🎨 Paint Coverage Calculator

Enter your wall dimensions to estimate how many gallons of paint you need. This is a general estimate — actual coverage varies by paint brand, surface texture and application method.

Total wall length (ft)

Wall height (ft)

Doors (each ~21 sq ft)

Windows (each ~15 sq ft)

Number of coats

Coverage per gallon (sq ft)

How the gallons are worked out

Wall length times height gives the gross area. Each door is deducted as 21 sq ft and each window as 15 sq ft, the remainder is multiplied by the number of coats, and that total is divided by the coverage figure. The result is rounded up to the nearest eighth of a gallon, with the whole-can count shown alongside.

gross area = wall length x wall height
net area   = gross - (doors x 21) - (windows x 15)
gallons    = net area x coats / coverage per gallon

The defaults - 60 ft of wall at 9 ft, two doors, three windows, two coats at 350 sq ft per gallon - give 540 sq ft gross, 87 sq ft of openings, 453 sq ft net, 906 sq ft to cover and 2.63 gallons. That is three cans.

The coverage figure is the optimistic one

350 sq ft per gallon is what a paint achieves rolled onto a smooth, sealed, previously painted wall. Real surfaces take more:

New or patched drywall drinks the first coat. Prime it - primer is cheaper than topcoat and stops the finish going patchy over the filler.
Texture costs area. Knockdown or orange peel can pull coverage down to 250-300 sq ft, and heavy stipple or masonry further still.
Spraying uses noticeably more than rolling once overspray and the paint left in the lines is counted.
A colour change may need three coats - deep reds and yellows over white, or anything over a dark wall, whatever the tin says.

Buy the round number up rather than the exact figure. Two gallons and a quart is really three gallons, and if you are near five, a single five-gallon bucket is mixed as one batch: cans tinted separately can differ just enough to leave a band down a long wall, which is the painting equivalent of a pattern mismatch. Keep the leftover sealed and labelled for touch-ups, since a re-tint months later rarely matches exactly.

This figure covers walls only. Ceilings, trim, door faces and the insides of window reveals are all extra, and normally a different paint - flat for ceilings, a scrubbable finish for trim. In a home built before 1978, rules on disturbing lead paint govern how you may sand or strip it, and those override any decorating plan.

Frequently asked questions

How much paint do I need for a 12x12 room?

A 48 ft perimeter at 8 ft is 384 sq ft, less about 51 sq ft for a door and two windows, so 333 sq ft. Two coats is 666 sq ft, which is 1.9 gallons - buy two, and a third if the walls are new or the colour is changing.

How many square feet does a gallon of paint cover?

About 350 sq ft on a smooth, sealed wall, and 250 to 300 on texture, bare filler or porous surfaces. The number on the tin assumes the best case and a full film thickness, not a coat stretched thin.

Should I deduct doors and windows?

Yes, once they add up. This page takes off 21 sq ft per door and 15 per window, which is close for a standard door and an average double-hung. Ignore very small windows - the cutting-in around them uses more paint than the glass saves.