Paint Calculator — A Gallon Does Not Cover the Same Wall Twice

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 12 ft wall becomes 3.6576 m and returns to exactly 12. Containers stay US gallons and quarts, because that is how paint is sold and an imperial gallon is about a fifth larger.

The Surface

The dimensions asked for below follow from this answer. A single wall has its own equation here: one length times one height. The rectangular room equation applied to a single wall returns four times the area, and the two are easy to confuse because both are described as a wall.

The Coverage Figure

A coverage figure is either per coat, which is what a can gives, or finished over all coats, which some published sources give. The two differ by the coat count, so the same number can mean half the paint. Unknown is an answer: the figure is taken as per coat and the result reports what the other basis would have given.

Coats and Primer

Published guidance gives two coats as standard and calls one coat realistic only over a very similar colour on a well primed surface, which is two conditions rather than one. The count multiplies the whole paint volume.

Primer is a separate product with its own coverage rather than an extra coat of paint, so it is asked rather than folded into the coat count. Published guidance calls for it on new or bare drywall, bare wood, glossy surfaces being repainted, stains, and light paint over a dark colour.

Openings, Allowance and Containers

None is an answer and is offered as one, so an empty list is an answer rather than missing data. The standard sizes here match the Drywall Calculator, so one room gives one net area across both pages.

Published guidance recommends 10 to 15 percent for waste and touch ups, separately from the container rounding. It is applied to the area before the division, so it is not a second rounding on top of the container count. None is a valid answer rather than a blank one.

A volume is not an order until it is expressed in containers. Four quarts make a gallon, and a job needing a fraction over two gallons buys three gallons or two gallons and a quart, which are different quantities of paint. Whichever you pick, the other option is reported beside each order.

Trim and Prices, Optional

In feet, the total run of skirting, architrave and casing. Optional, and it never blocks the wall result.

In inches, the face height of the trim. It is divided by twelve inside the area equation, because trim is measured in inches and runs in feet.

In square feet per US gallon, from the trim product label. Without it the trim area is reported and no trim volume is computed, because trim paint is usually a different product at a different sheen.

Optional, in your own currency. Any cost on this page is arithmetic on the figure you enter here; the page holds no price data of its own.

Optional, in your own currency. It is what decides whether the smaller container is worth the extra trip down the aisle.

Overview

Every paint calculator divides an area by a coverage figure. The arithmetic is right and the coverage figure is where the answer is decided.

The number on the tin is 350 to 400 square feet per gallon, and published guidance is explicit about the wall it was measured on: flat, smooth and non-porous, which one source calls rare in reality. The same guidance adds that manufacturers test under controlled conditions on sealed surfaces, so real world coverage runs 10 to 15 percent below the label before anything else is considered.

Coverage is a property of the wall rather than of the paint, and two separate things move it.

Texture is one. Light texture takes 85 to 90 percent of the label rate, knockdown 70 to 80, popcorn 60 to 70.

The substrate is the other. A previously painted wall in good condition takes the full rate. New drywall without primer takes 60 to 70 percent, bare wood 50 to 60, masonry 40 to 60.

A wall can be both, and the two multiply. A lightly textured new drywall wall is about 228 square feet per gallon against a 400 label, which is not far off half.

Two other things separate an area from an order. Primer is a separate product that covers less rather than more, and a coverage figure has two possible bases that differ by the coat count, so the same number can mean half the paint.

What to Look at First

Read the effective coverage rate before the gallon count. The volume is a consequence of that rate, and a reader who cannot see which rate was used cannot tell whether the figure is generous or short. Then read the orders, which are given one product group at a time. There is no total gallons figure anywhere on this page, because walls, ceiling, trim and primer are four different products at four different rates and a single number across them cannot be taken to a shop.

How to Use This Calculator

  1. Say what the wall is made of and what has been done to it. Texture and substrate are asked separately because they multiply: texture is how much surface the wall really has, the substrate is how much the material drinks, and a wall can carry both.

  2. Enter a figure from your can if you have one. A label rate for the product you are actually buying beats any published table, and the page will use it and say so.

  3. Say which basis that figure is on. Per coat is what a can gives. Finished over all coats is what some published sources give, and the two differ by the coat count.

