Paint Calculator

How to use the paint calculator

Measure your room with a tape measure and round to the nearest half foot — precision past that doesn’t change how many cans you buy. You need three numbers: the room’s length, its width, and the wall height. For wall height, measure floor to ceiling in a corner; most US homes built after 1995 have 9 ft ceilings on the first floor and 8 ft upstairs, while older homes are usually 8 ft throughout. If you’re only painting one accent wall, skip the room model and do the math directly: multiply the wall’s length by its height, double it for two coats, and divide by your coverage. A typical 12 × 8 ft accent wall is 192 sq ft for two coats — comfortably inside one gallon.

Count every door and full-size window in the room, including closet doors. The calculator subtracts 21 sq ft per door and 15 sq ft per window — the standard takeoff values painters use — because you won’t be rolling paint over glass and door slabs. Count a sliding glass door as two doors (about 42 sq ft of glass and frame). If your room has a large picture window or a wall of glass, count it as two or three windows to keep the deduction honest.

Leave coats at 2 unless you have a specific reason not to. Two coats is the default for a reason: one coat almost never covers evenly, even with "paint and primer in one" products, and the second coat is what gives you uniform color and a washable film. Going from a dark color to a light one, or painting over bare drywall? You may want a dedicated primer first (see the FAQ) — primer is a separate product and isn’t included in this gallon count.

The Advanced section holds two settings most people can skip. Coverage defaults to 350 sq ft per gallon, the midpoint of the 300–400 range printed on most US paint cans; drop it toward 250–300 for textured walls, stucco, or unprimed surfaces that drink paint, and raise it toward 400 for smooth, previously-painted drywall. Turn on "Include ceiling" only if you’re painting the ceiling the same color as the walls — ceilings in a different white should be calculated as their own job so you don’t mix can counts.

Read the result as the number of gallons to buy, already rounded up to whole cans. The "actual" figure in parentheses is the exact math — if it’s just barely over a whole number (say 2.1), consider whether a quart can cover the overage instead of a full gallon. And keep the leftover: a labeled, tightly-sealed can stores for years and makes touch-ups painless.

The formula

No black box — here is the exact math this calculator runs, the same takeoff a professional painter does on a notepad:

wall area = 2 × (length + width) × height − (doors × 21) − (windows × 15)
ceiling area = length × width   (only if painting the ceiling)
total area = (wall area + ceiling area) × coats
gallons = total area ÷ coverage, rounded UP to a whole can

Worked example with the default room: a 12 × 10 ft room with 8 ft walls has 2 × (12 + 10) × 8 = 352 sq ft of gross wall. One door (−21) and two windows (−30) bring that to 301 sq ft. Two coats doubles it to 602 sq ft, and at 350 sq ft per gallon that’s 1.72 gallons — so you buy 2.

The wall-area deduction is floored at zero: a closet with two doors can’t produce negative square footage. Rounding always goes up because paint is sold in whole cans, and running out 90% of the way through a wall guarantees a visible lap mark.

Frequently asked questions

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