Paint Calculator – How Much Paint Do I Need?
Walls, ceiling and woodwork worked out separately, with doors and windows deducted and the cheapest mix of 2.5L and 5L tins.
Paint Calculator
What sort of room?
A typical UK double bedroom: 4m × 3m with a 2.4m ceiling, one door and one window.
You need
7.4litres
30.2 m² of wall and 12 m² of ceiling, 2 coats
- Wall paint
- 4.6 L
- Ceiling paint
- 1.8 L
- Woodwork paint
- 0.9 L
- Deducted for openings
- 3.4 m²
How we worked this out
Your shopping list
1 × 5L tins of wall emulsion
£28.00
1 × 5L tins of ceiling paint
Ceiling paint is usually sold in 5L — a part tin keeps for touch-ups.
£22.00
1 × 2.5L tins of woodwork paint (satin or gloss)
Includes skirting, architrave and both sides of each door.
£20.00
1 × tubs of ready-mixed filler
£6.00
How far does a tin of paint go? (two coats at 13m²/L)
| Tin size | Total coverage | Covers a room of roughly |
|---|---|---|
| 2.5 L | 16.3 m² | Small cloakroom or a feature wall |
| 5 L | 32.5 m² | A 4m × 3m bedroom |
| 7.5 L (5 + 2.5) | 48.8 m² | A 5m × 4m living room |
| 10 L (2 × 5) | 65 m² | A large through lounge |
| 15 L (3 × 5) | 97.5 m² | Hall, stairs and landing |
How this calculator works
Wall area is the perimeter of the room multiplied by the wall height: 2 × (length + width) × height. From that we subtract 1.9m² for each door and 1.5m² for each window — both editable, because a patio door or a bay window is a lot bigger than a standard opening.
We then multiply the net wall area by the number of coats and divide by the coverage rate, which defaults to 13m² per litre. Tick "new plaster" and we add a whole extra coat — the mist coat — because bare plaster absorbs far more than a painted wall.
Ceilings are simply length × width. Woodwork is worked out from the skirting run around the room, less the door openings, plus an allowance for both sides of each door, at the higher 16m² per litre coverage that satin and gloss achieve.
Finally we convert litres into tins, favouring 5L over 2.5L because the larger tin is nearly always better value per litre.
Worked example: a 4m × 3m bedroom
Perimeter is 2 × (4 + 3) = 14m. At a 2.4m ceiling height that's 33.6m² of wall. Deduct one door (1.9m²) and one window (1.5m²) and you're left with 30.2m².
Two coats at 13m² per litre: 30.2 × 2 ÷ 13 = 4.6 litres. One 5L tin does it, with a little left for touch-ups. The 12m² ceiling needs 12 × 2 ÷ 13 = 1.85 litres, so a 2.5L tin of ceiling white. Woodwork — about 13m of skirting plus one door — comes to roughly 1.2 litres, which is a single 2.5L tin of satin.
Total: around £65–£75 of paint for the room, plus filler, a roller set, brushes, dust sheets and masking tape.
Tips from the trade counter
- Buy all your paint in one go. Tinted colours vary slightly between batches. Opening a second tin halfway up a wall can show.
- Box your paint. If you do need two tins of the same colour, pour them into one bucket and stir so the whole room is identical.
- Dark to light needs three coats. Or use a tinted undercoat — cheaper and faster than a third coat of expensive colour.
- Don't skimp on the mist coat. Thinned trade matt is cheap; stripping peeling paint off new plaster is not.
- Keep a labelled part tin. Write the room and date on the lid. You'll want it within a year.
Rooms that need a bit more thought
Bathrooms and kitchens need a moisture-resistant emulsion, and bare plaster in those rooms still needs its mist coat first. Rented properties and communal areas in flats may have rules on colour changes — check your tenancy or lease.
If your house was built before 1980 and you're sanding old woodwork, assume the paint may contain lead. Wet-sand it, wear a mask, and don't dry-sand or burn it off.
Frequently asked questions
How much paint do I need for a room?
A typical 4m × 3m room with a 2.4m ceiling has about 31m² of wall after deducting a door and a window. At 13m² per litre and two coats, that's roughly 4.8 litres — one 5L tin.
How far does 1 litre of emulsion go?
Around 12–14m² per coat on a smooth, sealed surface. Matt goes further than silk, and textured or porous surfaces can drop coverage by a third.
How many coats of paint do I need?
Two as standard. One coat only works when you're repainting the same colour on a sound surface. Dramatic colour changes and strong reds or yellows often need three.
What is a mist coat and do I need one?
A mist coat is matt emulsion thinned with about 20% water, used to seal fresh plaster. Without it the plaster drinks the topcoat and the finish peels. Tick the new plaster box and we'll add it.
Is a 5L tin cheaper than two 2.5L tins?
Almost always, yes — usually 15–25% less per litre. Our tin optimiser picks the cheapest sensible combination for the litres you need.
How much paint for a ceiling?
The ceiling area is simply room length × width. A 4m × 3m ceiling is 12m², which needs about 1.85 litres for two coats at 13m² per litre.
How much paint for skirting and doors?
Woodwork paint covers about 16m² per litre. Allow roughly 0.1 litres per metre of skirting for two coats, and about 0.4 litres for both sides of a standard door.
Should I subtract windows and doors?
Yes, for anything bigger than about 1m². We deduct 1.9m² per door and 1.5m² per window by default, and both figures are editable.