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Paint Calculator

Enter your room dimensions, how many coats you plan and the openings you are not painting. The calculator returns the gallons needed for walls, ceiling and trim separately, because they rarely use the same product.

📐 Room & surface details
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Pricing (optional)
Bar chart showing how many gallons of paint are needed for rooms from 10x10 to 20x20 feet, one coat
Wall area — not floor area — drives paint quantity, which is why a room twice the size rarely needs twice the paint.

How the paint math works

Paint is a wall area problem, not a floor area problem. Add the length of all four walls to get the perimeter, multiply by ceiling height, and you have gross wall area. A 12×12 room with 8 ft ceilings has 144 sq ft of floor but 384 sq ft of wall.

Then subtract openings. A standard interior door is about 21 sq ft and an average window around 15 sq ft. In a small room with two doors and two windows, that is over 70 sq ft you are not painting — roughly a fifth of a gallon per coat.

Coverage is typically 350 to 400 square feet per gallon on a smooth, previously painted surface. Divide your net wall area by that figure, then multiply by the number of coats. Two coats is the honest default; one coat only works when you are repainting a similar colour in the same sheen.

Because wall area scales with the perimeter rather than the floor, large rooms are more paint-efficient than small ones. Doubling a room's dimensions quadruples the floor but only doubles the wall — which is why a 20×20 room needs barely twice the paint of a 10×10.

What throws the estimate off

Texture and porosity eat paint. New drywall, bare wood and heavily textured walls can drop coverage to 250–300 sq ft per gallon. Prime first and the topcoat goes much further.

Big colour changes need more coats. Going dark-to-light, or covering a strong red or deep blue, often takes three coats without a tinted primer — and a tinted primer is usually cheaper than a third coat of finish paint.

Round up to the container you can actually buy. Paint comes in quarts and gallons, and the per-gallon price usually drops sharply at the gallon and five-gallon sizes.

Paint calculator FAQ

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

A 12×12 room with 8 ft ceilings has about 384 sq ft of wall before openings. After subtracting a door and two windows, one coat needs roughly a gallon and two coats needs about two gallons at 350 sq ft per gallon.

How many square feet does a gallon of paint cover?

Around 350 to 400 square feet on a smooth, previously painted wall. Expect 250 to 300 on new drywall, bare wood or textured surfaces unless you prime first.

Do I need one coat or two?

Two is the realistic default. One coat only covers reliably when you are repainting a very similar colour in the same sheen. Large colour changes often need a tinted primer plus two finish coats.

Do I subtract windows and doors when calculating paint?

Yes. A standard door is about 21 sq ft and a typical window about 15 sq ft. In a room with a couple of each, that is enough to change how many gallons you buy.

Other project calculators

Every calculator on this site works the same way — enter real measurements, get quantities you can hand to a supplier.

Working through a whole project? The full project calculator keeps all seven in one place, and the in-depth guides cover estimating materials accurately and reducing material waste.