  4. Answer the primer question honestly. Primer is a separate product with its own coverage, it is one coat rather than two, and skipping it on a porous wall costs more than using it.

  5. Read the volumes by product group. Walls, ceiling, trim and primer are four different products, and there is no single gallons figure that covers them.

  6. Buy the containers in one trip. Paint is mixed in batches and a second tin returns a colour that is close and not the same.

The container counts are arithmetic on a rate, and the rate is the weakest figure on the page. It is published trade guidance rather than a tested spreading rate for the product you are buying, so check the tin you actually pick up against the rate used here before you leave the shop.

Inputs & Outputs

Inputs

•Unit System : Options: US / Imperial (ft, sq ft, gal, qt), SI / Metric (m, sq m, L)
•What Is Being Painted : Options: Not selected, The walls of a rectangular room, The ceiling, The walls and the ceiling, A single wall, A surface entered as an area
•Room Length
•Room Width
•Wall Height
•Single Wall Length
•Single Wall Height
•Surface Area
•Wall Texture : Options: Not selected, Smooth, Light texture or orange peel, Knockdown or heavy texture, Popcorn
•Wall Substrate : Options: Not selected, Previously painted and in good condition, New or bare drywall, Bare wood, Concrete, brick or masonry
•Ceiling Texture : Options: Not selected, Smooth, Light texture or orange peel, Knockdown or heavy texture, Popcorn, The same as the walls
•Ceiling Substrate : Options: Not selected, Previously painted and in good condition, New or bare drywall, Bare wood, Concrete, brick or masonry, The same as the walls
•Coverage Basis : Options: Not selected, The figure on my can, per coat, A published label figure, 350 to 400 per gallon, A finished figure covering all coats, Unknown
•Your Coverage Figure
•Which End of the Published Band : Options: Not selected, 400 square feet per gallon, the top of the band, 350 square feet per gallon, the bottom of the band
•Coats : Options: Not selected, One, a touch up or the same colour again, Two, the standard for a repaint, Three, A stated number, Not sure, work it out from the colour change
•Stated Coat Count
•Colour Change : Options: Not selected, The same colour or close to it, Light over dark, Dark over light, A strong red, blue, green, yellow or orange, Not decided
•Primer : Options: Not selected, Yes, a separate primer coat, No, A paint and primer in one, Not decided
•Primer Coverage Figure
•Primer Coats : Options: One, which is the standard, Two, by choice
•Openings : Options: Not selected, None, Deduct the doors and windows entered, Deduct doors only, A stated area
•Doors
•Windows
•Stated Opening Area
•Waste And Touch Up Allowance : Options: Not selected, None, 10 percent, 15 percent, A stated percentage
•Stated Allowance
•Container Sizes Available : Options: Not selected, Gallons only, Gallons and quarts
•Trim Length
•Trim Height
•Trim Coverage Figure
•Price Per Gallon
•Price Per Quart

Outputs

•The areas: wall, ceiling and trim separately, with the openings treatment named and the allowance shown as its own multiplier
•The coverage rate: the label figure, the real world factor, the texture factor and the substrate factor, with the published band behind each and the rate range they produce
•The paint: the unrounded volume for each product group with its arithmetic shown, and never as one total
•The primer: its own rate, its own volume with no paint coat count in it, and its own containers
•The coats: what the count was based on, and why no coat is modelled as covering further than the first
•The containers: the order per group in gallons and quarts, with the overbuy in absolute and percentage terms and the other container option beside it
•The trim: the area, and the volume only where a trim coverage figure was entered
•The boundaries: what this page reports and what it deliberately does not

Paint Formula

Every quantity here comes from one area, one label rate corrected by three factors, and one coat count, with each product group carried separately to its own containers.

Areas

room_wall_sf = 2 * (len_ft + wid_ft) * height_ft
wall_sf = wall_len_ft * wall_h_ft
area_sf = surf_sf
ceil_sf = len_ft * wid_ft
trim_sf = trim_len_ft * trim_h_in / 12

A single wall is one length times one height. The rectangular room equation applied to a single wall returns four times the area, and the two are easy to confuse because both are called a wall.

The ceiling area is reported on its own and is never folded into the wall figure, because the ceiling is a separate product at a separate rate.

