Paint Calculator
How many gallons of paint your room needs from its dimensions, doors, windows and coats.
How it works
wall area = 2 × (length + width) × height − 21 sq ft per door − 15 sq ft per window · gallons = ⌈area ÷ 350⌉ × coats
The four walls form a band around the room: perimeter times height. A standard door blanks out about 21 sq ft and a typical window about 15, so those are subtracted. A gallon of wall paint covers roughly 350 sq ft per coat; the gallon count is rounded up per coat because you can’t buy 0.8 of a can.
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 need 560 sq ft of coverage — 1 gallon per coat rounded up, so buy 2 gallons (or 7 quarts).
Frequently asked questions
Does this include the ceiling?
No — ceilings are usually a different paint (flat ceiling white). A ceiling adds length × width more area: 120 sq ft for this room, well within one extra quart or gallon per coat.
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 premium paint-and-primer over a similar shade.
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 is a useful buffer rather than an error.
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This calculator is for educational purposes. Double-check important results before acting on them.