How the paint calculator works
The calculator adds up every surface you enter, subtracts the openings, multiplies by the number of coats and divides by the coverage from the tin. Then it rounds up to whole tins of the size you choose.
- Surface area. Each wall is width × height; a ceiling is length × width. Add as many surfaces as the room has.
- Openings. Doors, windows and anything else that will not be painted, with a count for repeats.
- Coats. Net area × coats is the total area the paint has to cover.
- Litres. Total area ÷ coverage in m² per litre.
- Tins. Litres ÷ tin size, rounded up, with the litres you will actually own.
- Primer. Optional: one coat (or more) of primer, sealer or mist coat over the same net area at its own coverage, rounded up to its own tins.
An optional extra percentage covers roller loading, touch-ups and the paint left in the tray. Five to ten percent is common on a first job.
Measuring a room for paint
For a simple room, add the four walls: two of length × height and two of width × height. If you only want the walls, leave the ceiling out; if you are painting the ceiling in the same paint, add it as a fifth surface. Measure the height from skirting to ceiling, not from the floor.
Bay windows, chimney breasts and alcoves add wall area. Measure each face separately rather than treating the wall as flat.
Reading the tin
Manufacturers quote coverage as a range, for example 10 to 12 m² per litre per coat. The higher number assumes a smooth, sealed surface and a careful roller. For bare plaster, textured wallpaper or a colour change from dark to light, use the lower number. Some brands quote coverage per tin rather than per litre; divide by the tin size to get the figure the calculator needs.
Imperial units
Switch to imperial and the calculator takes feet for the walls, square feet per US gallon for coverage and gallons for the tin. The calculation still runs in metres and litres, so the result matches the metric one.