Builder AI Tools › Materials calculators
Paint calculator
Enter the room size. The calculator takes off doors and windows, works out walls and ceiling separately, and suggests which tins to buy.
- Paint needed
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- Wall area
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- Walls
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- Ceiling area
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- Ceiling
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- Suggested tins
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Rounded up to whole 2.5 L steps. Buy walls and ceiling colours separately if they differ.
How much paint does a room need?
Wall area is the perimeter of the room times the height: 2 × (length + width) × height. Take off about 1.9 m² for each door and 1.4 m² for each window. Multiply by the number of coats and divide by the coverage on the tin.
For a 4 × 3.5 m bedroom with 2.4 m ceilings, one door and one window, the walls are 2 × 7.5 × 2.4 − 1.9 − 1.4 = 32.7 m². Two coats at 12 m² per litre is 5.5 litres, so a 5 L and a 2.5 L tin. The ceiling adds 14 m², or 2.3 litres for two coats.
Coverage: what the tin says and what you get
Manufacturers quote coverage on smooth, sealed, primed walls. Real coverage is lower on:
- New plaster, which soaks up paint. Apply a mist coat of watered-down matt emulsion first (the calculator can add it as an extra coat).
- Textured walls, Artex and rough render: knock 20–30% off the coverage figure.
- Big colour changes, especially dark to light, which may need a third coat.
Trade emulsions often cover more per litre than budget DIY paint and go on in fewer coats, which matters when labour is the bigger cost.
Woodwork
Skirting, architraves and doors use gloss, satinwood or eggshell, which covers roughly 14–16 m² per litre. A typical room needs well under 1 litre per coat for skirting and one door, so a 750 ml tin usually does two coats.
Tips for decorators
- Buy all the tins of one colour together and box them (mix them in one bucket) to avoid shade differences.
- Keep a little back for touch-ups after snagging.
- Price the job, not just the paint: use the labour calculator and the profit calculator.
Questions
How much does 5 litres of paint cover?
At a typical 12 m² per litre, 5 litres covers about 60 m² for one coat, or 30 m² for two coats, which is the walls of a small to medium bedroom.
Do I need a mist coat on new plaster?
Yes. Bare plaster is very absorbent. A first coat of matt emulsion thinned with water (often around 10–20%, check the tin) seals it so the top coats go on evenly.
Is two coats always enough?
Usually, on a similar colour. Going from dark to light, or onto patchy filler, often needs a third coat.
Does the calculator include doors and skirting?
No, only walls and ceilings in emulsion. Woodwork uses a different paint; most rooms need under 1 litre.
Results are estimates for planning and ordering. Check quantities with your supplier and product data sheets, and anything structural or regulated with a qualified professional. See our terms.