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How much paint do I need?
Length, width, ceiling height. Take off the doors and windows, decide on the coats, and it tells you the litres or gallons and which tins to buy so you are not back at the shop with a half-painted wall.
The tin says one coat covers 12 m² a litre. The wall disagrees about the coat, and usually about the 12.
Your room
Start from a typical room
The tin says something different about coverage
Most emulsions: 10–12 m² per litre. A US can's "400 sq ft per gallon" is about 9.8.
You need
5.9 L
Buy 1 × 5 L + 1 × 1 L.
- Walls, minus 2 openings
- 32.6 m² · 5.9 L
Rounded up to the half litre. Most people are short on the second coat and long on the first — this plans for 2 coats so the wall gets both.
How the number is worked out
Wall area is the perimeter times the height. Two lengths plus two widths, multiplied by the ceiling height. A 4 × 3.5 m room with a 2.4 m ceiling has 36 m² of wall before you take anything off.
Then the openings come off. A door is about 1.9 m², a window about 1.5 m². Skirting, coving and the odd radiator are left in — you cut in around them with the same paint, and the slack covers the brush you leave in the tray.
Coats multiply it; coverage divides it. Two coats of 36 m² at 11 m² per litre is 6.5 litres. Textured walls and bare plaster get 15 percent added, because they drink paint. A big colour change gets one primer coat on top of that.
Finally, tins. The number is rounded up to the half litre and turned into 5 L, 2.5 L and 1 L tins, because that is what the shelf holds. Buy the bigger tin when the split is close: leftover paint touches up scuffs for years, and a second trip does not.
Questions people ask before they buy paint
- How much paint do I need for a 12 × 12 room?
- With 8 ft ceilings, one door and one window: about 350 sq ft of wall after openings, which is roughly 0.9 gallons per coat. Two coats is 1.75 gallons, so buy two gallons. In metric, a 3.7 × 3.7 m room with 2.4 m ceilings needs about 32 m² of wall per coat, or 6.5 litres for two coats — a 5 L tin and a 2.5 L tin.
- Is one coat ever enough?
- On the same colour over sound, previously painted walls, sometimes. Over a colour change, over filler, or on new plaster, no — the first coat is the undercoat whatever the tin says. This calculator defaults to two because the second coat is the one people run out of.
- How much does a litre of paint cover?
- Most interior emulsions cover 10 to 12 square metres per litre per coat on a smooth, painted wall. That is the same figure as the 350 to 400 square feet per gallon printed on a US can. Textured walls and bare plaster take about 15 percent more, which the calculator adds if you tell it.
- Do I need primer?
- For a big change — dark to light, light to dark, red or yellow over anything, or new plaster — yes, one coat of primer or a mist coat first. It costs one extra coat of the wall area and saves a third coat of the expensive colour. The calculator adds it when you switch on 'big colour change'.
- Should I subtract doors and windows?
- Yes, and it matters most in small rooms. A standard door is about 1.9 m² (20 sq ft) and a typical window about 1.5 m² (16 sq ft). In a box bedroom that is a tenth of the wall area; in a large living room it is a rounding error. The calculator subtracts them either way.
Next
The quantity was the easy part
Whether the colour is right is the expensive question. Upload a photo and repaint your own walls in the browser, free and unlimited, before a single tin is opened.