Work out how many gallons of paint you need from wall area, coats and coverage, with a quick way to estimate your wall area.
| Input | Result |
|---|---|
| 350 sq ft, 2 coats, 350/gal | 2 gallons |
| 282 sq ft, 2 coats, 350/gal | 2 gallons (1.61 exact) |
| 600 sq ft, 1 coat, 400/gal | 2 gallons (1.50 exact) |
Paint manufacturers publish a spread rate measured on smooth, primed, non-porous test surfaces, and the published figures from Sherwin-Williams and Benjamin Moore both land in the same band: 350–400 ft² per US gallon per coat for interior latex on primed drywall. Real walls are worse than the test panel. Fresh unprimed drywall drinks paint and drops coverage to roughly 250–300 ft²/gal; heavy knockdown or popcorn texture does the same because there is simply more surface to wet; stucco and bare masonry can fall further still. Exterior work typically sits at 250–350 ft²/gal.
Because the honest answer is a range, this calculator asks you to pick the rate rather than guessing one for you. If your can prints a spread rate, use that number, it is specific to the product you are holding.
Coats are not optional in the arithmetic. Two coats is standard over primer, and the manufacturer figure is per coat, so a 400 ft² wall with two coats is 800 ft² of coverage to buy. The calculator rounds up to whole gallons because that is how paint is sold.
Perimeter × wall height, minus doors and windows. A 12×10 ft room with 8 ft ceilings: (12+12+10+10)×8 = 352 sq ft, minus about 2 doors (40) and 2 windows (30) → ≈282 sq ft of paintable wall.
Almost always yes, one coat rarely hides the old color evenly. Dark-to-light changes may need a tinted primer plus two coats.
Paint is sold by the gallon and color batches vary slightly, buying the rounded-up amount ensures one uniform batch. Leftover paint is useful for touch-ups.
Ceilings use the same math: length × width of the room, one coat of ceiling paint usually suffices. Add it to your wall area separately.