How to Calculate Paint for a Room
There is nothing worse than running out of paint when you are three-quarters of the way through a wall. It forces you to stop, drives up the cost, and can even result in subtle color differences if you buy a different batch. On the flip side, buying too many tins is a waste of money.
Our Room Paint Calculator takes the guesswork out of decorating. It calculates the total wall surface area, subtracts the space taken up by standard doors and windows, and tells you exactly how many litres (and standard 2.5L tins) to buy.
The Wall Area Formula
To calculate the paintable area of a room manually, you first need to find the total perimeter of the room (Length + Length + Width + Width). You then multiply that perimeter by the room's height.
Total Surface Area = Perimeter × Room Height
Next, you must subtract the areas you won't be painting. As a general rule of thumb, subtract 2m² for every standard door and 1.5m² for every standard single window. The resulting number is your true paintable area.
Understanding Paint Coverage
Not all paints cover walls equally. You must check the back of the tin for its specific coverage rate:
- Standard Matt Emulsion: Usually covers 12 m² to 14 m² per litre.
- Premium or Trade Emulsion: Often covers up to 14 m² to 16 m² per litre due to higher pigment levels.
- Thick or Textured Paint: Often drops down to 8 m² to 10 m² per litre.
Why Two Coats Minimum?
Always calculate enough paint for a minimum of two coats. A single coat of emulsion will rarely provide a solid, even finish, especially if you are painting over a darker color or uneven plaster. Don't try to stretch one tin over a whole room; the finish will appear patchy when the light hits it.