Paint tins quote a theoretical coverage rate measured on a smooth, previously painted, non-porous surface — real jobs rarely match that exactly. This tool starts from the manufacturer's stated rate for the paint type you pick, then applies the porosity, coat count, and a wastage allowance for cutting-in, roller loading, and spills, the same way a decorator's quote would.
A 4m × 5m bedroom with a 2.7m ceiling, one door and one window, painted in two coats of standard emulsion on a porous new-plaster surface, needs about 12 litres — verified against a published worked calculation using the same method.
Bare plaster is highly absorbent — the first coat acts as a "mist coat" that seals the surface rather than building colour, which is why trade guidance recommends thinning it 10–20% with water and treating it as a separate step from the finish coats.
Standard emulsions have limited opacity in pale colours — going from a deep colour to white or a light shade commonly needs a third coat, or a stain-blocking/high-opacity primer as a base, to avoid the old colour showing through.
Yes — airless spray typically uses 20–30% more paint than a roller for the same wall, since overspray and film thickness both increase consumption. This tool assumes roller/brush application.
Always round up to the next full tin size, and consider one extra small tin for touch-ups later — colour can drift slightly between batches, so matching a discontinued or old batch months later isn't guaranteed.