Paint Volume with Coats Calculator
Buying the right amount of paint saves both money and a frustrating mid-job trip to the store. The calculation is straightforward: multiply the area you are painting by the number of coats you plan to apply, then divide by the coverage your paint achieves per gallon. This calculator does that and returns the gallons you need, which you then round up at the till since paint is sold in whole cans. Two inputs matter most. The number of coats multiplies the paint directly, and two coats are standard for an even, durable finish, especially when changing colour or painting new surfaces. Coverage per gallon, usually printed on the can at around 350 to 400 square feet for a smooth primed wall, should be reduced for rough, porous or unprimed surfaces that drink in more paint and cover less. For a tighter estimate you can subtract large windows and doors from the wall area first, though leaving the full area in gives a useful safety margin. Primer is a separate product, so run it on its own pass with one coat and its own coverage. Every figure here is computed deterministically from the area times coats divided by coverage formula below, with a worked example that reconciles exactly to the calculator.
Paint needed is area times coats over coverage: gallons = area x coats / coverage per gallon. Painting 400 sq ft with 2 coats at 350 sq ft per gallon needs about 2.29 gallons, so buy 3.
Paint volume formula
gallons = (area x coats) / coverage per gallon
area = surface area to be painted
coats = number of coats to apply
coverage = square feet one gallon covers
Multiplying area by coats gives the total surface to cover, and dividing by coverage per gallon gives the gallons. Round up to whole cans when buying.
Worked example
Painting 400 square feet with 2 coats, using paint rated at 350 square feet per gallon.
- Total area to cover: 400 x 2 = 800 square feet.
- Gallons: 800 / 350 = 2.29 gallons.
- Round up to buy: 3 gallons.
You need about 2.29 gallons, so buy 3. These are the calculator's default inputs, so the result above matches the widget exactly.
Paint volume calculator: frequently asked questions
How much paint do I need?
Multiply the area to be painted by the number of coats, then divide by the coverage per gallon. For 400 square feet, 2 coats and a paint that covers 350 square feet per gallon, that is 400 times 2 divided by 350, which is about 2.29 gallons. Round up when buying, since you cannot buy a fraction of a can.
What coverage should I assume per gallon?
Manufacturers usually state coverage on the can, commonly around 350 to 400 square feet per gallon for one coat on a smooth, primed surface. Rough, porous or unprimed surfaces absorb more paint and cover less area per gallon, so reduce the coverage figure you enter for textured walls, bare drywall or masonry.
Why account for two coats?
A single coat rarely gives even, full colour, especially over a different existing colour or on new surfaces. Two coats are standard for a durable, uniform finish. The number of coats multiplies the paint needed directly, so two coats need twice the paint of one, which is why it is a key input here.
Should I subtract doors and windows?
For a rough estimate you can paint over the full wall area as a safety margin. For a tighter estimate, subtract the area of large windows and doors from the wall area before entering it. Keeping a small surplus is wise so you do not run short partway through and risk a visible difference between cans.
Does primer count separately?
Yes. Primer is a separate product with its own coverage, so calculate it on its own pass: enter the same area, one coat, and the primer's coverage per gallon. Priming bare or patched surfaces improves adhesion and can reduce the number of finish coats needed, which may lower your total finish paint.
Official sources
- Consumer product and home material guidance: US Food and Drug Administration (FDA). As at 25 June 2026.
Reviewed by the CalculatorHub team, edited by James Graham, 25 June 2026. See our methodology. This is general information, not financial, tax, legal or investment advice.