Home & build
Estimate paintable area and the number of gallons to buy.
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Paint calculator guide
Paint is bought by the gallon but rooms are measured in feet, and the number of gallons depends on how much wall is left after doors and windows are subtracted. This calculator finds the perimeter wall area from room length, width, and wall height, deducts a standard 21 sq ft for each door and 15 sq ft for each window, and divides the paintable area by 350 sq ft per gallon, rounding up to whole cans. That coverage sits at the conservative end of the 350–400 sq ft per gallon most manufacturers print on the label for smooth, previously painted drywall. The result is for one coat of wall paint only; it does not include the ceiling, trim, closets, or a second coat, and it does not account for a colour change from dark to light, textured plaster, or bare drywall, which all absorb more. Use it to buy the right number of cans for the first coat, then double for two coats.
Wall area (sq ft) = 2 × (length + width) × height. Paintable area = wall area − doors × 21 − windows × 15. Gallons = paintable area ÷ 350, rounded up.
Worked example: a 12 ft × 10 ft room with 8 ft walls has a perimeter of 2 × (12 + 10) = 44 ft and a gross wall area of 44 × 8 = 352 sq ft. One door and two windows deduct 1 × 21 + 2 × 15 = 51 sq ft, leaving 352 − 51 = 301 sq ft of paintable wall. At 350 sq ft per gallon that is 301 ÷ 350 = 0.86 gallons, which rounds up to 1 gallon for a single coat. Two coats would need 1.72 gallons, so 2 gallons. The calculator displays 1 gallon.
Room length, width, and wall height are in feet; doors and windows are counts. The door deduction is 21 sq ft (a 3 × 7 ft door) and the window deduction is 15 sq ft (a 3 × 5 ft window). Coverage is fixed at 350 sq ft per US gallon, about 8.6 m² per litre. If you buy in litres, 1 US gallon is 3.785 L, and a UK 5 L can covers roughly 60 m² at typical spreading rates.
The paint calculator returns a quantity to buy, not a guarantee of coverage. Substrate condition, waste from cuts, spillage, and settling all push real consumption above the theoretical figure, which is why most trades order a margin on top. Check the coverage rate printed on the product you actually purchase, because it varies between brands and mixes. On this page the figure rests entirely on room length, room width, wall height, doors and windows, so start there if the paint calculator returns something you did not expect.
Good to know: the gallon count is per coat, and most colour changes need two. Ceilings are excluded: a 12 × 10 ft ceiling adds 120 sq ft, roughly a third of a gallon. A patio door or large picture window is much bigger than the 15 sq ft assumption, so add up unusual openings by hand. Bare drywall and fresh plaster need a primer coat at lower coverage, and deep colours over light walls often need a tinted primer. Buy all cans from the same batch.
Construction quantities are planning estimates. Confirm product yield, waste, site conditions, structural requirements, and local code with the supplier or qualified professional.
Sources
How it works
Estimate paintable area and the number of gallons to buy. This tool explains the calculation so you can adjust the assumptions to match your situation.
Worked example
With Room length = 12 ft · Room width = 10 ft · Wall height = 8 ft · Doors = 1 · Windows = 2 → 1 gallons (estimated paint for walls). Change an input above and this example updates with your numbers.
Common questions
With 8 ft walls, one door, and two windows, a 12 × 10 ft room has about 301 sq ft of paintable wall, which needs 1 gallon per coat at 350 sq ft per gallon. Plan on 2 gallons for two coats, plus another gallon if you are painting the 120 sq ft ceiling.
Most interior wall paints cover 350–400 sq ft per gallon on smooth, sealed surfaces, according to manufacturer data sheets. Rough or porous surfaces, bare drywall, and heavy texture can drop coverage to 250–300 sq ft per gallon.
Usually yes. Two coats give even colour and full hiding, especially over a different colour or patched areas. Some paint-and-primer products claim one-coat coverage over a similar colour, but changing from dark to light nearly always needs two coats.
Add the length and width of the room, double it for the perimeter, and multiply by the wall height. Subtract about 21 sq ft per standard door and 15 sq ft per window. A 12 × 10 × 8 ft room is 352 sq ft gross, 301 sq ft after one door and two windows.
Ceiling area is length × width. A 12 × 10 ft ceiling is 120 sq ft, about a third of a gallon per coat at 350 sq ft per gallon. Ceiling paint is usually flat and sold separately, so add it to your list rather than to the wall total.