Paint Calculator
Coverage-rate based estimation for interior, exterior and enamel paint — know exact litres and cans before you buy.
How this is calculated
Wall area is the full perimeter of the room times height — the actual painted surface, not the floor area. Doors and windows are subtracted because they aren't painted with wall paint.
Wall area = 2 × (Length + Width) × Height
Openings = (Doors × Door area) + (Windows × Window area)
Paintable area = Wall area − Openings
Primer (L) = Paintable area ÷ Primer coverage
Topcoat (L) = (Paintable area × Coats) ÷ Finish coverage
Coverage rates vary by brand and wall texture — these are the standard manufacturer-quoted ranges per coat on a reasonably smooth, primed surface.
Why does the can-size breakdown matter more than the raw litre number?
Paint is sold in fixed can sizes (1L, 4L, 10L, 20L) — you can't buy 8.15 litres exactly. Rounding the raw number yourself usually over- or under-buys by a full can. The breakdown above picks the combination that wastes the least.
Do I really need a primer coat?
On new plaster, always — it seals surface porosity so the topcoat doesn't get soaked in unevenly, which is the single biggest cause of patchy, blotchy walls. On a simple recolour over sound existing paint, it can be skipped.
Why is my actual usage higher than this calculator shows?
Coverage figures are manufacturer-quoted on a smooth, primed, properly-diluted surface. Rough texture, uneven plaster, dark-to-light colour changes, and thinner-than-recommended dilution all cut real coverage — carry 10% extra on any wall with visible texture.
Does colour (light vs dark) change how much paint I need?
Going from a dark existing colour to a light new one, or vice-versa, often needs an extra coat regardless of what the can says — factor a 3rd coat into your budget for major colour changes.