How much does a gallon of paint cover?

About 350 square feet on a smooth, sealed, similarly coloured wall. On textured surfaces, bare drywall or a dramatic colour change, expect 250–300 and buy accordingly.

Do I need to subtract doors and windows?

For one or two openings it barely matters and buying slightly long is sensible. In a room with a wall of glass it matters a great deal, which is why the calculator asks.

Paint Calculator

Work out how many gallons of paint a room needs from its dimensions, coats and openings.

Draft — figures checked, awaiting an editorial read. Not indexed.

Paint to buy— gal

Paintable area: — ft²
Paint needed (exact): — gal
Gross wall areaBefore doors and windows are subtracted.: — ft²
Primer (one coat): — gal

Formula: Wall area = 2 × (length + width) × height, minus openings. Gallons = area × coats ÷ coverage per gallon.

What this calculates

This estimates how much paint a room needs, in whole gallons, allowing for the number of coats and for the doors and windows you will not be painting. Interior paint is the classic case of a calculation people do badly in both directions: buy for the gross wall area and you finish with most of a gallon left over, but forget that two coats means twice the paint and you are back at the shop mid-job with a wall half-finished and a colour that may not match batch to batch.

How it works

Wall area is the room perimeter — twice the length plus twice the width — multiplied by the ceiling height. From that, standard opening sizes are subtracted: about 21 square feet per door, being a 3 by 7 foot opening, and about 15 square feet per window. If you are painting the ceiling as well, its area is the room length times width and is added back. That paintable area is then multiplied by the number of coats and divided by the coverage figure on the can, which defaults to 350 square feet per gallon.

Worked example

A 14 by 12 foot room with 8 foot ceilings, one door and two windows, two coats. The perimeter is 2 × (14 + 12) = 52 feet, so gross wall area is 52 × 8 = 416 square feet. Subtract 21 for the door and 30 for the two windows and you have 365 square feet of paintable wall. Two coats at 350 square feet per gallon is 365 × 2 ÷ 350 = 2.09 gallons — so you buy three gallons, or more sensibly two gallons plus a quart.

Common mistakes

Forgetting to multiply by the number of coats is the most common and it halves the answer. Trusting the coverage claim on the can is the next: 350 square feet assumes a smooth, previously painted, similarly coloured wall. Bare drywall, textured plaster and deep colour changes all drink far more, sometimes 250 square feet per gallon or less. And primer is a single coat regardless of how many finish coats you plan, which is why it is reported separately here.

Common questions

How much does a gallon of paint cover?: About 350 square feet on a smooth, sealed, similarly coloured wall. On textured surfaces, bare drywall or a dramatic colour change, expect 250–300 and buy accordingly.
Do I need to subtract doors and windows?: For one or two openings it barely matters and buying slightly long is sensible. In a room with a wall of glass it matters a great deal, which is why the calculator asks.