Free paint calculator. Enter wall area, coats and coverage to find out exactly how many litres of paint and how many cans you need.
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Enter the total wall area in square metres, how many coats you plan, and the coverage printed on your paint can in m² per litre. The calculator multiplies area × coats ÷ coverage to get the litres needed, then rounds up to whole cans in your chosen can size. Add roughly 10% extra for waste and touch-ups on top of the result.
Paint needed (litres) = wall area × number of coats ÷ coverage per litre. A 40 m² wall with 2 coats at 10 m²/L needs exactly 8 litres. Cans = litres ÷ can size, always rounded up, 8 litres in 4-litre cans is 2 cans, but 8.2 litres is 3 cans, because paint is only sold in whole cans. The calculator uses your can's real coverage figure, which beats any rule of thumb.
Measure the width of each wall, add the widths together and multiply by the ceiling height, that is your gross wall area. Subtract roughly 2 m² per door and 1.5 m² per window for a tighter estimate, or skip the subtraction for a built-in safety margin. For ceilings, add length × width separately, a ceiling typically adds 40–50% to a room's paint need, and it is the part people most often forget.
Standard interior emulsion covers about 10–12 m² per litre per coat on a smooth sealed wall; primers and undercoats do 8–10, gloss 12–15, and exterior masonry paint only 6–8. Rough, textured or bare surfaces can drink 30–40% more than the can claims. Always check the specific coverage printed on your can, it varies by brand and product line, and it is the number this calculator is built around.
Two coats is the professional standard for walls in good condition. Use one coat only to refresh the same colour on a smooth surface. Plan three coats for dark-to-light colour changes, very saturated colours like deep reds, or fresh plaster, which also needs a thinned mist coat first so the paint bonds instead of flaking a year later.
Paint comes in 1, 2.5, 4, 5, 10 and 20-litre cans; the calculator rounds your litres up to the fewest whole cans in your chosen size. Add a 10–15% waste factor on top of the result for cutting in, roller absorption and touch-ups, unopened cans can usually be returned. Painting two rooms the same colour? Calculate them together: the leftovers combine, and you often need fewer cans in total than doing each room separately.
As a planning rule, one litre of interior emulsion covers about 10–12 m² per coat on a smooth sealed wall, roughly 0.085 to 0.1 litres per square metre. Double it for the standard two coats, and use the lower coverage figure for rough or porous surfaces.
Two coats is the professional standard for most walls. One coat only works for refreshing the same colour on a smooth surface. Go to three coats for dark-to-light changes, strong saturated colours, or new plaster (which needs a thinned mist coat first).
For the tightest estimate, subtract about 2 m² per standard door and 1.5 m² per window. Many people skip the subtraction instead, the over-estimate then doubles as the waste margin. Either approach works; just be consistent.
Yes. The first coat on fresh plaster should be emulsion thinned with around 20% water so it soaks in and bonds rather than sitting on the surface. Count it as one of your coats, then add two full-strength coats on top. Skipping it is the usual reason new plaster starts flaking.
Usually no, if the existing paint is sound, clean and you are staying with a similar colour, skip it. Use primer (or a stain-blocking primer) for bare plaster, water stains, dark-to-light colour changes, or glossy surfaces, which should be sanded first.
Always round up to whole cans, you cannot buy part of a can, and running out mid-wall risks a visible shade difference between batches. The calculator does this rounding for you based on the can size you select.
The ceiling area is simply length × width, and it typically adds 40–50% to a room's total paint need. A 4 m × 3 m bedroom ceiling is 12 m², about 2.4 litres for two coats at 10 m²/L. Calculate it separately so it is not forgotten.
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