Paint Calculator
Paint is sold by coverage, not by wall. This works out the paintable wall area of a room, subtracts doors and windows, multiplies by the number of coats, and turns it into gallons and litres.
| Room length | ft |
| Room width | ft |
| Wall height | ft |
| Coats | coats |
| Doors | doors |
| Windows | windows |
Result
Your result will appear here.
How it works
Wall area is the perimeter of the room multiplied by its height. Doors and windows are then subtracted, because paint is not normally wasted on glass or on a door that may already be finished. The result is multiplied by the number of coats, since the second coat uses almost as much paint as the first.
Coverage is the bridge between area and tins. A typical interior emulsion covers about 350 square feet per gallon (roughly 8.6 square metres per litre) on a smooth, previously painted surface. Textured, porous or unpainted surfaces absorb much more.
Formula
Areas are in square feet because paint coverage is conventionally quoted per gallon that way.
- Perimeter = 2 × (length + width)
- Gross wall area = perimeter × wall height
- Paintable area = gross wall area − (doors × 21) − (windows × 15)
- Total area = paintable area × number of coats
- Gallons = total area ÷ 350, rounded up
Worked example
A 12 × 12 ft room with 9 ft walls: perimeter 48 ft, gross area 432 sq ft, minus two doors (42 sq ft) and two windows (30 sq ft) leaves 360 sq ft. Two coats is 720 sq ft, which at 350 sq ft per gallon is 2.06 gallons — so buy 3 gallons.
Assumptions
Each door is counted as 21 sq ft (3 × 7 ft) and each window as 15 sq ft (3 × 5 ft). Coverage of 350 sq ft per gallon assumes a smooth, sealed surface; new drywall, plaster, render or a strong colour change can consume 20–40% more. Ceilings, trim, primer and the inside of cupboards are not included.
Reading the result
Round up, never down, and keep the leftover: a partly used gallon is the cheapest touch-up paint you will ever own, and colour matching two years later is never exact. If you are painting a strong colour over a light one, budget an extra coat, or a tinted primer, and add roughly a third to the estimate. Buying one larger tin is usually cheaper per litre than several small ones, but only if you will genuinely use it.
For general information only. Check the coverage printed on the paint you buy.
How to use the Paint Calculator
It takes a few seconds, and every figure is calculated in your browser.
- Enter the room length.
- Enter the room width.
- Enter the wall height.
- Enter the number of coats.
- Enter the number of doors.
- Enter the number of windows.
- Press Calculate — the result appears in the dark panel beside the form.
- Use Copy, CSV, Share or Print under the result if you want to keep it.
Paint Calculator — frequently asked questions
What formula does the Paint Calculator use?
Areas are in square feet because paint coverage is conventionally quoted per gallon that way. Perimeter = 2 × (length + width) Gross wall area = perimeter × wall height Paintable area = gross wall area − (doors × 21) − (windows × 15) Total area = paintable area × number of coats Gallons = total area ÷ 350, rounded up
How does the Paint Calculator work?
Wall area is the perimeter of the room multiplied by its height. Doors and windows are then subtracted, because paint is not normally wasted on glass or on a door that may already be finished. The result is multiplied by the number of coats, since the second coat uses almost as much paint as the first. Coverage is the bridge between area and tins. A typical interior emulsion covers about 350 square feet per gallon (roughly 8.6 square metres per litre) on a smooth, previously painted surface.
What assumptions and limitations should I know about?
Each door is counted as 21 sq ft (3 × 7 ft) and each window as 15 sq ft (3 × 5 ft). Coverage of 350 sq ft per gallon assumes a smooth, sealed surface; new drywall, plaster, render or a strong colour change can consume 20–40% more. Ceilings, trim, primer and the inside of cupboards are not included.
Can you walk through a worked example?
A 12 × 12 ft room with 9 ft walls: perimeter 48 ft, gross area 432 sq ft, minus two doors (42 sq ft) and two windows (30 sq ft) leaves 360 sq ft. Two coats is 720 sq ft, which at 350 sq ft per gallon is 2.06 gallons — so buy 3 gallons.
Last updated 10 November 2026 · Formula, worked example and assumptions are documented above, and every result is calculated in your browser.