Trim height is entered in inches and divided by twelve inside the equation, because trim is measured in inches and runs in feet.

Openings and the Allowance

open_sf = open_w_ft * open_h_ft
net_sf = gross_sf - open_total_sf
area_adj_sf = net_sf * (1 + waste_pct / 100)

A door is taken as 3 feet by 7 feet, which is 21 square feet, and a window as 3 feet by 4 feet, which is 12. Those are the figures the Drywall Calculator uses, so a reader who runs both pages on one room gets one net area rather than two.

The allowance is applied to the area, before any division. It is a separate thing from the container rounding and the two are never run together.

The Coverage Rate

cover_sf_gal = label_sf_gal * real_f * tex_f * sub_f

Three factors multiply because they describe different things. The real world factor is the gap between a label claim and achieved coverage, which published guidance puts at 10 to 15 percent. The texture factor is how much surface area the wall really has. The substrate factor is how much paint the material absorbs.

A lightly textured new drywall wall at a 12.5 percent real world factor carries all three, at 0.875, 0.875 and 0.65, which is 0.498 of the label rate.

The real world factor applies to any label figure, including one you take off your own can, because the gap is between a label claim and achieved coverage and a can figure is a label claim printed from the same controlled test. It would not apply to a rate measured off a finished job of your own, and there is no input for that here.

The base rate for the surface factors is the label figure. Where the basis is a published one, the band runs 350 to 400 and you choose which end the order is placed against; the other end is reported beside it with its own container count, and no midpoint is taken between the two.

The Basis

cover_sf_gal = finish_sf_gal * coats

A coverage figure stated as finished over all coats already carries the coats inside it, so converting it to a per coat figure multiplies rather than divides. A finished 175 square feet per gallon over two coats is 350 per coat.

Where the basis is unknown the figure is taken as per coat, which is what a can gives, and the result states the assumption and what the other basis would have produced.

Volume by Product Group

group_gal = area_adj_sf * coats / cover_sf_gal
primer_gal = area_adj_sf / prime_sf_gal

The volume equation runs once per product group. Walls, ceiling and trim have their own rates and their volumes are never summed. Primer is a fourth group and is never summed with any of them.

The primer equation has no coat count in it. Primer is one coat unless a second is chosen, and multiplying it by the paint coats is the commonest way the figure is inflated.

Containers

quarts = part_gal_left * 4
overbuy_pct = (bought_gal / need_gal - 1) * 100

The part gallon left is what remains after the whole gallons are taken out. Taking them out is a rounding down, which is a function rather than an arithmetic expression: the whole gallons are the volume rounded down to the nearest gallon, and the remainder in quarts is that figure times four, rounded up, because a part quart cannot be bought either. Four quarts make a gallon, so a remainder that rounds to four quarts is bought as one more gallon instead.

Containers round up once, at the end, and never down.

Unit Conversions

L = gal * 3.785412
L = qt * 0.946353
sq m = sq ft * 0.09290304
sq m per L = sf per gal * 0.024542

A US gallon is not an imperial gallon. An imperial gallon is 4.546 litres, about a fifth larger, so a coverage figure from a British source is stated per imperial gallon and converts differently. This page uses US gallons throughout.

Coverage Is a Property of the Wall

This is the part of the calculation every other paint calculator leaves out.

The number on the tin is 350 to 400 square feet per gallon. Published guidance is explicit about the wall it was measured on: flat, smooth and non-porous, which one source calls rare in reality. Another qualifies it as assuming proper application thickness and technique.

The same guidance adds a second thing. Manufacturers test coverage under controlled conditions on sealed surfaces, so real world coverage runs 10 to 15 percent below the labelled rate before anything about your particular wall is considered. A 400 label is 340 to 360 in practice on the best wall you will ever paint.

Then the wall itself takes two more bites, and they are separate questions.

Texture is the first. It is how much real surface area the wall carries, and published figures give it as a percentage of the label rate: smooth 100 percent, light texture or orange peel 85 to 90, heavy texture or knockdown 70 to 80, popcorn 60 to 70.

The substrate is the second. It is how much paint the material underneath absorbs: previously painted and in good condition 100 percent, new drywall without primer 60 to 70, bare wood 50 to 60, concrete or masonry 40 to 60.

