How much paint do I need? Working it out properly
6 min read · Updated 2026-10-12 · BuildCalc
Paint is cheap next to labour, but running out three-quarters of the way through a wall is a nuisance, and a second trip rarely produces an exact colour match if the tin came from a different batch. The good news is that the quantity is easy to work out precisely, provided you allow for the two things most people skip: the number of coats and the coverage claim on the tin.
Start with the paintable area, not the floor area
Paint goes on walls, so measure walls. Add the four wall lengths together, multiply by the ceiling height, and you have the gross wall area. A room twelve feet by fourteen with eight-foot ceilings has roughly four hundred and sixteen square feet of wall. If the ceiling is being painted too, add length times width for that.
Now deduct the openings. A standard interior door is about twenty square feet and a typical window between ten and twenty, so a room with three openings loses forty to sixty square feet of wall you do not have to cover. Leave a small allowance for framing and reveals, but do make the deduction — over a large room it can save a whole tin.
Coverage and coats decide the tins
Every tin states its coverage, usually between three hundred and fifty and four hundred square feet per gallon for interior emulsion on a smooth, sealed wall. That figure falls sharply on bare plaster, fresh drywall or rough masonry, which absorb the first coat. If you are painting a new surface, assume the lower end of the range, or plan a dedicated primer.
Multiply the paintable area by the number of coats before dividing by coverage. One coat is enough only when you are refreshing the same colour; a colour change normally needs two, and going from a dark colour to a light one sometimes needs three. Buying for one coat when you need two is the most common mistake in the whole job.
Trims, ceilings and the practical extras
Skirting, door frames and window reveals are usually painted in a different, harder-wearing product, and they are measured in linear feet rather than square feet — which you already have, since it is the room's perimeter. A litre of trim paint typically covers more area than wall paint because the surface is smaller and smoother, but it takes more coats over bare wood.
Finally, round up. Paint is sold in whole containers, and a little left over is genuinely useful for touching up scuffs later. Store it sealed, labelled with the room and the date, and kept out of frost.
Run your own numbers
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Common questions
Does one gallon really cover 400 square feet?
On a smooth, previously painted wall, yes. On bare plaster, fresh drywall or textured surfaces the first coat can cover half that, so allow more paint or use a primer.
Should I deduct windows and doors?
Yes, using roughly twenty square feet per opening. It is a small saving in a single room but a real one across a whole house.
BuildCalc provides general information and planning estimates only, not professional construction, engineering or electrical advice. Material quantities are illustrative and depend on your own project, products and local codes. See our full disclaimer.