Buying the right amount of paint comes down to two numbers most people get wrong: the true wall area after subtracting doors and windows, and the number of coats. This guide walks through measuring, coverage, primer, ceilings and trim, and why you should always round up.
Paint is estimated from the area you are covering, so the first job is to measure the walls in square metres. For a room, add up the length of every wall to get the perimeter, then multiply by the ceiling height.
Wall area (m²) = Perimeter (m) × Height (m) − doors and windows
A 4m by 5m room has a perimeter of 18 metres. At a 2.4 metre ceiling height that is 18 × 2.4 = 43.2 square metres. Subtract roughly 5 square metres for a standard door and window, leaving about 38 square metres of actual wall to paint. Our square metre calculator handles the arithmetic.
Paint tins state a spread rate, commonly around 10 to 16 square metres per litre per coat, depending on the paint and surface. The critical multiplier that people forget is the number of coats.
Paint needed (L) = Wall area × Number of coats ÷ Coverage per litre
Our 38 square metre room, painted with two coats at 10 square metres per litre, needs 38 × 2 ÷ 10 = 7.6 litres — so a 10 litre tin, with a little spare for touch-ups. One coat is rarely enough over a different colour, so always plan for at least two.
Bare plasterboard, patched repairs, timber, and dramatic colour changes usually need a primer or undercoat first, which is a separate product and an extra "coat" to estimate. Fresh plaster in particular drinks paint and should be sealed first, or your topcoats will look uneven and you will use far more than expected. Factor the primer into both your quantity and your budget.
Do not forget the other surfaces. The ceiling is a separate area (length times width — 20 square metres for our room) usually painted in a dedicated ceiling paint. Trim, doors, architraves and skirting use a different enamel or trim paint in much smaller quantities but should be counted so you are not caught short. Each surface type is its own small calculation.
Real-world coverage is always a little worse than the tin's ideal figure. Textured or rough walls have more surface area than their flat dimensions suggest and soak up more paint. Some paint is lost in the tray, on the roller, and in cutting in. A sensible approach is to take the calculated figure and round up to the next tin size rather than buying exactly, so a mid-job shortfall does not send you back to the store mid-coat.
Calculate each surface — walls, ceiling, trim — separately, apply the right number of coats, and round up to whole tins. Buying all the wall paint in one batch also matters: like tiles, paint can vary slightly between batches, and a top-up tin mixed later may not match perfectly. Measure once, buy for two coats plus a margin, and you will rarely go wrong.
How much paint do I need for a room?
Calculate the wall area (perimeter times ceiling height, minus doors and windows), multiply by the number of coats, and divide by the paint's coverage per litre. A 38 square metre room with two coats at 10 square metres per litre needs about 7.6 litres.
How many coats of paint should I plan for?
At least two. One coat rarely covers evenly, especially over a different colour. Estimating for a single coat is the most common paint-buying mistake — dark-to-light changes and vivid colours can even need three coats.
What is paint coverage per litre?
Most paints state a spread rate of around 10 to 16 square metres per litre per coat, depending on the paint and surface. Rough or textured walls have more real surface area and cover at the lower end, so use a conservative figure.
Do I need primer or undercoat?
Bare plasterboard, patched repairs, timber and dramatic colour changes usually need a primer or undercoat first. Fresh plaster in particular should be sealed, or topcoats look uneven and you use far more paint than expected.
Should I include the ceiling and trim?
Yes, but calculate them separately. The ceiling is its own area (length times width) usually painted in ceiling paint, and trim uses a different enamel in smaller quantities. Each surface type is its own small calculation.