Work out how much paint to buy for a room. Subtracts doors and windows, and lets you add the ceiling.
5.0
4.0
2.50
2
1
1
Fine tuning
8.3 L
Paint needed
4
Tins to buy
× 2.5 L
45.0 m²
Wall area
41.5 m²
Paintable area
Estimating paint without a second trip
Paint coverage is quoted per litre on smooth, previously painted wall. Almost nothing in a real room is that, which is why coverage figures on the tin are optimistic and why the second coat is not optional.
How it works
- Multiplies wall length by height to get gross area, then subtracts doors and windows.
- Divides by the coverage rate and multiplies by the number of coats.
- Adds a waste allowance for roller absorption, tray residue and the bit left in the tin.
net area = (perimeter × height) − doors − windows litres = net area ÷ coverage × coats × (1 + waste) typical coverage per litre smooth previously painted wall 10–12 m² fresh plaster or filler 6–8 m² (it drinks the first coat) textured or woodchip 5–7 m² standard door ≈ 1.8 m² standard window ≈ 1.5 m²
Worked example
A 4 × 3 m bedroom with 2.5 m ceilings, one door and one window, two coats on previously painted wall.
- perimeter = (4 + 3) × 2 = 14 m
- gross wall area = 14 × 2.5 = 35 m²
- subtract door 1.8 and window 1.5 → 31.7 m²
- at 11 m²/litre: 31.7 ÷ 11 = 2.88 litres per coat
- two coats = 5.76 litres, plus 10% waste = 6.34 litres
Buy 7 litres — a 5 L tin plus a 2.5 L, since paint is not sold by the litre. Rounding down here is how people end up matching a batch that has been discontinued.
Reading the result
- Buy all of it in one go, from the same batch. Colour varies measurably between production batches, and a wall finished from a second tin bought a month later can show the join in daylight.
- Fresh plaster absorbs far more than the tin suggests. Thin the first coat as a mist coat, or expect to use half again as much.
- A dark colour going over a light one — or vice versa — often needs three coats regardless of what the tin claims. Price for three if the change is dramatic.
- Ceilings are extra. This calculates walls only; add length × width for the ceiling and treat it as its own surface, usually with a different paint.
Common questions
- Should I subtract doors and windows?
- Yes, but do not be too precise about it. Subtracting them and then adding 10% waste lands close to right. Ignoring both roughly cancels out, which is why rough estimates often work — until the room has three windows.
- How much extra should I allow?
- 10% for a simple room, 15% if there are many corners, alcoves or a lot of cutting in. Roller sleeves and trays hold a surprising amount that never reaches the wall.