Paint Coverage Calculator: Litres and Cans
Paint Coverage Calculator
Room dimensions to litres and cans, with door and window openings deducted and the number of coats accounted for. Works in metric or imperial, including the North American habit of quoting coverage in square feet per gallon.
Standard openings are assumed at 0.9 by 2.1 m per door and 1.5 by 1.2 m per window, which is close enough for ordering paint. Coverage varies with surface, colour change and application: 10 to 12 square metres per litre is typical for a smooth primed wall, less on bare drywall or rough render. Buy the extra can before the job, not during.
How to use this tool
Enter the room in metres or feet, set the number of coats, say how many doors and windows there are, and put in the coverage figure from the tin. The tool returns litres and, more usefully, whole cans, because paint is not sold by the litre you need.
Coverage can be entered either way. Metric tins quote square metres per litre, North American tins quote square feet per US gallon, and the selector converts, so there is no need to do the awkward 350 square feet per gallon conversion by hand.
Why the openings and the second coat both matter
A standard door removes about 1.9 square metres and a typical window about 1.8. In a small room with a door and two windows that is 5.5 square metres off a 33.6 square metre wall area, roughly a sixth of the job, and ignoring it is how people end up with a leftover can they did not want. The tool deducts them at standard sizes, which is close enough for ordering.
The second coat is not optional arithmetic. Two coats over a similar colour is the normal specification, and skipping it in the estimate halves the paint. Colour changes, bare drywall and rough render all drink more than the tin promises, so a coverage of 10 to 12 square metres per litre is realistic for smooth primed wall and less for anything else.
A worked check you can repeat on paper
A 4 by 3 metre room with 2.4 metre walls: the perimeter is 14 metres, so gross wall area is 33.6 square metres. Deduct one door and two windows, 5.49 square metres, leaving 28.11. Two coats at 11 square metres per litre is 28.11 times 2 divided by 11, which is 5.11 litres, so two 3.78 litre cans with about 2.45 litres spare. Buying one can would leave the second coat unfinished by a wide margin.
Mistakes that catch people out
Estimating for one coat. The commonest error and it is a factor of two. Fix: the coats box defaults to 2.
Using floor area instead of wall area. Walls are perimeter times height; a 4 by 3 room has 12 square metres of floor and 33.6 of wall. Fix: enter room dimensions and let the tool build the wall area.
Trusting the coverage claim on a colour change. Going dark to light can need three coats regardless of the tin. Fix: reduce the coverage figure or add a coat.
Buying paint in two trips. Batch colour varies between production runs. Fix: buy all of it at once; the tool tells you the whole-can count so you can.
How this was checked
The area arithmetic was verified on the worked example above, the opening deductions confirmed against standard door and window sizes, and the unit conversion checked so 350 square feet per US gallon returns 8.59 square metres per litre. Can counting was tested at the boundary: an exact multiple never adds a spare can, while a fraction over always rounds up.
Related Calculators and Reference
Wall and floor areas for irregular shapes are the 2D Area Calculator, and converting the room dimensions themselves is the Length Unit Converter. If the same room is getting a floor, the Tile and Flooring Calculator handles the boxes. New carpet for the same room is the Carpet Calculator. Papering the walls instead is the Wallpaper Calculator, and the outside walls of the house are the Siding Calculator.