Paint calculator
Enter the room dimensions and this returns gallons, counting two coats and subtracting the doors and windows you are not painting. It works from 350 square feet per gallon, the conservative end of the 350 to 400 range Sherwin-Williams publishes.
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The formula
Wall area is the perimeter multiplied by ceiling height, minus the openings. Perimeter is twice the length plus twice the width, so a 14 by 12 room at 9 feet gives 52 feet of perimeter and 468 square feet of wall before subtractions.
Openings come off at 20 square feet per door and 15 per window — a standard 3-by-6-foot-8 door and an average double-hung window. Multiply what remains by the number of coats, divide by 350, and round up, because paint is sold in whole cans.
| Step | Arithmetic | Example, 14 × 12 × 9 ft room |
|---|---|---|
| Perimeter | 2 × (length + width) | 52 ft |
| Gross wall area | perimeter × ceiling height | 468 sq ft |
| Openings | (doors × 20) + (windows × 15) | 50 sq ft |
| Net area | gross − openings | 418 sq ft |
| Total to cover | net × coats | 836 sq ft at two coats |
| Gallons | total ÷ 350, rounded up | 3 gallons |
Why 350 and not 400
Sherwin-Williams publishes 350 to 400 square feet per gallon, and both numbers are true — of smooth, primed, previously painted drywall. Texture, bare patches, skim coat and any porous surface pull the real figure toward the bottom of that band or below it. Estimating at 400 and discovering 350 means a second trip to the store mid-wall, which is worse than a spare quart.
When you also need primer
Primer is a separate calculation and covers less: HomeAdvisor prices it at $10 per gallon and it typically spreads over 200 to 300 square feet rather than 350. Three situations genuinely need it — bare drywall or patched repairs, a dramatic color change in either direction, and any surface with stains or smoke damage that would bleed through. Painting a beige wall a slightly different beige does not.
If you are pricing the job rather than the paint
This page answers how much paint to buy. What a painter charges is a different question with a different answer, and it lives on its own page: what interior painting costs covers the national average, the per-square-foot rates, and the labor share that makes this the one renovation where doing it yourself changes the number dramatically.
The short version, if you only need the shape of it: the paint is the small part. Materials for the example room above come to a few tens of dollars, and almost everything a professional quote contains is time — preparation, cutting in, and waiting between coats.
Questions people ask
How much does a gallon of paint cover?
How many coats of paint do I need?
Should I subtract doors and windows?
How much paint do I need for two coats?
Does paint quality change how much I need?
- One gallon of interior paint covers about 350–400 square feet — Sherwin-Williams, Paint Calculator FAQ, 2026 · checked 2026-08-14
- Primer covers less than finish paint, roughly 200–300 square feet per gallon — Sherwin-Williams, Paint Calculator FAQ, 2026 · checked 2026-08-14