About this calculator
Buying too little paint means a second trip mid-project; buying too much wastes money. This calculator estimates the paint you need for a room’s walls based on its size, the doors and windows to subtract, and the number of coats — then adds a sensible margin so you don’t run out.
How it works
The room is treated as four walls: the perimeter multiplied by the ceiling height gives the gross wall area. Standard door and window sizes are subtracted, the result is multiplied by the number of coats, and then divided by the paint’s coverage rating. A 10% allowance is added for waste and touch-ups. A door counts as about 1.9 m² (21 ft²) and a window about 1.4 m² (15 ft²).
Frequently asked questions
What does this calculator include?
The four walls of a rectangular room, minus doors and windows, across the number of coats you choose.
Does it include the ceiling?
No — this estimates walls only. Measure and add the ceiling separately if you plan to paint it.
Why add 10%?
Spills, uneven or porous surfaces and future touch-ups. It is safer to have a little extra than to run out mid-job.
Metric or imperial?
Both. The toggle switches between metres and litres, or feet and gallons.