Short answer
The formula
Dulux UK sets out the method in three steps, and it works for any brand:
- Measure. Multiply the height and width of each surface to get square metres. Subtract windows and doors the same way.
- Multiply by coats. Dulux says some products need one coat and more durable finishes need two, so multiply the area by the number of coats.
- Divide by coverage. Divide by the square metres per litre listed on your tin or in the product information.
So:
Litres = (wall area - doors and windows) × coats ÷ coverage (m² per litre)
The coverage figure is the one that changes most between products. Dulux's calculator notes that paint requirements may vary with the surface, type of paint, sheen and colour. That is why the tin, not a general rule, is the number to use.
How to measure a room
Walls are the bulk of the job, and a room's walls are easiest to measure as one strip:
- Perimeter: add up the length of every wall. A 4 m × 3.5 m room has a perimeter of 4 + 3.5 + 4 + 3.5 = 15 m.
- Height: floor to ceiling, for example 2.4 m.
- Wall area: perimeter × height = 15 × 2.4 = 36 m².
Then measure each door and window and take its area off. Measure yours rather than guessing, because sizes vary.
Do the ceiling separately, because it often takes a different paint. For the same room the ceiling is 4 × 3.5 = 14 m². Benjamin Moore's calculator, for example, says its results assume two coats and do not include the ceiling.
Works out material quantities and measurements for building jobs, metric first with an imperial mode.
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Construction Calc's Paint estimator takes a surface width and height, the number of coats and the coverage per litre (or per gallon in imperial mode), and gives you litres or gallons. It does not subtract doors and windows for you, so enter the area after you have taken them off, or treat each wall as its own surface. Its default coverage is 10 m² per litre per coat, a deliberately cautious figure. Replace it with the number on your tin. You can save the result to a named job alongside the rest of the room.
Worked example
A bedroom 4 m × 3.5 m with walls 2.4 m high, one door measuring 0.8 × 2.0 m and one window measuring 1.2 × 1.2 m. Two coats.
| Step | Working | Result |
|---|---|---|
| Wall area | 15 × 2.4 | 36.0 m² |
| Door | 0.8 × 2.0 | 1.6 m² |
| Window | 1.2 × 1.2 | 1.44 m² |
| Area to paint | 36 - 1.6 - 1.44 | 32.96 m² |
| Two coats | 32.96 × 2 | 65.92 m² |
Now divide by the coverage on the tin:
| Coverage on the tin | Litres needed |
|---|---|
| 10 m² per litre | 6.6 L |
| 12 m² per litre | 5.5 L |
| 14 m² per litre | 4.7 L |
The coverage changes the answer by almost 2 litres, which can be the difference between one tin size and the next. For comparison, Dulux's in-store rule of thumb is about 5 litres for all the walls in a 4 m × 4 m room, which sits within this range.
Rough guides for when you are in the shop
If you do not have measurements with you, Dulux UK offers these quick estimates:
- A 750 ml tin for a door or the skirting in a small room.
- A 1 litre tin for a 2.5 m interior wall.
- Around 2.5 litres for all the walls in a 2 m × 2 m room.
- About 5 litres for all the walls in a 4 m × 4 m room.
Treat these as a sense check, not a shopping list. Measuring and using your tin's coverage is more reliable, and Benjamin Moore describes its own calculator results as a rough estimate too.
Things that change how much you need
- Number of coats. This doubles or halves the answer, so decide it first. Dulux's guide links the number of coats to the product you choose.
- The surface. Dulux's calculator says requirements vary with the surface. If yours is rough or very absorbent, allow some extra on top of the tin's figure.
- Colour change. The same calculator lists colour as a factor. A big change in colour may need an extra coat.
- Sheen and paint type. Also listed by Dulux. Check the coverage figure on the specific product, not a different one from the same brand.
- Woodwork. Doors, frames and skirting are usually a separate paint. Measure them separately.
US units
In the US, coverage is printed as square feet per gallon. The formula is the same:
Gallons = (wall area in sq ft - openings) × coats ÷ coverage (sq ft per gallon)
Construction Calc's imperial mode works in feet and gallons, with a default coverage of 350 sq ft per gallon per coat that you should replace with the figure on your can.
Buying tips
- Round up to whole tins and keep the leftover for touch-ups.
- Buy all the paint for one colour at once if you can, so the whole wall comes from the same purchase.
- Measure the room you have, not a standard size. Alcoves, chimney breasts and sloping ceilings all change the area.
For other materials on the same job, see how many bricks per m2 or, for a garden project, how much concrete for a fence post.
