Each coat removes the same fraction of what is still showing, not the same amount. A paint that hides 80% of the substrate per coat leaves 20% after one coat, 4% after two and 0.8% after three — so the residue falls off geometrically and there is always, strictly, some of the old colour left. What matters is whether the remainder is visible, and that is a perceptual question with a standard answer: a colour difference below about ΔE 1 is invisible, and below ΔE 2 is detectable only with both colours side by side under good light.
Which is why the starting colour matters more than the paint does. Magnolia over white starts a short distance away, so the same paint finishes it in two coats. White over navy starts about as far apart as two colours can be, and now wants three — not because it covers worse, but because 4% of navy still showing is plainly visible while 4% of magnolia is not. Strong reds and deep greys are worse again: they are the colours with the least pigment opacity and the longest distance to travel, which is why a red feature wall famously eats paint. A tinted primer closes most of that distance in one cheaper coat, and that is what primer is actually for.
The default room — 4 × 5 m, 2.7 m high, one door, one window — is 45.3 m² of wall. At two coats it wants 11.0 L of new-plaster emulsion: five 2.5 L tins, about £100.
Now paint it white over a navy wall. One coat leaves a colour difference of ΔE 7.8 — obviously unfinished. The second gets to ΔE 1.4, which is the state every decorator knows: looks fine head-on, and shows along the wall in raking light. Only the third reaches ΔE 0.35, which nobody can see.
Three coats is 15.9 L, seven tins, £140. The two-coat guess was not £40 cheaper — it was a room that has to be painted again. And drop the hiding power to a contract matt at 65% and the same job needs a fourth coat, which is the whole argument against buying on price per litre: a paint that needs an extra coat costs you a coat of labour as well as a third more material.
A theoretical rate measured on a smooth, previously painted, non-porous surface. The tool starts from the stated rate for the paint type you pick.
New or porous surfaces absorb paint before any film builds. The first coat is treated at a porous rate; later coats at the tin’s own.
Each coat multiplies the volume. Pale colours over deep ones commonly need a third coat, or a stain-blocking primer.
Cutting in, loading a roller and covering sockets all consume paint that never reaches the wall area. 10% is a floor.
Manufacturer figures assume a smooth, previously painted, non-porous surface. Real coverage is typically 10–20% lower once porosity, texture and technique are counted: exactly what the porosity factor corrects for.
A new or porous surface absorbs paint into itself before any film builds on top. The first coat effectively primes the surface, and only the second coat contributes fully to colour and coverage. On bare plaster it is a “mist coat”: thinned 10–20% with water and treated as a separate step.
Matt emulsion typically covers slightly more area per litre than silk, since the flattening agents reduce how far it penetrates; gloss and satinwood, being thinner, often cover more still. Airless spray uses 20–30% more paint than a roller for the same wall; this tool assumes roller or brush.
Cutting in around trim, loading a roller, and covering sockets and pipework all consume paint that never reaches the official wall area. Trade guidance treats 10% as a floor, rising to 15% for rooms with many obstructions.
Rounding each room up separately can overbuy: three rooms each needing about 1.2 L of the same paint is three tins bought one by one but two when totalled first. The shopping list totals litres by paint type across every room, then rounds once. Always round up to the next tin, and keep a small one for touch-ups: colour can drift between batches.
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.
This page has changed in 191 archived releases, but each of those was a site-wide pass, so none is attributable to this tool on its own and none is listed here. That is not a claim that the tool never changed — a release that reworked many pages at once may well have altered this one too. The changelog has them.