Paint Calculator
Estimate how much paint a room needs from its size, doors, windows and coats, in litres or gallons, plus cans to buy.
🔒 Runs entirely in your browser — nothing is uploadedHow paint quantity is calculated
Paint is sold by volume but applied over an area, so the estimate starts with the surface you are covering. The four walls of a rectangular room have a combined area of wall area = 2 × (length + width) × height. Doors and windows are not painted, so their areas are subtracted: paintable area = wall area − doors − windows. If you choose to paint the ceiling, its area length × width is added. A standard door is about 1.9 m² (21 ft²) and a typical window about 1.4 m² (15 ft²), which are the defaults in the calculator; adjust them if yours are larger.
The amount of paint follows from the coverage rate, which tells you how many square metres or square feet one litre or gallon will cover in a single coat. The formula is paint needed = paintable area × coats ÷ coverage. With the typical defaults of 10 m² per litre or 350 ft² per gallon, a room with 60 m² of surface and two coats needs 60 × 2 ÷ 10 = 12 litres. Finally, the number of cans is ceil(paint needed ÷ can size), rounded up because you cannot buy part of a can.
Tips for accurate paint estimates
Coverage depends on the paint and the surface. Dark walls, fresh plaster and rough or textured finishes soak up more paint, and a coat of primer or a tinted undercoat can reduce the number of finish coats needed. Always use the coverage figure printed on the tin, because premium paints often cover more area per litre than budget products. Remember that ceilings, skirting boards, door frames and radiators usually need different paint, so plan those separately.
Buy all the paint for one wall or room at the same time, and mix cans together before you start, because colour can vary slightly between batches. Keep a little paint for future touch-ups. This calculator works entirely in your browser, with no data sent anywhere, and its results are estimates for planning rather than a guarantee of exact consumption.
How to use
- Enter the room sizeType the room length, width and wall height in metres or feet.
- Subtract doors and windowsEnter how many doors and windows there are and their approximate area so they are not painted in the estimate.
- Set coats and coverageChoose the number of coats and the coverage printed on the can; tick the ceiling box if you will paint it too.
- Read litres and cansSee the paintable area, the paint required and how many cans of your chosen size to buy.