Paint Calculator (Gallons for a Room)
Calculate how many US gallons of paint you need for a room. Uses the standard 350 sq ft/gallon coverage rate common on US paint cans, adjusted for number of coats.
Last updated: September 2026
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About this calculator
The formula is V = ceil((A × N) / C), where V is gallons of paint, A is total wall area in square feet, N is the number of coats, and C is coverage per gallon in square feet. Rounding up matters — paint is sold by the gallon, and running short mid-coat forces an unwanted second trip and a possible batch mismatch.
The default 350 sq ft/gallon is the number most US latex-paint manufacturers print on the can for smooth, previously painted drywall. Real-world coverage is usually lower: rough or porous surfaces (fresh drywall, textured walls, popcorn ceilings) absorb 20–30% more paint, and darker colors going over lighter ones often need three coats rather than two.
To compute wall area, sum perimeter × ceiling height, then subtract windows and doors (a typical door is 20 sq ft, a window 15). For a 10 × 12 ft room with 8 ft ceilings and one door plus two windows, walls ≈ (10+12) × 2 × 8 = 352, minus 20 + 30 = 302 sq ft. Add the ceiling if you're painting it too (10 × 12 = 120 sq ft, so 422 total).
Budget one extra quart per project for touch-ups — paint dries with age and touching up years later with a fresh gallon rarely matches. For interior latex, the shelf life of an unopened can is around 10 years; once opened, 1–2 years if resealed carefully.
How to use
Example — a 10 × 12 ft bedroom, 8 ft ceilings, two coats. Wall area ≈ (10 + 12) × 2 × 8 = 352 sq ft; subtract 40 sq ft for a door and a window → 312 sq ft. Two coats at 350 sq ft/gal coverage: ceil((312 × 2) / 350) = ceil(624/350) = ceil(1.78) = 2 gallons. Buy two 1-gallon cans (or one 1-gallon plus a quart if you want a small reserve). Example — a 15 × 20 ft living room with 9 ft ceilings, two coats, fresh drywall (lower effective coverage). Wall area ≈ (15 + 20) × 2 × 9 = 630 sq ft; subtract 60 sq ft for a large window and door → 570 sq ft. Coverage drops to about 275 sq ft/gal on unprimed drywall. ceil((570 × 2) / 275) = ceil(4.14) = 5 gallons. Compare with the metric paint-coverage calculator at /en/calculators/construction/paint-coverage/ if you're shopping in litres.
Frequently asked questions
Is 350 sq ft/gallon accurate?
It is the coverage rate most US latex-paint manufacturers print on the can, and it applies to smooth, previously painted drywall in one coat under ideal conditions. Real-world coverage is often 250–300 sq ft/gal on textured walls, fresh drywall, or rough surfaces. Always check the specific brand and product — premium paints often quote 400 sq ft/gal while budget lines quote 300. If in doubt, buy a bit extra and keep the receipt.
How many coats do I really need?
Two coats is standard for most jobs and produces the most uniform finish. One coat is fine for touch-ups in a matching color or over an already-painted wall of similar color. Three coats are needed when going from dark to light, when the wall is fresh drywall or unprimed, or when you're using a very thin (e.g. some 'ceiling white') paint. 'One-coat' claims on premium paints require heavy application and rarely produce a fully uniform result.
What about primer?
Primer is a separate purchase and usually covers 200–300 sq ft per gallon. Use it on fresh drywall, over stains, when changing colors dramatically, or when switching from oil-based to latex. Modern paint-and-primer-in-one products can skip a separate primer coat in many cases, but for fresh drywall a dedicated primer is still recommended.
How do I calculate the wall area?
Add up the length of each wall (that's the perimeter) and multiply by the ceiling height. For a 10 × 12 ft room, perimeter = 2 × (10 + 12) = 44 ft. With an 8 ft ceiling, walls = 44 × 8 = 352 sq ft. Subtract about 20 sq ft per standard door and 15 sq ft per standard window. Don't forget the ceiling (length × width) if you're painting it too; ceiling paint typically covers about the same as wall paint.