How much paint do I need for one room?
Work out the net wall area — the wall area minus doors and windows — multiply by two coats, and divide by the coverage. Here is the sum spelled out on a real room.
Short answer: multiply the room's perimeter by the ceiling height, subtract doors and windows, and multiply by two (because you need two coats). Divide the result by the coverage stated on the tin — typically 6-8 sq m per litre per coat for ordinary wall paint — and add 5-10% for wastage. A perfectly ordinary room measuring 4.20 × 3.60 m with a 2.50 m ceiling comes out at 34 sq m of net wall area and needs about 9-12 litres for two coats. The instructions on the tin always take precedence over any rule of thumb.
The sum in four steps
Paint is bought by wall area, not floor area. A narrow room has more wall area per square metre of floor than a square room of the same size, and a room with a high ceiling has more than one with a low ceiling. So there are only four figures you need: the perimeter, the height, the openings, and the coverage.
- Perimeter × height = gross wall area.
- Minus doors and windows = net wall area. That is the surface you'll actually be painting.
- Times the number of coats — almost always two.
- Divided by the coverage, plus 5-10% wastage, gives the number of litres to buy.
The example: a room measuring 4.20 × 3.60 metres
Take a perfectly ordinary bedroom. Internal measurements 4.20 m and 3.60 m, 2.50 m from floor to ceiling. There is one door and two windows.
Step 1 — the perimeter. (4.20 + 3.60) × 2 = 15.60 metres of wall all the way round. 15.60 × 2.50 = 39.0 sq m of gross wall area.
Step 2 — the openings. The door measures 0.90 × 2.05 m = 1.85 sq m. The two windows measure 1.20 × 1.30 m = 1.56 sq m each, so 3.12 sq m. Total 4.97 sq m, rounded to 5.0 sq m. The net wall area becomes 39.0 − 5.0 = 34.0 sq m.
Step 3 — two coats. 34.0 × 2 = 68 sq m of painted surface. That is the figure your paint needs to cover — not the 34.
Step 4 — litres. If the tin says 8 sq m per litre: 68 ÷ 8 = 8.5 litres. If it says 6: 68 ÷ 6 = 11.3 litres. Add 5-10% for wastage and you land on 9-12.5 litres. In practice you'd buy a single 10-litre tin if the walls are smooth and prepared — and 10 + 2.5 litres if they're rough, wallpapered, or changing colour dramatically.
Notice how little the door and windows actually moved the total: barely 13% of the area. That's often under one litre of paint. If you only have one door and one small window, you can safely skip the deduction and let it be your safety margin instead.
When do you subtract doors and windows — and when not?
The rule of thumb among painters: only subtract openings larger than about 0.5 sq m. A door and a normal window count. A power socket, a small basement window or a radiator recess doesn't — you paint around them anyway, and the edges use more paint than the area saves.
If the frame and door leaf are also going to be painted, that goes on top, not subtracted. The frame is a narrow, double-sided surface that drinks more paint per square metre than a flat wall, and it's typically painted with a different type of paint from the wall.
Coverage: what the number on the tin means
Coverage is printed on every tin as "spreading rate" or "sq m per litre" and applies per coat. Rough figures, as they're typically stated:
- Wall paint, sheen level 5-10: 6-8 sq m per litre per coat.
- Ceiling paint, matte: 7-10 sq m per litre per coat — the ceiling is smooth and typically absorbs less.
- Primer: 8-12 sq m per litre.
- Wood paint for doors, frames and mouldings: 8-12 sq m per litre, but narrow pieces use disproportionately more.
Those figures are the manufacturers' own, measured under good conditions: a smooth, primed, single-colour surface rolled by a practised hand. Your wall is rarely that surface. The instructions on the tin always take precedence over an article — this one included. Read it before you calculate.
Why two coats, and when three?
Two coats is standard because one coat almost never covers evenly. Even when the colour looks solid wet, it dries with patches where the roller has spread thinner. Two coats also give the even sheen that's what the eye actually reads as "properly painted".
Allow for three coats in three cases: you're going from a dark colour to a light one, you're painting a strong colour (especially red, orange and yellow, where the pigment covers poorly), or the wall is freshly filled and absorbs unevenly. Three coats on the 4.20 × 3.60 m room means 34 × 3 = 102 sq m and so around 13-18 litres.
New plaster, freshly filled patches and never-painted surfaces also need priming first. The primer counts as its own coat, but typically uses less paint per square metre than the topcoat.
A quick rule of thumb — and its limits
If you just need a figure in your head, for a roughly square room with a 2.50 m ceiling: net wall area ≈ floor area × 2 to 2.5. The 4.20 × 3.60 m room has 15.1 sq m of floor, and 15.1 × 2.25 ≈ 34 sq m — which matches the 34 sq m of net wall area the full sum gave.
The rule doesn't hold in a long, narrow hallway, in a room with a high ceiling, or in a room with many angles — there the factor can easily become 3 or 4. If the room has a sloped ceiling, it doesn't hold at all. If the figure needs to be used for more than a rough estimate, measure the perimeter.
Where HouseSense comes in
The perimeter is the figure most often measured wrong — people forget a recess, count a chimney breast as a straight wall, or measure from the outside instead of the inside. HouseSense works out the wall area from the floor plan itself: you scan the room with the iPhone's LiDAR or draw it on graph paper, and the takeoff shows the wall area per room with doors and windows subtracted, following the same rules as the sum for the 4.20 × 3.60 m room. Press and hold on a single wall and the elevation folds open — the wall seen from the front with its openings and measurements — so you can see exactly what surface the figure covers.
The takeoff can be exported as a PDF if a painter needs to quote. But the sum is yours: net wall area × number of coats ÷ coverage + wastage. It works with the app, with a tape measure, and with a napkin.