A wall can be both, and the two multiply. A lightly textured new drywall wall is 0.875 times 0.65, which is 0.569 of the label rate, or about 228 square feet per gallon against a 400 label. Put the real world factor in front of it and the rate is 199. That is below the lowest band a single list of surfaces offers, and it is why the two questions are asked separately here.

A calculator that defaults to the label figure has answered for a wall it was never told about, and it will be wrong by close to double on the surfaces where being wrong matters most.

Primer Is Not a Coat of Paint

Three things about primer run against what a reader expects and they all point the same way.

It covers less. Published figures give primer 200 to 300 square feet per gallon against 350 to 400 for paint, because it is designed to soak into the surface rather than sit on it. A reader who assumes the cheap undercoat stretches further has it backwards. Some manufacturers claim higher for particular products, which is a product claim rather than a general rate and belongs on the can rather than in a table.

It is one coat, not two or three. The primer volume carries no paint coat count. Multiplying primer by the coats is the commonest way a primer figure is inflated, and it turns one and a half gallons into three.

And skipping it costs more. Published guidance works a case on four hundred square feet of new drywall: one primer coat plus two paint coats at thirty five plus seventy, which is one hundred and five, against four coats of paint at one hundred and forty. Another source puts it without figures, that skipping primer on porous surfaces just means an extra coat of more expensive topcoat.

So the decision that feels like saving the cost of a primer container spends it on topcoat instead, and finishes worse.

This page reports primer as its own product with its own rate, its own volume and its own containers, and never sums it into a paint figure. A combined total would hide all three.

One Figure, Two Bases, Half the Paint

A coverage figure is either per coat or finished over all coats, and the two differ by the coat count.

A can label gives a per coat figure: this much area, one coat. Some published sources give a finished figure: this much wall, done, over however many coats it takes.

A finished rate of one hundred and seventy five square feet per gallon over two coats is three hundred and fifty per coat. The same number on the other basis means twice the paint.

Sources print both and do not always say which, so a reader who takes a finished figure as a per coat figure buys half of what they need, and finds out on the second coat.

This page asks which basis a figure is on, and where the answer is a finished figure it converts and shows the multiplication rather than refusing the input. Refusing it would mean diagnosing the exact error the page exists to prevent and then declining to accept the number that carries it.

Where the basis is unknown the figure is taken as per coat, which is what a can gives, and the result says so and reports what the other basis would have produced.

Paint Cannot Be Topped Up

Every other quantity on this site can be bought again. A yard of gravel, a sheet of board, a bag of concrete: run short, go back, buy more, and what arrives is the same thing.

Paint is not. Published guidance recommends buying all the paint for a room at once, from the same batch, because a second tin is mixed separately and returns a colour that is close and not the same. On a wall in daylight, close is visible.

That changes what a shortfall means. Running out of gravel costs a delivery charge. Running out of paint costs a wall, because the patch that finishes it will read as a patch.

It also changes what the overbuy is for. The extra half gallon is not waste. It is touch up paint from the only batch that will ever match what is on the wall, and it is worth keeping labelled with the room it came from.

So this page rounds up, reports the overbuy in both absolute and percentage terms rather than hiding it, and states what it is for. A calculator that minimised the overbuy would be optimising the one quantity where being slightly over is the point.

A Quart Is a Quarter of a Gallon

A volume is an intermediate. The order is containers.

One published worked example shows what goes wrong. A textured wall needing three coats: it gives 1.34 gallons per coat and three coats as four gallons total. Three times 1.345 is 4.03, which rounds up to five containers. The shortfall is a thirtieth of a gallon, which is about ten square feet of wall, and it turns up at the end of the last coat.

The rule is the one this project applies everywhere: full precision through the arithmetic and one round up at the end, because a part container cannot be bought.

The container size then changes the answer again. A job needing 2.19 gallons buys three gallons and wastes 0.81, or two gallons and one quart and wastes 0.06. That is a real difference on a small room, and it is invisible if the page reports gallons.

So this page reports the containers rather than the volume, shows both the gallons only and the gallons and quarts options, and gives the overbuy for each. The reader can then decide whether the smaller container is worth the extra trip down the aisle, which is a decision and not an arithmetic result.

