Garden Paint calculator
Paint is sold by coverage, typically 350 to 400 square feet per gallon on a smooth, primed surface. Two things blow that estimate: forgetting the second coat, and a textured or previously unpainted wall that drinks more than the label promises.
How this is calculated
Wall area = perimeter × height. Subtract 21 sq ft per door and 15 sq ft per window. Gallons = (area × coats) ÷ coverage per gallon.
Source: Coverage of 350-400 sq ft per gallon is the standard manufacturer figure; door and window areas are US standard sizes
Worth knowing: Coverage on the can assumes a smooth, primed, similarly-colored surface. Bare drywall, textured walls, and any dark-to-light color change will use appreciably more — add 10-20% and prime first. Buy all of a color in one trip: separate batches can differ subtly, and the seam shows on a large wall.
Common questions
Should I subtract windows and doors?
Yes, but do not get precise about it. A standard interior door is about 21 sq ft and an average window about 15. If the room is mostly glass, measure properly; otherwise these figures are close enough and err slightly toward buying extra.
Do I really need two coats?
Almost always. One coat over an existing similar color can work with a high-build paint, but any color change, any patched drywall, and any sheen change will show through. Two thinner coats also level better than one thick one.
How much extra should I buy?
Round up to the next whole can and keep the remainder. Touch-ups are inevitable, and paint from the same batch matches exactly where a fresh can mixed months later may not.
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