Buying too little means a second trip and a risk of a slightly different batch. Buying far too much means a shelf of half-used tins. The calculation itself takes a minute, and there is a calculator on every product page that will do it for you — but it is worth knowing what it is doing.
The sum
Measure the perimeter of the room and multiply by the wall height. That is your gross wall area. Subtract roughly 1.8 m² for each door and 1.5 m² for each average window.
Divide by the coverage figure on the tin — Zigma emulsions run between 10 and 13 m² per litre per coat — then multiply by the number of coats. Two is standard over a primed wall.
What eats the extra
Coverage figures are measured on a smooth, sealed, primed surface. Real walls are rarely that.
Bare plaster, new blockwork and rough render can take 15 to 25 per cent more on the first coat because the surface drinks it. Textured surfaces have far more actual area than their measured area suggests. And a strong colour going over a much lighter one, or vice versa, often needs a third coat regardless of what the arithmetic says.
Buy the big tin
Paint is cheaper per litre in larger packs, and having a litre left over for touching up is worth more than the small saving from cutting it fine.
It also removes the batch problem: tinted paint is mixed per order, and while the plant matches closely, the safest guarantee of an identical shade across a whole room is that it came out of one container.
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