The method

How this calculator works

Measure the wall area

The calculator finds the room perimeter—two lengths plus two widths—and multiplies it by wall height. This works well for ordinary rectangular rooms. For an L-shaped room, calculate each rectangular section separately or measure each wall and combine the widths.

Standard allowances are subtracted for doors and windows. If you have a very large glazed wall, wide bifold doors or unusually small openings, use the result cautiously and make a manual area adjustment.

Apply coats and real product coverage

Paint coverage varies with the product, surface and application method. The label may promise 12–14 m² per litre, while a porous fresh-plaster wall can use more. Enter the manufacturer’s coverage for the finish you plan to use, then choose the likely number of full coats.

Two finish coats are a sensible default for a colour change. Deep colours, damaged walls and dramatic light-to-dark changes may need a third coat or an appropriate primer first.

Leave enough for touch-ups

A 10% allowance covers normal losses in trays, rollers and cutting-in, plus minor measurement error. Textured walls or an inexperienced painting team may justify 15%. Buying slightly more from the same batch can also avoid a subtle colour difference later.

Paint container sizes vary. Round the estimate up to a sensible combination of available tins rather than down. Unopened excess may be returnable, depending on the retailer and whether the colour was mixed to order.

Worked example

Example: a 4 m × 3 m room with 2.4 m walls

  1. The perimeter is 14 m, giving 33.6 m² of gross wall area.
  2. Subtract one door and two average windows: 33.6 − 4.9 = 28.7 m².
  3. For two coats at 10 m²/L, divide 57.4 m² by 10 to get 5.74 L, then add 10% waste: 6.31 L.
  4. Paint is rounded up to the next half litre, so the amount to buy is 6.5 L.

Practical answer: Buy 6.5 litres (6.3 L before rounding). In practice that is likely to mean a 5 L tin plus a 2.5 L tin, so check which sizes your paint comes in; the leftover is useful for touch-ups.

Before you buy

Common measuring mistakes

  • Using floor area as wall area instead of measuring the room perimeter and height.
  • Counting one coat when the colour or surface will realistically need two.
  • Using a generic coverage rate when the chosen product states a different figure.
  • Subtracting every small opening precisely, then running short because no waste allowance was included.