How much paint do I need?
Buying paint is a quick calculation, not a guess. Here is how to work out the wall area, allow for coats and coverage, and turn it into tins, with a worked example in both metric and US units.
Calculate the area of a room or space, plus perimeter and material cost.
Running out of paint halfway up the last wall, or carting three unopened tins back to the shop, both come from the same thing: buying by guesswork. Working out how much paint you need is a short, reliable calculation. You find the area to be painted, take off the bits you are not painting, divide by how far a litre or gallon goes, and multiply by the number of coats. Here is the method step by step, with a worked example in both metric and US units. For the area step, a free square footage calculator does the measuring maths for you.
Step 1: measure the wall area
Paint covers walls, so you want wall area, not floor area. For a room, the quickest way is to measure the distance all the way around the room (the perimeter) and multiply by the wall height.
So wall area = perimeter × height. For a room 4 m by 3 m with walls 2.4 m high, the perimeter is 4 + 3 + 4 + 3 = 14 m, and the wall area is 14 × 2.4 = 33.6 square metres. In US units, a 13 ft by 10 ft room with 8 ft walls has a perimeter of 46 ft and a wall area of 46 × 8 = 368 square feet. If the room is an odd shape, measure each wall separately and add them up, the same way you would for any area calculation.
Step 2: subtract doors and windows
You do not paint the glass or the door, so take their area off. A standard interior door is roughly 1.6 to 2 square metres (about 18 to 21 square feet), and an average window is around 1.5 square metres (about 16 square feet), but measure yours if you want to be exact.
In the metric example, say the room has one door (1.8 m²) and one window (1.5 m²). Subtract: 33.6 - 1.8 - 1.5 = 30.3 square metres to paint. Do not bother subtracting skirting boards or small gaps; the rounding at the end absorbs them. Only subtract the big openings.
Step 3: divide by the paint's coverage
Every tin states its coverage, how much area one litre or gallon covers in a single coat. It is usually printed as "spreading rate" on the tin. A typical emulsion covers about 10 to 12 square metres per litre; a US gallon covers about 350 to 400 square feet. Rough or very porous surfaces drink more, so use the lower number if your walls are bare plaster or heavily textured.
Dividing the area to paint by the coverage gives the paint for one coat. Metric example at 11 m² per litre: 30.3 ÷ 11 = 2.75 litres per coat. US example at 375 sq ft per gallon, with a door and window removed (368 - 21 - 15 = 332 sq ft): 332 ÷ 375 = 0.89 gallons per coat.
Step 4: multiply by the number of coats
Almost every job needs two coats for an even, durable finish, and you should assume two unless you are painting a very similar colour over itself. A big colour change (dark over light, or light over dark) can need a primer or a third coat.
So double it. The metric room needs 2.75 × 2 = 5.5 litres; the US room needs 0.89 × 2 = about 1.8 gallons. Now round up to whole tins, because you buy paint in fixed sizes. The metric job rounds to a 5 litre tin plus a 1 litre tin; two 2.5 litre tins come to only 5 litres, which would leave you short, so take the extra litre as well. The US job rounds up to 2 gallons. That leftover is not waste, it is your touch-up paint for scuffs later.
Do not forget the ceiling and the primer
If you are painting the ceiling too, that is a separate area: length × width (the floor area), so 4 × 3 = 12 m² for the example room, needing its own paint at the same coverage. Trim, skirting and doors use a different paint (usually gloss or satin) and much less of it, so buy a small separate tin.
Primer or undercoat is worth its own calculation on bare plaster, new wood, or a drastic colour change; it uses roughly the same coverage as topcoat for one coat. A good primer can save you a whole coat of expensive colour, so it often pays for itself.
A quick all-in example
Pulling the metric room together: walls 33.6 m², minus a door and window = 30.3 m², at 11 m²/litre = 2.75 litres per coat, two coats = 5.5 litres, round up to 6 litres (a 5 L plus a 1 L tin). Add the ceiling at 12 m² = 1.1 litres per coat, two coats = 2.2 litres of ceiling paint, round to a 2.5 L tin. That is the whole room decided in four lines of arithmetic instead of a hopeful trip to the shop.
Paint needed for common room sizes
As a quick reference, here is the wall paint for a room with 2.4 m high walls and two coats, before you subtract doors and windows (so round down slightly once you do). It assumes a coverage of 11 m² per litre.
| Room size | Wall area | Paint for 2 coats | Buy |
|---|---|---|---|
| 3 m × 3 m | 28.8 m² | 5.2 litres | 5 L + 1 L |
| 4 m × 3 m | 33.6 m² | 6.1 litres | 5 L + 2.5 L |
| 4 m × 4 m | 38.4 m² | 7.0 litres | 5 L + 2.5 L |
| 5 m × 4 m | 43.2 m² | 7.9 litres | 2 × 5 L |
These are walls only; a ceiling adds its floor area (length × width) at the same coverage. Textured or bare surfaces drink more, so treat these as a floor, not a ceiling, and round up.
Common mistakes to avoid
- Using floor area instead of wall area. Paint goes on walls; measure perimeter times height, not the floor.
- Forgetting the second coat. One coat almost never covers evenly. Budget for two.
- Rounding down. Always round up to whole tins; running out mid-wall can leave a visible lap mark, and batch-to-batch colour variation means a tin bought later may not match exactly, so buy all your tins at once and, for a big area, mix them together before you start.
- Ignoring the surface. Bare plaster, new wood and bold colour changes soak up more, so use the lower coverage figure and consider a primer.
Measure the walls, take off the doors and windows, divide by the coverage on the tin, double it for two coats, and round up. A free square footage calculator handles the area step if the room is an awkward shape, and the same measuring approach works for flooring, tiling and wallpaper too.