Nails & Numbers

Home › Paint › Paint Calculator

Paint Calculator

Coverage from Sherwin-Williams data sheets, checked against 1604 / wet mils

The coverage number on a can of paint is geometry. A US gallon is 231 cubic inches, so spread one mil thick it covers 1,604 sq ft, and every published figure is that number divided by the wet film the sheet assumes. It contains no allowance for the roller, the tray or the overlap. This calculator shows the published figure, the arithmetic behind it and your own allowance as three separate numbers, because they are three different things.

Interior paint quantity

ft
ft
ft
no.
no.
ft
ft
%
sq ft/gal

Coverage comes from the manufacturer's product data sheet, named in the table below. If you have the can in front of you, use the override field: the label beats this page.

Specification summary

Generated from on . Reopen that address to reproduce these figures exactly.

Entered

Result

This figure is drawn from the room you type in above.
The four walls unrolled, with the openings taken out

Painting prices

As stated per row
ItemLowHighUnit
Paint, value tier$15$35per gallon
Paint, mid tier$35$50per gallon
Paint, premium$50$60per gallon
Primer$20$30per gallon
Walls, painted$1$2per sq ft
Ceilings, painted$1$2.50per sq ft
Trim$1$3per linear ft
Labor$30$60per hour

Source: This Old House, interior painting cost. Last price check: September 8, 2026.

The per gallon figures are the ones this page can use directly, since the calculator returns gallons. The per square foot figures are installed prices including labor and are measured against floor area rather than the wall area this page computes.

Wallpaper prices

As each row states
ItemLowHighUnit
Wallpaper installation, typical job$316$820total
Installation labor$1$7per sq ft of wall
Hanging labor$25$35per roll
Wallpaper, material$25$35per roll
Peel and stick$0.70$1.50per sq ft
Grasscloth$2.50$3.90per sq ft
Flocked$2.30$5.90per sq ft
Foil or mylar$2.15$2.70per sq ft
Custom designed$5$100per sq ft
Installer$25$80per hour

Source: HomeAdvisor, how much does wallpaper installation cost. Last price check: September 16, 2026.

The source prices a 210 sq ft bedroom's labor at $210 to $1,470, which is its $1 to $7 per sq ft; its typical job is $316 to $820.

Worked example

A 14 ft by 12 ft bedroom with 9 ft ceilings, two doorways and two 3 ft by 5 ft windows, painted in SuperPaint Interior Latex Flat, two coats, ceiling included, at the default 10% allowance.

  1. Perimeter. 2 × (14 + 12) = 52 ft. Not four times the longest wall, which is the commonest way this goes wrong.
  2. Gross wall. 52 × 9 = 468 sq ft.
  3. Openings. Two doorways at 36 × 80 in is 40 sq ft, two windows at 3 × 5 ft is 30. 70 sq ft out, leaving 398.
  4. Ceiling. 14 × 12 = 168 sq ft, so 566 sq ft a coat.
  5. Coverage. The sheet says 350 to 400. Sizing from 350 and taking 10% off for waste gives 315 sq ft per gallon.
  6. Paint. 566 ÷ 315 = 1.8 gal a coat, 3.6 gal for two, so buy 4.

Now run it again with the allowance at zero and the coverage at the published 400. The same room comes out at 2.8 gallons, which rounds to three cans. One decision about a waste allowance nobody publishes moved the answer by a whole gallon, and that is the decision most calculators make silently on your behalf.

The formula

Three lines of arithmetic, and the third is the one worth arguing about:

A = 2(L + W)h − openings + (Lw if the ceiling)    gallons = A n / (c (1 − k))
L, W, h
room length, width and ceiling height, ft
openings
doorways at 36 by 80 in, plus each window measured
n
coats. Coverage is published per coat, never for the job
c
published coverage, sq ft per gallon, from the data sheet
k
waste allowance. Ours, defaulting to 0.10, and no manufacturer publishes one

The published c is itself 1,604 divided by the wet film thickness the sheet names, which is why the fifth column of the table above reproduces the second so closely. Where it does not, the gap is the manufacturer's own allowance and the table says so.

