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.
Rounding each room up separately can genuinely overbuy — three rooms each needing a bit over one tin's worth of the same paint might round to three tins bought individually, when the combined total only needs two. The shopping list totals litres by paint type across every room first, then rounds once, which is exactly how you'd actually shop for a multi-room job.