Paint calculator

How much does a gallon of paint cover?

About 350 to 400 square feet per coat on a smooth, primed wall, by Sherwin-Williams’ figures, and a gallon of primer covers 200 to 300. Two coats on the walls of a 12 × 12 room with an 8 ft ceiling come to 768 sq ft, which at 350 is 2.19 gallons: two gallons and a quart.

Coverage is the one number in the working that comes from the tin rather than the tape, and it varies more than people expect. British tins are quoted in metric and claim a good deal more — “up to” 14 m² per litre for retail matt emulsion and up to 17 for the trade version — because those figures assume a smooth, sealed wall in a similar colour. The default here is 350 sq ft per US gallon, the cautious end of the US range. It is stored that way whichever unit you are working in, so switching the page between feet and metres re-labels the field and leaves your answer exactly where it was.

How the number is worked out

Paint covers surface, so the first job is to find the surface. For each room the calculator takes the wall run — the perimeter of the shape you measured — and multiplies it by the ceiling height. Doors and windows come off. If you are painting the ceiling too, the floor area goes back on, because a flat ceiling is the same size as the floor under it.

That gives the surface one coat has to cover. Multiply by the coats, divide by what a container covers, and you have a volume. Then comes the step most calculators skip: paint is not sold by the gallon-and-a-bit. The answer is rounded up into containers you can actually buy.

The worked example

A 12 ft by 14 ft room with an 8 ft ceiling, one door and two windows, two coats:

Buying two gallons would leave you a third of a wall short. Buying three wastes most of a gallon. The quart is the step paint is really sold in, so that is the step the rounding uses.

Rooms whose walls cannot be worked out

A triangle entered as a base and a height has an area but no known sides, and the same is true of a trapezoid or a parallelogram without its slanted edges. Wall length cannot be recovered from those numbers, so the calculator says so and leaves that room’s walls out rather than inventing them. Its ceiling is still counted — the floor area is known either way.

Taking the rooms with you

If you have already measured on the square footage calculator, the button at the bottom of it opens this page with those rooms in place. Nothing is uploaded: the measurements travel inside the link, after the #, which browsers never send to a server.