Why a coverage number needs its film thickness beside it

Coverage is not a property of paint. It is a property of paint and a chosen thickness, and the thickness is the half everybody drops. Quote 400 sq ft per gallon without saying 4 mils wet and you have published half a fact.

The arithmetic is fixed and public. 231 cubic inches in a US gallon, divided by a thousandth of an inch, is 231,000 square inches, which is 1,604 square feet. Divide by the wet mils and out comes the coverage. Every sheet in the table above passes that check, which is a good sign about the sheets and a bad sign about any calculator that treats coverage as a number handed down from nowhere.

What the check also exposes is that the published range means something different on almost every sheet. On the primer there is no range at all and the single figure is the theoretical one. On SuperPaint the top of the range is theoretical and the bottom is an unexplained 12% held back. On Duration both ends are film thickness and nothing is held back. On block filler the theoretical figure sits in the middle. Same manufacturer, same document format, four different conventions.

That is the case for showing the arithmetic rather than the answer. A reader who can see 1,604 divided by 4 can tell which kind of number they are holding. A reader given only 'about 400' cannot.

Published coverage against the film thickness it assumes
ProductPublished sq ft/galWet milsDry mils1604 / wet milsVolume solidsWhat the range meansDocument
SuperPaint Interior Latex Flat (A86)350 to 40041.440135%top end is the film, bottom is a 12% allowanceSherwin-Williams document 101.02A, 08/2026
SuperPaint Interior Latex Semi-Gloss (A88)350 to 40041.540138%top end is the film, bottom is a 12% allowanceSherwin-Williams document 101.30B, 8/2026
PVA Interior Latex Primer-Sealer (B28W08020)40040.840120%single figure, the theoretical film exactlySherwin-Williams document 108.45B, 09/2021

Read 2026-09-08. The fifth column is 1,604 divided by the wet mils in the third, and it is the theoretical coverage of a US gallon at that film thickness. Compare it with the second column.

Download this table as CSV. Same cells as printed above, nothing added.

The number on the can is geometry, and you can check it yourself

A US gallon is 231 cubic inches. That is a definition, not a measurement. Spread it one mil thick, one thousandth of an inch, and it covers 231 ÷ 0.001 = 231,000 square inches, which is 1,604 square feet. So the coverage of a gallon of any liquid at any film thickness is:

sq ft per gallon = 1,604 ÷ wet mils

Every data sheet in the table below prints its coverage figure and the wet film thickness it assumes. Do the division and the coverage figure falls straight out:

  • PVA primer: 4.0 mils wet. 1,604 ÷ 4 = 401. Published: 400.
  • SuperPaint: 4 mils wet. 1,604 ÷ 4 = 401. Published: 350 to 400.
  • Duration: 5.3 to 6.4 mils wet. 1,604 ÷ 6.4 = 251, 1,604 ÷ 5.3 = 303. Published: 250 to 300.
  • Block filler: 16.0 mils wet. 1,604 ÷ 16 = 100. Published: 75 to 125.

Which means the number on the can is theoretical coverage. It is what the gallon covers if every drop of it lands on the wall at exactly the stated thickness. There is no allowance in it for the roller nap, the tray, the cut-in overlap you go over twice, the texture of the surface, or the inch left in the can. Those are yours to add, and this calculator makes you add them on purpose rather than burying a fudge factor.

Four sheets, one manufacturer, four different meanings

Here is the part that is genuinely worth knowing. The relationship between the printed range and the printed film thickness is not the same on any two of these sheets, and nothing on the sheet tells you which kind you are holding.

