Home & Construction⏱ 4 min read
How to Calculate How Much Paint You Need for a Room
Buying too much paint wastes money; too little means a second trip mid-job. Here is the exact calculation for walls, ceiling, and woodwork, including the adjustments that actually matter.
Most paint calculators give you a single number and ignore the real variables: number of coats, surface porosity, and the waste from cutting in around edges. Here is a calculation that actually holds up.
Wall Area Calculation
Measure each wall and subtract doors/windows:
Wall area = (Length x Height) - (Door area) - (Window area)
Standard room (4m x 3.5m, 2.4m ceiling height):
Perimeter = 2 x (4 + 3.5) = 15m
Total wall area = 15 x 2.4 = 36 m2
Subtract 1 door (2.0 x 0.8m): -1.6 m2
Subtract 2 windows (1.2 x 1.0m each): -2.4 m2
Net wall area: 36 - 1.6 - 2.4 = 32 m2
Coverage Rates and Coat Allowances
Typical paint coverage (manufacturer stated):
Matt emulsion (smooth surface): 12-15 m2/litre
Silk emulsion: 10-13 m2/litre
Eggshell (woodwork): 12-14 m2/litre
Gloss: 12-15 m2/litre
Masonry paint: 5-8 m2/litre (rough render)
Real-world adjustment:
Manufacturer rates are measured on smooth, primed surfaces.
Apply these factors to manufacturer rate:
New plaster / unprimed: x 0.60 (very thirsty surface)
Previously painted (same colour): x 0.90
Cutting in around edges: x 0.85 (brush coverage is less)
Overall practical rate: multiply stated rate by 0.75-0.85
Full Calculation Example
Room: 32 m2 net wall area, 2 coats needed
Paint: 12 m2/litre (stated), practical rate x 0.80 = 9.6 m2/litre
Litres needed per coat: 32 / 9.6 = 3.33 litres
For 2 coats: 3.33 x 2 = 6.67 litres
Add 10% buffer: 6.67 x 1.10 = 7.33 litres
Paint sold in: 2.5L and 5L tins
Order: 1 x 5L + 1 x 2.5L = 7.5L total (rounds up cleanly)
Ceiling (4m x 3.5m = 14 m2), 2 coats:
14 / 9.6 x 2 x 1.10 = 3.21 litres -- buy 1 x 2.5L + 1 x 1L
(Or use leftover wall paint if it is white/off-white)
Woodwork (Doors, Skirting, Window Frames)
Standard door: 1.2 m2 per face (both faces = 2.4 m2)
Skirting board: measure linear metres x 0.1m height
4m x 3.5m room perimeter = 15m x 0.1m = 1.5 m2
Window frame: approximately 0.5-0.8 m2 each
Woodwork for standard room (2 doors, 2 windows):
Doors: 2 x 2.4 m2 = 4.8 m2
Skirting: 1.5 m2
Windows: 2 x 0.6 m2 = 1.2 m2
Total: 7.5 m2
Eggshell at 12 m2/litre practical (x 0.85) = 10.2 m2/litre
2 coats: 7.5 / 10.2 x 2 = 1.47 litres -- buy 1 x 750ml + 1 x 1L
(Woodwork often needs 3 coats over raw wood -- multiply by 3/2)