Paint Calculator
How many gallons of paint and primer a room needs from its size, doors, windows, coats, ceiling, surface texture and the coverage on the can, with the can mix to buy and the cost.
How it works
wall area = 2 × (length + width) × height − door area × doors − window area × windows (+ length × width for the ceiling) · coverage = can coverage ÷ surface factor (1 smooth, 1.15 light texture, 1.35 rough) · gallons = area × coats ÷ coverage, rounded up to whole cans · primer = area ÷ primer coverage
The four walls form a band around the room: perimeter times height. Each door (about 21 sq ft) and window (about 15 sq ft) is subtracted; change those sizes under More options if yours differ, or switch to “Total wall area” for an odd-shaped space. The ceiling can be added with the same paint. The total area is multiplied by the number of coats and divided by the coverage printed on the can — 350–400 sq ft per gallon per coat for most wall paints — then reduced for textured or porous surfaces, which soak up more paint. The result is rounded up to whole gallons, and the can mix shows the cheapest way to buy it in 5-gallon buckets, gallons and quarts. Primer is one coat at its own coverage. The trim estimate assumes baseboards around the room plus casings of about 17 ft per door and 14 ft per window, roughly 4 in wide.
Worked example
A 12 × 10 ft room with 8-ft ceilings: perimeter 44 ft × 8 = 352 sq ft of wall. Two doors (42 sq ft) and two windows (30 sq ft) leave 280 sq ft. Two coats at 350 sq ft per gallon need 280 × 2 ÷ 350 = 1.6 gallons — buy 2 gallons (a second gallon usually costs less than 3 quarts, so the can mix suggests 2 gallons rather than 1 gallon + 3 quarts). Adding the 120 sq ft ceiling makes 400 sq ft and 2.29 gallons, so 3 gallons; a coat of primer on the walls and ceiling at 300 sq ft per gallon is another 1.33 gallons, so 2 gallons of primer.
Frequently asked questions
Does this include the ceiling?
Only if you switch on “Paint the ceiling too”, which adds length × width using the same coats. Ceilings are often a different flat white paint; the notes then show roughly how much of that you would need on its own.
Do I really need two coats?
Usually yes for a true, even color — especially over a darker or patchy wall. One coat can suffice when repainting the same color, or with a paint-and-primer over a similar shade. Going from dark to light often needs a tinted primer plus two coats.
How much does a gallon of paint cover?
Most interior wall paints list 350–400 sq ft per gallon per coat on smooth, primed walls. Textured walls, fresh drywall and porous masonry can cut that by a quarter or more, which is what the surface option allows for. Always check the coverage on your can and enter it under More options.
Should I subtract baseboards and trim?
Their area is small and walls always take a bit more paint than the math says (roller loading, touch-ups), so leaving them in the wall area is a useful buffer. If you paint the trim in a different color, the trim estimate gives the quarts for that.
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This calculator is for educational purposes. Double-check important results before acting on them.