ProductPublished sq ft/galWhat the range actually is
SuperPaint Interior Latex Flat (A86)350 to 400top end is the film, bottom is a 12% allowance
SuperPaint Interior Latex Semi-Gloss (A88)350 to 400top end is the film, bottom is a 12% allowance
PVA Interior Latex Primer-Sealer (B28W08020)400single figure, the theoretical film exactly
SuperPaint Exterior Latex Flat (A80)350 to 400top end is the film, bottom is a 12% allowance
Duration Exterior Acrylic Flat (K32)250 to 300both ends are film thickness
PrepRite Interior-Exterior Latex Block Filler (B25W00025)75 to 125range straddles the theoretical film

Read that column again. On the primer, 400 is the ceiling and there is nothing held back. On SuperPaint, 400 is the ceiling and 350 is a roughly 12% allowance that has no film thickness printed against it. On Duration, both ends are film thickness and the allowance is zero. On block filler the theoretical figure sits in the middle, so the low end is a genuine warning about how much bare block drinks.

A calculator that divides your wall area by 400 and stops has not asked which of those four it is dividing by.

The Duration and block filler sheets are quoted in full on the exterior paint calculator, and every document behind this site is listed at sources.

Perimeter, not four times the biggest wall

The most expensive mistake on this page is not coverage, it is wall area. Two habits cause it: measuring the longest wall and multiplying by four, and forgetting that a rectangular room has two pairs of walls, not four of the same.

A 14 by 12 room is 2 × (14 + 12) = 52 linear feet of wall. Four times the long wall would be 56, which is 8% high. In a 20 by 10 room the same shortcut gives 80 against a true 60, which is 33% high and a whole gallon on a two-coat job.

The figure above draws the four walls unrolled into one strip at a single scale, with the doors and windows punched out where they fall, so you can see the perimeter rather than trust it.

Which openings to deduct, and which to leave in

Deduct the hole, not the trim. A 36 by 80 in doorway is 20 sq ft of wall you are not painting, and a 3 by 5 window is 15. Four of those in one room is 70 sq ft, which on this example is 15% of the wall.

What people over-deduct is anything they intend to paint anyway. A door slab and its casing are usually a different sheen in a different can, so they come out of the wall figure and go into a trim figure, not into thin air. Built-in cabinetry, a fireplace surround and a tiled splashback are genuine deductions.

Small openings are not worth chasing. A light switch is a rounding error next to the 10% you are already allowing for the roller.

One coat, two coats, and the thing coverage cannot tell you

Every coverage figure on this page is per coat. Two coats over the same wall takes twice the paint, because the second coat is the same film as the first.

Whether you need two is a hiding question, and it depends on what is already on the wall rather than on anything printed on the can. White over white can be one. A color change, bare drywall, or anything over a strong color is normally two, and a dark accent wall in a saturated red or yellow can be three regardless of what the tin says about coverage.

Primer does not count as a coat of paint. It goes on at the same 4 mils wet and leaves 0.8 mils of dry film against the topcoat's 1.4, because it is sealing rather than covering. That is in the volume solids table further down.

The waste allowance is ours, and we would rather say so

No data sheet on this page publishes a waste allowance. We looked. The coverage figures are theoretical film coverage, as the arithmetic above shows, and what happens between the can and the wall is left to you.

So the calculator defaults to 10%, and that figure is ours. It is not read from a document and it is not a code value. It is a round number for the paint that ends up in the roller nap, in the tray, on the brush, in the overlap where the cut-in meets the roller, and in the bottom of the can. Set it to zero if you want the theoretical answer, and raise it if you are spraying, where 20% to 30% overspray is ordinary.

Two things it does not cover, because they are not waste:

  • Texture. A knock-down or orange-peel wall has more surface than its length times its height. Nobody publishes a multiplier for that either. If your walls are heavily textured, the honest move is to paint one and measure what it took.
  • Color change. Going dark over white, or white over dark, is a coats problem, not a coverage problem. Add the coat rather than the allowance.

The sheets these figures were read from

Read 2026-09-08. Every figure on this page came from the manufacturer’s own product data sheet, quoted below as printed, with the document number and revision date the sheet carries.

