Tinker 
How to use it
Pick metric or imperial, then enter the room's length, width and wall height. Add the number of doors and windows, the coats you will apply, and the coverage printed on your paint tin.
The formula
Wall area = 2 × (length + width) × height. Subtract 1.9 m² (20 ft²) per standard door and 1.5 m² (15 ft²) per average window. Paint needed = area × coats ÷ coverage.
Worked example
A 4 m × 3 m room with 2.5 m walls has 2 × 7 × 2.5 = 35 m² of wall. Minus one door (1.9) and one window (1.5) leaves 31.6 m². Two coats at 10 m²/L need 31.6 × 2 ÷ 10 = 6.32 litres, or about 6.95 L with 10% spare, so buy 7.5 L (e.g. a 5 L and a 2.5 L tin).
Tips
This is an estimate. Rough, porous or freshly plastered walls absorb more paint, and strong colour changes may need an extra coat. Ceilings are not included; calculate them separately as length × width.
Frequently asked questions
How much area does a litre of paint cover?
Most emulsions cover about 10–12 m² per litre per coat; a US gallon covers roughly 350–400 ft². Always check the tin.
Do I need two coats?
Usually yes for even colour, especially over a different shade or new plaster. One coat may do for refreshing the same colour.
Should I subtract doors and windows?
Yes, for accurate totals. If you are painting doors or frames too, buy trim paint separately.