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
The four walls unrolled, with the openings taken out
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.
Perimeter. 2 × (14 + 12) = 52 ft. Not four times the longest wall, which is the commonest way this goes wrong.
Gross wall. 52 × 9 = 468 sq ft.
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.
Ceiling. 14 × 12 = 168 sq ft, so 566 sq ft a coat.
Coverage. The sheet says 350 to 400. Sizing from 350 and taking 10% off for waste gives 315 sq ft per gallon.
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
Product
Published sq ft/gal
Wet mils
Dry mils
1604 / wet mils
Volume solids
What the range means
Document
SuperPaint Interior Latex Flat (A86)
350 to 400
4
1.4
401
35%
top end is the film, bottom is a 12% allowance
Sherwin-Williams document 101.02A, 08/2026
SuperPaint Interior Latex Semi-Gloss (A88)
350 to 400
4
1.5
401
38%
top end is the film, bottom is a 12% allowance
Sherwin-Williams document 101.30B, 8/2026
PVA Interior Latex Primer-Sealer (B28W08020)
400
4
0.8
401
20%
single figure, the theoretical film exactly
Sherwin-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.
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:
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.
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.
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 colour change, bare drywall, or anything over a strong colour 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.
Colour 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.
Coverage: 75-125 sq. ft. per gallon @ 16.0 mils wet 7.7 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 colour, 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.
How every figure here is verified: Sources & Method. Who builds this: 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.
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, colour change and application method all move the answer, and none of them are on a data sheet.