DocumentRevisionCoverage, as printed
SuperPaint Interior Latex Flat, A86 SeriesSherwin-Williams document 101.02A, 08/2026Coverage: 350-400 sq. ft. per gallon @ 4 mils wet, 1.4 mils dry
SuperPaint Interior Latex Semi-Gloss, A88 SeriesSherwin-Williams document 101.30B, 8/2026Coverage: 350-400 sq. ft. per gallon @ 4 mils wet, 1.5 mils dry
PVA Interior Latex Primer-Sealer, B28W08020Sherwin-Williams document 108.45B, 09/2021Coverage: 400 sq.ft.per gallon @ 4.0 mils wet; 0.8 mils dry

Why all six are Sherwin-Williams. Not a preference. Sherwin-Williams publishes its data sheets at a stable public address with a document number and a revision date on every one, which is what makes a figure citable. Where a brand’s coverage number could only be found on a retailer listing or a review site, it is not on this page, because repeating a number does not source it. If you are using another brand, read the back of your can and type the figure into the override field. The label beats this page.

Frequently asked questions

How much paint do I need for a 12x14 room?
About 566 sq ft a coat with 9 ft ceilings, two doorways, two windows and the ceiling included. At the low published 350 sq ft per gallon with a 10% waste allowance that is 1.8 gal a coat, so 3.6 gal for two coats and you buy four.
How many square feet does a gallon of paint cover?
The published figure is normally 350 to 400 sq ft per gallon for an interior latex, and it is arithmetic rather than experience: a US gallon is 231 cubic inches, so at 4 mils wet it covers 1,604 / 4 = 401 sq ft. That number has no waste allowance in it.
Why does the can say 400 square feet when I only get 300?
Because 400 is theoretical film coverage. It assumes every drop reaches the wall at exactly the stated thickness, with nothing in the roller nap, the tray, the brush or the overlap where the cut-in meets the roller, and no extra for surface texture. The gap between 400 and what you actually get is the allowance you have to add yourself.
Do I subtract windows and doors when calculating paint?
Yes, deduct the opening itself. A standard 36 by 80 in doorway is 20 sq ft and a 3 by 5 ft window is 15. Do not deduct the door slab or the casing if you are painting them, because those go into a trim quantity instead.
Does primer count as one of my coats?
No. A PVA primer-sealer goes on at 4 mils wet and leaves 0.8 mils dry, against 1.4 mils for a SuperPaint topcoat at the same wet film. It is sealing porous drywall paper so the finish sits on the surface, not hiding color, so it is sized separately here.
Does a flat or semi-gloss finish change how far the paint goes?
Not on these sheets. SuperPaint Interior Latex Flat and Semi-Gloss both publish 350 to 400 sq ft per gallon at 4 mils wet. The dry film differs slightly, 1.4 mils against 1.5, but the coverage figure is the same.
How much extra paint should I allow for waste?
No manufacturer publishes a figure, which is why this page makes it a field you set rather than burying it. The default here is 10% for roller and tray loss on a brush and roller job. Spraying loses considerably more to overspray, and heavily textured walls have more surface than their measured area.
What does it cost to paint the interior of a 2000 sq ft house?
This Old House prices painted walls at $1 to $2 per square foot, measured against floor area and including labor, so the walls of a 2000 sq ft house are $2,000 to $4,000. The same rate gives $1,200 to $2,400 for a 1200 sq ft house and $3,000 to $6,000 for a 3000 sq ft house. Ceilings are a separate row at $1 to $2.50 per square foot, and trim is $1 to $3 per linear foot.
What is the labor cost for painting?
The same source gives painting labor at $30 to $60 per hour. Its per square foot rows are installed prices that already include labor, so the hourly figure is an alternative way of pricing the job rather than an addition to them.
How much does a 5 gallon bucket of paint cover?
