Paint Calculator
Find how much paint to buy for a room: enter its size and the number of coats. Advanced adds more rooms, ceilings, doors and windows, primer, woodwork, tin prices and cost.
Takes away one door and one window. Advanced lets you set how many, their sizes and the ceiling.
Two coats is normal. Use 3 for a big colour change.
Typical wall paint: about 10 m² per litre (about 400 sq ft per US gallon). Check your tin.
Rough or thirsty surfaces soak up more paint.
5–10% is usual. Keep some for touch-ups later.
Use primer on new plaster, bare wood or a big colour change. Typical: 8 m²/L (325 sq ft/gal).
Leave a size blank if you won't buy it. We pick the cheapest mix of the sizes you price. The same prices are used for every paint.
How your paint is worked out
Area of the walls, less doors and windows, × coats ÷ what one litre or gallon covers, then rounded up to tins.
- 1
Wall area
2 × (12 + 10)× 8= 352 sq ft
- 2
Take away doors, windows
352 − 1 × 21− 1 × 15= 316 sq ft
- 3
Coats ÷ coverage
316 × 2 ÷ 400= 1.58 gal× 1.1 = 1.74 gal
- 4
Round up to tins
1.74 gal needed= 2 × 1 gallon
Read more: the formula, worked examples in feet and metres, typical coverage and tips
Paint is sold by volume, but you use it by area. So the job is to find the area you will paint, take away the doors and windows, multiply by the number of coats and divide by how much area one litre or gallon covers. Then round up to the tins you can actually buy.
The formula
wall area = 2 × (length + width) × height (for a whole room) or width × height (one wall)
area to paint = wall area − doors × door size − windows × window size
ceiling = length × width
paint = area to paint × coats × surface factor ÷ coverage × (1 + extra % ÷ 100)
primer = (walls + ceiling) × 1 coat × surface factor ÷ primer coverage × (1 + extra %)
skirting = (room perimeter − door widths) × skirting height
door frames = doors × 2 sides × (2 × door height + door width) × frame width
tins = the cheapest mix of tin sizes that holds at least the paint needed
Worked example
In feet and US gallons. A room 12 ft long, 10 ft wide and 8 ft high has walls of 2 × (12 + 10) × 8 = 352 sq ft. Take away one 3 × 7 ft door (21 sq ft) and one 3 × 5 ft window (15 sq ft): 352 − 21 − 15 = 316 sq ft. Two coats at 400 sq ft per gallon is 316 × 2 ÷ 400 = 1.58 gallons. Adding 10% extra gives 1.58 × 1.1 = 1.738 gallons (6.58 litres). That is more than 1 gallon, so buy 2 one-gallon cans and keep the 0.26 gallon left over for touch-ups. One gallon and three quarts would also do, but quarts cost much more per litre.
In metres and litres. A room 4 m × 3 m × 2.5 m high has walls of 2 × (4 + 3) × 2.5 = 35 m². Take away a 0.8 × 2 m door (1.6 m²) and a 1.2 × 1.2 m window (1.44 m²): 35 − 1.6 − 1.44 = 31.96 m². Two coats at 10 m² per litre is 31.96 × 2 ÷ 10 = 6.392 litres, and with 10% extra 6.392 × 1.1 = 7.03 litres. The best tins are one 5 L and one 2.5 L (7.5 L), leaving about 0.47 L.
Typical coverage and sizes
| Thing | Typical figure |
|---|---|
| Wall paint (emulsion, latex), one coat | 10–12 m² per litre · 350–400 sq ft per US gallon |
| Primer or undercoat | about 8 m² per litre · 300–350 sq ft per gallon |
| Interior door | about 0.8 × 2.0 m · 3 × 7 ft (21 sq ft) |
| Window | about 1.2 × 1.2 m · 3 × 5 ft (15 sq ft) |
| Tins sold in litres | 750 ml, 1 L, 2.5 L, 5 L, 10 L |
| US cans | 1 quart (0.95 L), 1 gallon (3.79 L), 5 gallons (18.9 L) |
These are typical rule-of-thumb figures, checked 10 October 2026. Every paint is different, so use the coverage printed on your tin. The surface factors (+10% for light texture, +20% for new plaster or bare drywall, +30% for heavy texture, +50% for brick or block) are rough guides. Without your prices, the tin mix uses typical price steps where smaller tins cost more per litre.
Tips
- Buy all the paint for one room at the same time, so the colour matches. If you do need two batches, mix them together in a bucket first.
- Going from dark to light usually needs a primer plus two coats, or three coats.
- Ceilings and woodwork often use a different paint from the walls, so they are listed separately.
Common questions
How much paint do I need for a room?
Work out the wall area, take away doors and windows, multiply by the number of coats and divide by the coverage on the tin. A 12 × 10 ft room with 8 ft walls needs about 1.74 US gallons for two coats with 10% extra, so 2 one-gallon cans.
How much area does a litre of paint cover?
Typical wall paint covers about 10–12 m² per litre per coat, or 350–400 sq ft per US gallon. Primer covers less, about 8 m² per litre or 300–350 sq ft per gallon. Every paint is different, so use the coverage printed on your tin.
How many coats of paint do I need?
The worked examples use two coats, which is common for walls. Going from dark to light usually needs a primer plus two coats, or three coats. New plaster, bare drywall and textured walls soak up more paint, so pick the right surface type.
How do I calculate the wall area of a room?
Add the length and width, multiply by 2 and then by the height. A 4 × 3 m room 2.5 m high has walls of 2 × (4 + 3) × 2.5 = 35 m². Take away 1.6 m² for a door and 1.44 m² for a window to get 31.96 m².
How much does a door or window take off?
A typical interior door is about 0.8 × 2.0 m, or 3 × 7 ft (21 sq ft), and a typical window about 1.2 × 1.2 m, or 3 × 5 ft (15 sq ft). Enter your own sizes if yours are different, and the calculator takes them off the wall area.
Should I buy all the paint at once?
Yes, buy all the paint for one room at the same time so the colour matches. If you do need two batches, mix them together in a bucket first. Keep any leftover paint for touch-ups later. Ceilings and woodwork often use a different paint.
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