Paint Calculator
Enter the room dimensions and how many doors and windows. The calculator subtracts them and works out the paint for your chosen number of coats.
- Area to paint
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- Openings subtracted
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- Buy
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How much paint a room needs
Work out the wall area, subtract the doors and windows, then divide by the paint's coverage and multiply by the number of coats. Wall area is the room's perimeter times its height: a 4.5 × 3.6 m room with 2.4 m walls has 38.9 m² of wall. Subtract about 1.9 m² per door and 1.4 m² per window, and at 12 m² per litre two coats need roughly 5.7 litres. Most emulsion covers 10–14 m² per litre, so check the tin — dark colours and textured walls cover less.
How many coats you actually need
| Situation | Coats |
|---|---|
| Same colour, refreshing | 1 |
| Similar shade, previously painted | 2 |
| Light over dark | 2–3, plus primer |
| Strong colour, especially red or yellow | 3, over a tinted primer |
| New plaster | Mist coat (thinned paint) plus 2 |
| Covering stains or smoke | Stain-blocking primer plus 2 |
Reds and yellows genuinely need more coats than other colours, because the pigments used are less opaque. A grey-tinted primer underneath saves a coat and is cheaper than the topcoat.
Buying without over- or under-ordering
- Round up, and buy it all at once. Paint is mixed in batches and two tins of the same colour bought weeks apart can differ subtly. If you need 5.7 litres, buy 10 rather than topping up later.
- Add 10% for cutting in. Edges, corners and around fittings use more than flat area suggests.
- Textured or bare surfaces drink paint. Bare plaster, artex and rough render can cut coverage by a third.
- Keep the leftovers labelled with the room and date. Touch-ups are much easier than rematching.
- Ceilings need less than you think but are harder to keep even — a full roller and steady overlaps beat trying to stretch it.
How to use the Paint Calculator
- Enter the room’s length, width and wall height.
- Say how many doors and windows there are — standard sizes are subtracted automatically.
- Choose the number of coats. Two is standard.
- Read the amount needed and the suggested tin sizes.
Frequently asked questions
How much does a litre of paint cover?
Typically 10–14 m² for one coat on a smooth, previously painted wall. This calculator uses 12 m² per litre. Bare plaster, textured surfaces and dark colours all reduce it.
How many gallons for a 12x12 room?
A 12 × 12 ft room with 8 ft walls has 384 sq ft of wall. After a door and two windows, roughly 334 sq ft — about 0.9 gallons per coat, so 2 gallons covers two coats with some left over.
Do I subtract windows and doors?
Yes, and this calculator does it for you at about 1.9 m² per door and 1.4 m² per window. For unusually large windows or patio doors, measure them and reduce the room dimensions accordingly.
Do I need primer?
On bare plaster, wood, metal or over a dramatic colour change, yes. On a previously painted wall in a similar shade, no — two coats of emulsion is enough.
Why does the tin claim more coverage than this?
Manufacturer figures assume ideal conditions: smooth, sealed, previously painted surfaces and an even application. Real walls, cutting in and roller loss all reduce it, which is why this calculator uses a slightly conservative figure.
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Last reviewed September 4, 2026. Results are estimates for general information only.