What Is a Paint Calculation

A paint calculation turns a room into a list of containers, and the difficulty is that the divisor is not a constant.

The area is straightforward. Perimeter times height for the walls, length times width for the ceiling, minus whatever openings you decide to take out.

The coverage rate is where the calculation actually happens, and it is not a property of the paint. It is a property of the wall the paint is going onto. The tin states what one gallon does on a flat, smooth, sealed surface under laboratory conditions, and the wall in front of you has texture, has a history, and has never been in a laboratory.

Two independent things move that figure. How much real surface area the texture adds, and how much paint the material underneath absorbs. They compound, and together they can halve the label rate.

Then the answer has to become an order. Paint is sold in containers, not in gallons, so the volume rounds up to whole tins. And it is mixed in batches, so the whole order has to be bought at once, because a tin bought next week is a colour that is close and not the same.

So the useful output is not gallons. It is containers of a named product, computed at a rate that describes your wall, bought in one trip.

Key Facts

  • Published guidance gives most interior paints as 350 to 400 square feet per gallon, on a flat, smooth and non-porous surface, which one source calls rare in reality.
  • Manufacturers test coverage under controlled conditions on sealed surfaces, so real world coverage is typically 10 to 15 percent below the labelled rate.
  • Coverage by texture, as a percentage of the label rate: smooth 100 percent, light texture or orange peel 85 to 90, heavy texture or knockdown 70 to 80, popcorn ceiling 60 to 70.
  • Coverage by substrate: previously painted and in good condition 100 percent, new drywall without primer 60 to 70, bare wood 50 to 60, concrete or masonry 40 to 60.
  • Texture and substrate are separate questions and they multiply. A lightly textured new drywall wall is about 0.57 of the label rate, or 228 square feet per gallon against a 400 label.
  • Published guidance states that the first coat on a porous surface absorbs into the substrate and the second goes further, and advises planning the whole job at the first coat rate to avoid running short.
  • A coverage figure from the can of the paint being bought overrides every published table, including the ones on this page, and is reported as the reader's own.
  • Published guidance gives primer 200 to 300 square feet per gallon, less than paint rather than more, because it is designed to soak into the surface. Some manufacturers claim higher for particular products, which is a product claim rather than a general rate.
  • Primer is one coat. Multiplying a primer volume by the paint coat count is the commonest way the figure is inflated.
  • Published guidance works 400 square feet of new drywall as one primer coat plus two paint coats against four coats of paint alone, and gives the primer route as the cheaper of the two.
  • A coverage figure has two possible bases. Per coat is what a can gives; finished over all coats is what some published sources give. They differ by the coat count, so a finished 175 over two coats is 350 per coat.
  • Two coats is the published standard. One coat is realistic only over a very similar colour on a well primed surface.
  • Sources disagree on which colour direction is harder: one names dark over light, another gives light over dark as 80 to 90 percent needing two to three coats against dark over light at 90 to 100 needing two. Both agree that strong reds, yellows and oranges are the difficult case and often want a tinted primer.
  • Published guidance says to apply paint at the label rate and not thinner to make it go further, because thin application reduces hide, durability and colour accuracy.
  • Flat and eggshell finishes spread thinner and cover more area; semi-gloss and gloss build a thicker film and cover less.
  • Published guidance gives ceiling paint about 350 to 400 square feet per gallon on a smooth ceiling, and one source uses 300 for ceiling paint generally, so the widest published range is 300 to 400.
  • A door is taken as 3 by 7 feet, which is 21 square feet, and a window as 3 by 4 feet, which is 12. These are the figures the Drywall Calculator uses, so one room gives one net area across both pages.
  • Paint is sold in gallons and quarts, and four quarts make a gallon. A volume is not an order.
  • One published worked example gives 1.34 gallons per coat over three coats as 4 gallons total, where three times 1.345 is 4.03 and rounds up to 5 containers.
  • Published guidance recommends buying all the paint for a room at once, from the same batch, because a second tin returns a colour that is close and not the same.
  • Published guidance separately recommends adding 10 to 15 percent for waste and touch ups.
  • A 12 by 12 room at 8 feet has 384 square feet of wall before openings.
  • An imperial gallon is 4.546 litres, about a fifth larger than a US gallon, so a coverage figure from a British source is not interchangeable with one from a US label.

