Paint Quantity Calculator
Buying paint is a guessing game: too little and you stop mid-wall for a second tin that may not match the first batch, too much and you pay for paint you will never use. This calculator turns the wall area, the number of coats and the paint’s coverage into the litres you need and the cans you should buy.
Enter the wall area directly, or let the calculator work it out from the room’s length, width and height (it subtracts a typical door and window allowance). Set the number of coats — two is standard for a colour change — and the coverage stated on your paint tin (around 10 m² per litre for emulsion). The result includes a wastage buffer so a real-world tin count is enough.
It is the calculation that stops you buying the wrong amount of paint — enough for two coats with a wastage buffer, in one batch, so the whole room is finished without a mid-wall dash to the DIY store.
Enter your values to see the result.
How this is calculated
The wall area is calculated from the room’s length, width and height as 2 × (length + width) × height, the perimeter times the height. A typical opening allowance of one door (about 1.8 m²) and one window (about 1.6 m²) is subtracted, since those areas are not painted. The paintable area is then multiplied by the number of coats to get the total area to cover, and divided by the paint’s coverage in m² per litre per coat to get the litres needed.
A 10% wastage buffer is added to the litres to account for roller absorption, cutting-in, spillage and the difficulty of getting the last paint out of the tin. The number of cans is the buffered litres divided by the can size you enter, rounded up to a whole can — rounding up ensures you buy enough in one batch, since paint is tinted per batch and a second trip may yield a slightly different shade.
The model assumes a simple rectangular room with one door and one window. Rooms with more openings, alcoves or sloping ceilings need the openings area adjusted — you can subtract extra openings by reducing the wall height input or by calculating the wall area directly elsewhere. Coverage varies by paint type (emulsion, gloss, primer), surface texture and application method, so use the coverage printed on your specific tin rather than a generic figure.
Worked example
A 4 m × 3 m room, 2.4 m high, two coats, paint covering 10 m²/L, 2.5 L cans.
- 1Wall area = 2 × (4 + 3) × 2.4 = 33.6 m².
- 2Openings = 1.8 + 1.6 = 3.4 m²; paintable = 30.2 m².
- 3With 2 coats = 60.4 m² to cover.
- 4Litres = 60.4 ÷ 10 = 6.04; with 10% waste = 6.64 L.
- 5Cans = ceil(6.64 ÷ 2.5) = 3 cans (7.5 L).
Frequently asked questions
- How do I find my paint’s coverage?
- It is printed on the tin, usually as square metres per litre or square feet per quart. Emulsion wall paint typically covers 10–12 m² per litre per coat; gloss and primer differ. Use the figure for your specific product for the most accurate result.
- How many coats do I need?
- One coat for a refresh of the same colour, two for a colour change or over a darker shade, and sometimes three for a light colour over a dark one. The calculator multiplies the area by the number of coats you enter.
- Does this include ceilings or trim?
- Only if you include them in the wall area. For a ceiling, use the floor area as the paint area. For skirting and trim, measure their length and width separately and add them in.
- How do I paint a ceiling with this?
- Treat the ceiling as a wall at floor level: enter the floor length and width as the room dimensions, set the wall height to the value that gives the ceiling area, or simply use the floor area (length × width) as the paintable area and skip the perimeter calculation. Use the ceiling paint’s coverage figure, which often differs from wall emulsion.
- What if my paint covers more or less than 10 m²/L?
- Change the coverage field to the figure printed on your tin. Quality emulsion often covers 12–14 m²/L, while cheap or heavily pigmented paint may cover only 8. The result is only as good as the coverage figure you enter, so check the tin rather than guessing.
Related calculators
Method: wall area = 2 × (length + width) × height, minus a typical door and window, × coats ÷ coverage, plus 10% wastage. Coverage varies by paint and surface. Last updated: September 2026.
These results are general estimates from established formulas and conversion factors. Paint coverage, tile yields, concrete volumes, fuel economy, oven behaviour and clothing and shoe sizes vary by product, surface, vehicle, oven and brand. Always confirm with your supplier, the product label or the brand’s own size chart before buying or acting on a calculation.