Paint Coverage Calculator
Wall area minus openings, times the number of coats, rounded up to whole cans — and how much is left for touch-ups.
Quick answer: paint for one room, two coats
| Room perimeter | Wall to paint | Gallons needed | Gallons to buy |
|---|---|---|---|
| 36 ft | 273 ft² | 1.46 | 2 |
| 44 ft | 345 ft² | 1.84 | 2 |
| 54 ft | 435 ft² | 2.32 | 3 |
| 64 ft | 525 ft² | 2.8 | 3 |
| 80 ft | 669 ft² | 3.57 | 4 |
9 ft walls, one door (21 ft²) and two windows (15 ft² each) deducted, no ceiling, 375 ft² per gallon. Use the coverage on your can's data sheet: a thicker film covers less.
| Gross wall area | — |
|---|---|
| Deductions | — |
| Net area to paint | — |
| Area × coats | — |
| Paint required | — |
| Cans to buy | — |
| Left over | — |
The number on the can is arithmetic, not a measurement. A US gallon is 231 cubic inches exactly. Spread it one mil thick, 0.001 in, and it covers 231,000 in², or 1,604 ft². A spread rate is that constant divided by a film thickness and nothing more. International Protective Coatings prints the formula as volume solids (%) × 16.04 ÷ dry mils. Benjamin Moore's Regal Select N550 sheet gives 400–450 sq ft per gallon and, one row below, a wet film of 3.6–4.0 mils. 1,604 ÷ 4.0 = 401. 1,604 ÷ 3.6 = 446. Nothing in that range is set aside for what stays in the roller or what the wall drinks, and the footnote under it says so: "Depending on surface texture and porosity." Treat the printed figure as a ceiling.
What the ceiling leaves out is the roller. International Protective Coatings puts brush-and-roller over-application at 5% on simple structures and 10–15% on complex ones. That is the extra a competent painter lays on to reach the specified thickness. A room is simple. Add that 5% and Regal Select's 401 becomes 382 ft²/gal. Type 382, not 450. Sherwin-Williams prints ProMar 200 Zero VOC Flat at 350–400 for a single wet film of 4 mils. 1,604 ÷ 4 = 401, so the top of that range is the bare theoretical figure and only the bottom of it holds anything back. Where a range is printed, take the bottom.
Primer is a second product, not a third coat. Sherwin-Williams' published drywall system is "1 coat ProMar 200 Zero V.O.C. Latex Primer" and "2 coats ProMar 200 Zero V.O.C. Interior Latex". That is three passes over bare board, but two products with two spread rates. The coats selector multiplies one coverage figure and cannot price that. Run this twice, one coat then two. Self-priming paints are the exception; Regal Select is one, and still wants two coats on a bare substrate.
What I check on a job
What the perimeter and the order miss. Before I trust the gallons, I check what the perimeter actually walked. A room traced off the plan tends to skip the closet interiors, the short returns around a chase and the jambs of deep window openings, and each of those is wall that gets every coat. Then I check the openings. The fixed 21 and 15 ft² suit an ordinary door and window, but a cased opening, a pair of French doors or a window wall is a different size, so I measure it and enter whole doors and windows that add up close to it rather than counting it as one door. The ceiling toggle prices the ceiling at the wall's coverage; when the ceiling takes a different product with its own sheet, I run it separately, the way the primer note runs primer. The last thing that goes wrong is not the number at all: cans picked up on different days can come from different batches, so I buy the whole quantity at once.
Deductions use 21 ft² per door and 15 ft² per window, which are ordinary US sizes (about 3×7 and 3×5). Coverage varies by product and by what you are painting over. The figure on the can assumes a smooth, sealed, primed surface. Bare drywall, skim coat, textured plaster and masonry all drink more, sometimes half again. Take the number from the can you are actually buying and adjust it down for porous substrates rather than trusting the default here.
Worked example
Two coats on the walls of a room with a 44 ft perimeter and 9 ft ceilings, one door, two windows: how many gallons?
- Gross wall area = 44 × 9 = 396 ft².
- Less a door (21 ft²) and two windows (15 ft² each): 345 ft².
- Two coats = 690 ft²; at 375 ft² per gallon that is 1.84 gallons — two cans.
Two gallons, with about 0.16 of a gallon left for touch-ups. Add the ceiling and the page adds its area at the same coverage.
2 values on this page were driven in a browser and compared with independently derived figures on 2026-10-29 (2808 values across 228 calculators site-wide).
Questions
How much does a gallon of paint cover?
About 350–400 ft² per coat on a smooth wall; the calculator uses the figure on your can, 375 by default.
Do I subtract doors and windows?
Yes: a door is about 21 ft², a window about 15 ft². On a small room they are a tenth of the wall.
What spread rate should I enter if the sheet says 400–450 sq ft per gallon?
382. That range is arithmetic: 1,604 ÷ 4.0 mils of wet film is 401, with nothing for what stays in the roller. International Protective Coatings puts brush-and-roller over-application at 5% on simple structures; take that off 401 and you get 382. Where a range is printed, take the bottom.
Is primer a third coat?
No, it is a second product with its own spread rate. Sherwin-Williams' published drywall system is one coat of primer and two of finish. Run the page twice: one coat at the primer's rate, then two at the finish's.