Applications

  • Someone painting a newly boarded room finds the rate that applies to bare drywall rather than the one on the tin, and sees how far apart they are.
  • A reader with a textured wall in an older house finds that texture and the wall's history are two separate reductions that compound.
  • Someone deciding whether to prime learns that the primer route is the cheaper one on a porous wall, which is the opposite of how the decision feels.
  • A reader who found a coverage figure online checks which basis it is on before using it, and converts it if it is the other one.
  • Someone comparing this figure with another calculator's can see whether the difference is the surface, the basis, or the rounding.
  • A person buying for a small room sees whether a gallon and a quart beats two gallons, and by how much.
  • Someone who ran short last time learns that the shortfall was a batch problem rather than a quantity problem, and why the overbuy is worth keeping.
  • A reader running both this page and the Drywall Calculator on one room gets the same net wall area from both.

Example Calculations

Example 1. The same room, two walls, twice the paint

Given: a 12 by 12 room at 8 feet, two coats, the 400 end of the published label band, no openings deducted, no allowance, gallons only.

The wall area is 384 square feet.

On a previously painted smooth wall the texture and substrate factors are both 1.000, so the rate is 400 times the real world factor of 0.875, which is 350 square feet per gallon. The volume is 384 times 2 divided by 350, which is 2.19 gallons, and the order is 3 gallons.

On a lightly textured new drywall wall the texture factor is 0.875 and the substrate factor is 0.650, so the rate is 400 times 0.498, which is 199. The volume is 3.86 gallons and the order is 4 gallons.

Result: three containers against four, on one room, from two answers about the wall. Neither figure is a correction to the other. They are two different walls.

The real world factor is worth watching on the first of those. Without it the smooth wall reads 1.92 gallons and buys two containers, and with it the same wall buys three. That factor is not a rounding refinement. It is the difference between finishing the job and not.

Example 2. A finished figure read as a per coat figure

Given: the same room and the same smooth previously painted wall, two coats, and a coverage figure of 175 square feet per gallon that the reader found online without a basis.

Read correctly as a finished figure, which already covers both coats: the per coat rate is 175 times 2, which is 350. The three factors then take it to 306, the volume is 2.51 gallons and the order is 3 gallons.

Read as a per coat figure: the rate is 175, the factors take it to 153, the volume is 5.02 gallons and the order is 6 gallons.

Result: six containers against three. The same number on the two bases differs by the coat count, which is why the page asks which basis a figure is on rather than assuming.

The error runs the other way too. A per coat figure taken as finished buys half of what the job needs, and that one is found on the second coat.

Example 3. Primer multiplied by the coats

Given: the same room, new drywall, smooth, two paint coats, and a primer whose effective rate works out at 250 square feet per gallon.

The primer volume is 384 divided by 250, which is 1.54 gallons, and the order is 2 gallons. It carries no coat count, because one primer coat is one primer coat.

The paint runs at its own rate, 400 times 0.875 times 0.650, which is 228. The volume is 3.38 gallons and the order is 4 gallons.

Result: two containers of primer and four of paint, reported as two products. An engine that multiplied the primer by the paint coats returns 3.07 gallons of primer, which is twice what the job needs and a whole extra container of the wrong product.

There is also no gallons figure anywhere that adds them. Primer and paint are different products and 4.92 is a number that cannot be taken to a shop.

Example 4. Three gallons or two gallons and a quart

Given: the 2.19 gallon job from Example 1, with gallons and quarts both available.

Three gallons is 0.81 over, which is 36.7 percent.

Two gallons and one quart is 2.25, which is 0.06 over, or 2.5 percent.

Result: the second option is 0.75 gallons less paint for the same job. On a room that is most of a tin, and the smaller container exists for exactly this.

The overbuy is not waste in either case. It is touch up paint from the batch that is on the wall, and it is the only batch that will ever match it, which is why the page reports the overbuy rather than minimising it.

Example 5. The published example that rounds down

Given: 400 square feet of textured wall needing three coats, at an effective rate of 298 square feet per gallon.

400 divided by 298 is 1.345 gallons per coat. Three coats is 4.03 gallons.

