuilt six by eight and four by eight panels. Horizontal slat fences take four to six, because horizontal boards sag between posts. High wind areas reduce to six or less.
The Depth
Published sources give two readings of the same rule and their worked examples follow neither consistently, so there is no default here that is not a choice made for you. Both readings are reported whichever one you pick.
Over most of the northern half of the country the frost line sets the depth and the fence height drops out of the calculation entirely. Not known is an answer and is offered as one.
The Gates
None is an answer and is offered as one. A gate is not fence: the opening comes off the length before the sections are counted, and it splits one run into two.
The Waste Allowance
Published guidance gives ten percent for bad cuts, warped boards and mistakes, which are lumber problems. It runs on the rails and the pickets and on nothing else: a post, a hole and a bag are counted rather than cut.
The Hole and the Concrete
Optional. Nothing here changes the post count above. A four by four is a trade designation rather than a measurement, and the actual dressed size is what the hole rule and the concrete volume run on.
In inches. Leave empty to take the published rule of about three times the post width, which is 10.5 inches for a four by four and lands inside the published 10 to 12 inch band.
In inches, at the bottom of the hole. Published guidance gives it as improving drainage and reducing the concrete fill depth. The post sits on it.
In pounds. Published yields are 0.375 cubic feet for a fifty pound bag, 0.45 for a sixty and 0.6 for an eighty, and the page uses them only where you do not enter a yield of your own.
In cubic feet, printed on the bag. It beats the published figure outright, because the three published bag counts per post differ by the hole rather than by the concrete.
The Rails and the Pickets
Optional whole number. Leave empty to take the published height rule, which gives two rails for most privacy fences and three above six feet. A second published source instead gives the height in inches divided by twenty four, rounded up, and the page reports both.
In inches, the actual dressed width rather than the nominal name. Enter the gap as well, or the page reports no picket count.
In inches. A gap of nought is a valid answer and gives a tight board fence, so leaving the field empty is not the same as entering nought.
Prices
Optional, in your own currency. Any cost here is arithmetic on a price you entered.
Optional, in your own currency. Applied to the order quantity rather than the counted one.
Optional, in your own currency. Applied to the order quantity rather than the counted one.
Optional, in your own currency. Applied to the bag count computed from your hole.