Paint Calculator
Paint is worked out from wall area — the room's perimeter times its ceiling height — with the doors and windows taken off. Using floor area instead, which many calculators do, gives you about a third of what you need.
Paint needed
2tins
- Wall area
- 352 sq ft
- Less openings
- 44 sq ft
- Area to paint
- 308 sq ft
- With 2 coats
- 616 sq ft
- Square metres
- 28.6
- Doors are counted at 36 by 80 inches and windows at 3 by 4 feet. Measure your own if they are unusual.
- This is wall area — perimeter times height — not floor area. Add the ceiling separately if you are painting it.
- Coverage is whatever the tin says; 350 square feet per US gallon is typical for one coat on a smooth, previously painted surface. Bare plaster and strong colour changes take more.
About the paint calculator
A twelve by ten room has 120 square feet of floor and 352 square feet of wall. That gap is the single reason paint calculations go wrong, and it is why this one starts from the perimeter rather than the floor.
Doors and windows come off next. A standard door is about twenty square feet and an average window twelve, so a small room with a door and two windows has over forty square feet you would otherwise be buying paint to cover. Then the total is multiplied by the number of coats and divided by whatever the tin says it covers.
Coverage is an input rather than a fixed figure, because it is printed on the tin and varies enormously. Three hundred and fifty square feet per US gallon is typical for one coat on smooth, previously painted walls; bare plaster and heavy texture can take almost twice as much.
What it works out
- Tins needed, rounded up to whole tins
- Wall area, openings and the net area to paint
- Any number of coats, at your own coverage rate
The formula
Paint = ((2 × (Length + Width) × Height) − Openings) × Coats ÷ Coverage
The part people get wrong is the first step. A room's wall area is its perimeter multiplied by its ceiling height — not its floor area. A twelve by ten room has 120 square feet of floor but 352 square feet of wall, so working from the floor figure buys you roughly a third of the paint you need.
Doors and windows come off next. A standard door is about twenty square feet and an average window about twelve, and in a small room with two windows and a door that is over forty square feet you would otherwise be paying to cover.
Multiply what is left by the number of coats, then divide by whatever the tin says it covers. Two coats is normal; one is only ever enough when you are repainting a similar colour onto a sound surface.
- Length and Width
- The floor dimensions of the room, used to get its perimeter.
- Height
- Floor to ceiling. Eight feet is standard in most homes.
- Openings
- Doors at 36 by 80 inches and windows at 3 by 4 feet, subtracted from the wall area.
- Coats
- Two for most jobs. Three over a strong colour or bare plaster.
- Coverage
- Square feet one tin covers in one coat, as printed on the tin.
A worked example
A twelve by ten bedroom with eight-foot ceilings, one door and two windows, in two coats.
That works out to 2 tins.
- Wall area
- 352 sq ft
- Less openings
- 44 sq ft
- Area to paint
- 308 sq ft
- With 2 coats
- 616 sq ft
- Square metres
- 28.6
Questions
How much paint do I need for a 12x12 room?
With eight-foot ceilings, a 12 by 12 room has about 384 square feet of wall. Take off roughly 44 square feet for a door and two windows and you are painting 340 square feet, which is one US gallon per coat, so two gallons for two coats.
Does this include the ceiling?
No. This is walls only. A ceiling is the floor area — length times width — so add that separately, and remember ceiling paint is usually a different product.
How many coats of paint do I need?
Two is the normal answer. One coat only works when you are going over a sound, similar colour. Bare plaster, new drywall, or covering a dark colour with a light one usually needs a primer plus two coats.
How much does a gallon of paint cover?
About 350 square feet in one coat on a smooth, previously painted wall. Rough render, bare plaster and textured surfaces drink far more — sometimes as little as 200 square feet to the gallon.
Should I subtract doors and windows?
Yes, if there are more than one or two. In a room with a door and two windows that is over forty square feet, which is a meaningful part of a gallon. In a room with a single small window it barely matters and leaving it in gives you useful spare.
How much extra paint should I buy?
The calculation rounds up to whole tins, which usually leaves enough spare for touch-ups. Keep whatever is left sealed and labelled with the room — matching a colour years later is far harder than storing the tin.