One published worked example prints this as 4 gallons total.

Result: 5 containers. A part container cannot be bought, so the rounding is up and it happens once, at the end. The shortfall of 0.03 gallons is about 10 square feet of wall, and it turns up at the end of the last coat with no matching tin available.

Standards & References

This is not a code subject. There is no table with editions and no minimum quantity to test against, which is why this page reports a quantity and issues no pass or fail. The coverage rates here are published trade guidance and label ranges rather than tested spreading rates for any named product, and the figure on the can of the paint being bought overrides every table on this page. Published guidance states that manufacturers test coverage under controlled conditions on sealed surfaces, so real world coverage runs 10 to 15 percent below the labelled rate before any surface factor is applied. The texture and substrate factors are given by one published source as percentages of the label rate and are treated here as independent and multiplicative, which is the reading that follows from what they describe rather than a statement the source makes. Primer coverage is published as 200 to 300 square feet per gallon across several sources that overlap and do not agree, so the widest range is reported and its low end used; no midpoint is issued. Ceiling paint is given as 350 to 400 on a smooth ceiling by one source and 300 generally by another, and the same handling applies. Sources disagree on which colour direction needs more coats and this page reports both readings rather than choosing. The openings sizes match the Drywall Calculator so that one room gives one net area across both pages. Any cost is arithmetic on a price the reader enters. This page computes quantities; it does not select a product, brand, colour or sheen, does not cover spray application, and does not address surface preparation, which changes the coverage rate and is governed by the federal lead renovation rule on housing built before 1978.

Units

Areas are entered and reported in square feet with square metres alongside at 0.09290304. The reference room's 384 square feet of wall is 35.67 square metres.

Room dimensions and wall heights are in feet with metres alongside at 0.3048. Twelve feet is 3.658 metres and eight feet is 2.438.

Volume is in US gallons with litres alongside at 3.785412, and in US quarts with litres at 0.946353. Four quarts make a gallon, so 2.19 gallons is 8.31 litres and one quart is 0.946 litres.

A US gallon is not an imperial gallon. An imperial gallon is 4.546 litres, about a fifth larger, so a coverage figure from a British source is stated per imperial gallon and converts differently. This page uses US gallons throughout and says so.

Coverage rates are in square feet per gallon with square metres per litre alongside, converting by multiplying by 0.024542. A 400 square foot per gallon rate is 9.82 square metres per litre, 350 is 8.59, and 228 is 5.60.

Containers stay US gallons and quarts in both systems, because that is how the product is sold rather than a unit of measure, and the litres each order holds are reported beside the count.

The real world factor, the texture factor, the substrate factor, the coat count and the overbuy percentage are dimensionless and identical in both systems.

Trim height is entered in inches, or centimetres in metric at 2.54, and is divided by twelve inside the trim area equation, because trim is measured in inches and runs in feet.

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 a quantity and issues no pass or fail. There is no published minimum for a quantity of paint and the coverage figures are ranges rather than requirements.
  • The coverage rates are published trade figures rather than tested spreading rates for any named product. The figure on your can overrides all of them.
  • The texture and substrate factors are treated as independent and multiplicative. That follows from what each describes and it is not a statement any source makes directly.
  • Where a published figure is a band, the planning figure is the middle of that band and both ends are reported with the rate range they produce. Where two sources disagree, the widest range is reported, its low end is used and no midpoint is issued.
  • It does not model spray application, which differs materially from rolling and has its own losses.
  • It does not compute a trim volume without a trim coverage figure, because trim paint is usually a different product at a different sheen.
  • It plans every coat at the first coat rate. The first coat on a porous wall absorbs more and the later ones go further, and published guidance advises ignoring that difference in the conservative direction.
  • It does not settle which colour direction needs more coats, because published sources name opposite directions. A colour change in either direction is planned at three coats.
  • It does not select a product, brand, colour or sheen, and it does not evaluate hiding for a specific colour pair.
  • It offers gallons and quarts only. Other container sizes are not modelled in this version.
  • It does not cover surface preparation, which changes the coverage rate, and does not address the lead rule that governs disturbing paint in housing built before 1978.
  • Any cost is arithmetic on a price you entered.