At the published 350 to 400 sq ft per gallon, a 5 gallon bucket covers 1,750 to 2,000 sq ft per coat before any waste allowance, or 875 to 1,000 sq ft of wall at two coats. The 12x14 room example needs 3.6 gal for two coats, so one bucket covers it with paint left over.
How do I calculate square footage for paint?
Add the four wall lengths, multiply by the ceiling height, then deduct the doors and windows. A 14 by 12 room with 9 ft ceilings is 52 linear feet of wall and 468 sq ft before openings. Measure the ceiling separately, as length times width, if it is being painted too.
How much does it cost to paint a 12x12 room?
This Old House prices painted walls at $1 to $2 per square foot of floor area, so a 12x12 room is $144 to $288 for the walls, and $144 to $360 more if the ceiling is painted at its $1 to $2.50 rate. Doing it yourself, walls 9 ft high come to 432 sq ft before openings, which at 350 sq ft per gallon with a 10% allowance is 2.7 gal for two coats. Three gallons of mid tier paint at $35 to $50 each is $105 to $150.
What does it cost to paint a bathroom, kitchen or living room?
The source has no row for a particular room, so the same $1 to $2 per square foot of floor applies to each. A 15 by 20 ft living room is 300 sq ft of floor, which is $300 to $600 for the walls. In a bathroom or kitchen, tile and built-in cabinets come out of the wall area like any other deduction, and the source does not price painting kitchen cabinets at all.
How much does a quart of paint cover?
A quart is a quarter of a US gallon, 57.75 cubic inches, so at the published 350 to 400 sq ft per gallon it covers about 88 to 100 sq ft a coat. The source used here prices paint by the gallon only, $15 to $60 depending on tier, and gives no quart price.
How much does wallpaper cost to install?
HomeAdvisor's June 2026 guide gives $316 to $820 for a typical wallpaper job, with labor at $1 to $7 per square foot of wall or $25 to $35 a roll, and the wallpaper itself at about $25 to $35 a roll. Peel and stick paper is $0.70 to $1.50 per square foot and grasscloth $2.50 to $3.90. Measure the wall area the same way this page does for paint, net of doors and windows, then add the roll waste for pattern repeat.
How much does it cost to paint a bedroom?
This Old House's April 2026 article gives $1 to $2 per square foot for walls and $1 to $2.50 for ceilings, measured against floor area. A 12 by 12 ft bedroom is 144 sq ft, so $1 to $2 per square foot comes to $144 to $288 for the walls. A painter by the hour is $30 to $60.
How much does a 5 gallon bucket of paint cost?
This Old House's April 2026 article prices paint per gallon: $15 to $35 for value paint, $35 to $50 for mid tier and $50 to $60 for premium. Five gallons at those rates is $75 to $175, $175 to $250 and $250 to $300.
How much does an interior paint job cost?
This Old House's April 2026 article gives $1 to $2 per square foot for walls and $1 to $2.50 for ceilings, measured against floor area, and $30 to $60 an hour for a painter. Painting the inside of a 1,500 sq ft house is $1,500 to $3,000 for the walls at that rate. This page counts the gallons.
What is the labor cost to paint a room?
This Old House's April 2026 article gives $30 to $60 an hour for a painter and $1 to $2 per square foot for walls painted, materials included, measured against the room's floor area. A 12 by 12 ft room is $144 to $288 for the walls.

Check these numbers yourself

How every figure here is verified: Sources & Method. Who builds this: , and the method is on About. Found something wrong? Tell us and it gets fixed or removed.

Figures on this page last checked against the source documents on 2026-09-08. Codes are amended locally; confirm against the edition your jurisdiction enforces.

Related calculators

Guides that use this calculator

More of them: all guides.

The can beats this page. Coverage figures were read from the data sheets named above on 8 September 2026 and manufacturers revise them. The waste allowance is this calculator's default, not a manufacturer's figure. Surface texture, color change and application method all move the answer, and none of them are on a data sheet.