This is a guide. Surface texture, primer condition, and brand all affect coverage — always check your tin.
This calculator works out the total wall area of a room, deducts a standard allowance for doors and windows, then divides by your chosen coverage rate and multiplies by the number of coats. The result is rounded up to the nearest half-litre (or tenth of a gallon) so you don't run short.
Emulsion, gloss/satin, masonry and primer/undercoat all cover different areas per litre — emulsion spreads furthest on a typical wall, while masonry paint's thicker, textured formulation covers less per litre. The slider snaps to a sensible default for whichever paint type you pick, but stays adjustable so you can match the exact figure printed on your tin, which is always the more accurate number.
A standard door is assumed to take up about 1.85 m² of wall area, whichever way it opens. Windows are grouped into three size bands — small (0.5 m²), medium (1.2 m²) and large (2.4 m²) — since exact window sizes vary too much to ask for individually. These are fixed, typical figures; an unusually large window or a bay window will cover more than the "large" band accounts for.
Paint is sold in fixed tin sizes, and running short mid-wall means a delay while you buy more and a visible seam where an old and new batch of paint meet. Rounding the calculated amount up to the nearest half-litre (or tenth of a gallon) gives a small buffer without meaningfully overbuying.
This is a guide, not an exact figure. Surface texture (bare plaster absorbs far more than a previously painted wall), primer condition, and brand-to-brand differences in coverage all affect how far a tin actually goes — always check the coverage rate printed on your specific tin and adjust the slider to match.