Common Mistakes to Avoid

  • Taking the number on the tin as the rate for your wall. It is measured on a flat, smooth, sealed surface under controlled conditions, and real world coverage runs 10 to 15 percent below it before any surface factor.
  • Answering only one of the two surface questions. Texture and substrate are separate and they multiply, and a lightly textured new drywall wall carries both.
  • Using a coverage figure without knowing its basis. Per coat and finished over all coats differ by the coat count, so the same number can mean half the paint.
  • Multiplying primer by the coat count. Primer is one coat unless a second is chosen, and the multiplication turns one and a half gallons into three.
  • Assuming primer stretches further because it is cheaper. It covers 200 to 300 against 350 to 400 for paint, because it is designed to soak in.
  • Skipping primer to save money on a porous wall. Published guidance works it as one primer coat plus two paint coats being cheaper than the four coats of paint it takes without.
  • Adding paint and primer into one gallons figure. They are different products and the sum cannot be bought.
  • Adding wall, ceiling and trim paint into one figure. Three products at three rates.
  • Applying the wall rate to the ceiling. Ceiling paint has its own published figure and the ceiling is often the surface in a room that has never been painted.
  • Rounding a volume down to whole containers. A part container cannot be bought, and a published worked example gets this wrong at 4.03 gallons.
  • Applying paint thinner than the label rate to make it go further. That costs hide, durability and colour accuracy rather than saving paint.
  • Buying the second gallon next week. Paint is mixed in batches and the second tin is a colour that is close and not the same.
  • Treating the overbuy as waste. It is touch up paint from the only batch that will ever match the wall.
  • Using a British coverage figure with US gallons. An imperial gallon is about a fifth larger, so the rates are not interchangeable.
  • Computing a single wall with the rectangular room equation. That returns four times the area, and both are described as a wall.

Frequently Asked Questions

How much paint do I need for a 12 by 12 room?
384 square feet of wall at 8 feet. On a previously painted smooth wall against a 400 label the effective rate is 350 square feet per gallon once the real world factor is applied, so two coats is 2.19 gallons and the order is three containers. On lightly textured new drywall the rate falls to 199 and the same room needs four.
How many square feet does a gallon of paint cover?
350 to 400 on a flat, smooth, sealed surface, which is what the label is measured on. Real world coverage runs 10 to 15 percent below the label even on a good wall. Texture then takes 10 to 40 percent off that, and the substrate takes another 30 to 60 percent off on bare or porous walls.
Do I need primer?
Published guidance calls for it on new or bare drywall, bare wood, glossy surfaces being repainted, stains, and light paint over a dark colour. On a porous wall it is also the cheaper route: one primer coat plus two paint coats works out below four coats of paint alone.
How much does a gallon of primer cover?
200 to 300 square feet, less than paint rather than more, because primer is designed to soak into the surface. Some manufacturers claim higher for particular products, so the can is the better source. This page uses the low end of that published range and issues no midpoint.
Do I multiply primer by the number of coats?
No. Primer is one coat unless you are applying a second by choice. Multiplying it by the paint coat count is the commonest way a primer figure ends up two or three times too large.
How many coats of paint do I need?
Two is the published standard. One is realistic only over a very similar colour on a well primed surface. Strong reds, yellows and oranges often need three even over primer, and a tinted primer is worth asking about for those.
Why does my figure differ from another calculator?
Usually one of four things. The other calculator defaulted to the label rate rather than asking about your wall, or it left out the 10 to 15 percent real world gap, or it used a coverage figure on the other basis, or it rounded the container count down instead of up.
Should I buy all the paint at once?
Yes. Paint is mixed in batches and a tin bought later is a colour that is close and not the same, which shows on a wall in daylight. Buy the whole room in one trip and keep what is left as touch up, because it is the only batch that will ever match.
Why does this calculator ask about texture and substrate separately?
Because they are two different things and they multiply. Texture is how much real surface area the wall has, and the substrate is how much paint the material underneath absorbs. A wall can be both textured and bare, and a lightly textured new drywall wall lands near half the label rate once both are applied.

Related Calculators

Explore similar calculators that might be useful for your project:

Drywall Calculator

Square Footage Calculator

Concrete Calculator

Gravel Calculator