How to use the paint calculator
- Measure each room’s length, width and wall height. Use “Add a room” for every extra room.
- Count the doors and windows. Their standard sizes (21 sq ft and 15 sq ft) can be changed under More options for patio doors or bay windows.
- Enter the number of coats and the coverage printed on the can.
- Choose a can size and, for a cost, your price per gallon or litre. Tick primer for new drywall, patched plaster or a big colour change.
- Read the cans to buy at the top of the results, then copy, share or print the shopping list.
How the paint estimate is worked out
Each room is treated as four walls, so the wall area is the perimeter multiplied by the height. Openings are then taken away, because glass and door leaves do not get wall paint.
Area to paint = wall area − doors × door area − windows × window area
Paint = area to paint × coats ÷ coverage × (1 + waste %)
Cans = paint ÷ can size, rounded up to a whole can
When you tick ceilings, the floor area of each room (length × width) is added once. Primer is worked out the same way with one coat and its own coverage. The cost is the number of cans × can size × your price, so a part-used last can is still paid for in full.
Worked example
A 12 × 10 ft bedroom with 8 ft walls, one door and two windows, painted with two coats of a paint that covers 350 sq ft per gallon, plus 10% extra, bought in 1-gallon cans at $42 a gallon:
- Wall area: 2 × (12 + 10) × 8 = 352 sq ft.
- Doors and windows: 1 × 21 + 2 × 15 = 51 sq ft, so the area to paint is 352 − 51 = 301 sq ft.
- Paint: 301 × 2 ÷ 350 = 1.72 gallons; adding 10% gives 1.89 gal.
- Rounded up to whole cans that is 2 × 1 gal, which at $42 per gallon costs $84.00.
Tips and common mistakes
- Use the coverage on your own can. Rough plaster, textured finishes and bare drywall soak up more paint than a smooth painted wall, so enter a lower coverage figure for them.
- Keep the waste allowance. Paint left in the tray, on the roller and in the can adds up. Ten percent suits most jobs; raise it for uneven walls.
- Buy everything from one batch. Tinted paint can vary slightly between mixes. If cans come from two batches, pour them together in a large bucket first.
- Remember trim is separate. Skirting boards, frames and doors usually take a harder-wearing gloss or satin paint, which this calculator does not include.
- Check older homes for lead. In the United States, homes built before 1978 may have lead-based paint. Test before sanding or scraping, and use a lead-safe certified contractor if old paint will be disturbed.
US, UK and Canada differences
In the United States, interior paint is sold by the US gallon (3.785 litres), the quart and the 5-gallon bucket, and labels give coverage in square feet per gallon. In the UK, wall paint is usually called emulsion and comes in 1, 2.5, 5 and 10 litre tins, with coverage in square metres per litre. Canadian stores sell 3.78 L cans, often still called a gallon, and 946 mL cans, and many labels show both square metres and square feet.
Switching units converts the numbers you typed instead of resetting them, and the can sizes change to the matching tins.
Metric units and other countries
In metric mode, enter room sizes in metres and coverage in m² per litre from the tin. In Pakistan, India and the Gulf states paint is also sold in litre-based tins, so metric mode fits there too; set your currency in the header and type your local price per litre. Example prices appear only for the US, UK, Canada, the eurozone and Australia, because a converted price would not match local shops.