Toolrack

Paint Calculator

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a standard door is ~1.6 m², a window ~2 m²
from the tin — usually 8–12
CANS TO BUY—
LITRES NEEDED—
AREA PAINTED—
LEFT OVER—

Everything is computed in this tab — nothing is sent anywhere.

Work out how much paint a room needs and how many cans to actually buy. Openings are subtracted before anything else, coats and coverage come from the tin, and rounding happens only where it happens in real life — at the till.

How the estimate is built

The wall area is the room's perimeter times the ceiling height — for a 4 × 5 m room at 2.7 m, that is 18 × 2.7 = 48.6 m². Openings come off first, the remainder is multiplied by the number of coats, and the result is divided by the coverage printed on the tin. Only then do litres become cans, rounded up once.

That single late rounding is the whole discipline. Round per wall or per coat and the errors stack, each one buying a little paint nobody needs; round once at the end and the leftover line tells you honestly what will still be in the can when the room is done — useful for touch-ups, and for deciding whether the next size down would do.

Frequently asked questions

Why subtract windows and doors first?

Because the paint never touches them, and the order changes the answer. Subtracting 3.6 m² of openings before multiplying by two coats removes 7.2 m² of painted work; subtracting after the multiplication would only remove 3.6 and quietly buy you extra paint.

How many coats should I plan?

Two is the trade default whenever the colour changes. One coat covers only when repainting the same colour on sound, primed walls; going light over dark can take three. Bare plaster drinks the first coat — count it as a primer, not a coat.

Where does the coverage number come from?

The tin. Manufacturers print square metres per litre for one coat on a smooth, primed surface — typically 8 to 12. Textured or unprimed walls cover less; if in doubt, use the low end of the printed range.

Why is the can count what matters?

Because shops sell cans, not litres. A 9-litre job in 2.5-litre cans is four cans and a litre left over. Budgeting from litres leaves you short at the till — or short a can on a Sunday with the walls half done.

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