How Much Paint Do You Need for a Room?

Paint for walls is a function of perimeter and ceiling height, not the floor rectangle. Two rooms with the same floor area can need very different paint if one has taller ceilings.

Perimeter is 2 × (length + width). Multiply by height for gross wall area, then subtract a standard allowance for each door and window. The Paint Calculator uses 21 ft² (about 2 m²) per door and 15 ft² (about 1.4 m²) per window.

Coverage on a can assumes a smooth, primed surface and one coat at the recommended spread rate. Two coats is the usual job for a colour change. Dark-to-light changes and new plaster or drywall often need more.

A common planning rate is about 350 ft² per gallon (around 12 m² per litre) per coat. Textured walls, unprimed board, and cheap paint all reduce that number. Rounding up to the next tin is cheaper than a second trip when the shop has a different batch.

Ceiling paint is a different product and a different calculation — use the floor area, not the wall formula. Trim and doors are usually a separate enamel or acrylic and should not be folded into the wall estimate.

Sample pots are for deciding colour on the actual wall, in the actual light. They are not a substitute for calculating the full room once you have chosen a colour.

A 12 ft × 10 ft room with 8 ft ceilings, one door, and two windows, two coats: gross walls 352 ft², deductions 51 ft², net 301 ft², times two coats is 602 ft². At 350 ft² per gallon that is about 1.7 gallons — round up to 2 gallons.

Run the same dimensions in the Paint Calculator to see the litre equivalent if you are buying in the UK.