What this calculator does
The Paint Area Calculator works out how much wall surface you actually need to paint. Most paint-quantity mistakes come from painting the full wall rectangle and ignoring the openings — doors and windows don't get painted, so they should be subtracted. This tool gives you the net paintable area in square metres so you can buy the right amount of paint.
How to use it
Enter the total length of all the walls you are painting and the wall (ceiling) height. Then enter how many doors and windows there are along with the typical width and height of each. The calculator multiplies length by height for the gross area, subtracts the total door and window area, and shows the net paintable result. All measurements are in metres and the result is in square metres.
The formula explained
The core equation is $$A = (L \times H) - \Sigma(\text{door area}) - \Sigma(\text{window area})$$ First, gross wall area is \(L \times H\). Each door contributes its width \(\times\) height, multiplied by the number of doors; the same applies to windows. Those opening areas are subtracted from the gross area. If openings exceed the wall area the result is clamped to zero.
Worked example
Suppose your walls total 12 m long and 2.4 m high: gross area = \(12 \times 2.4 = 28.8 \text{ m}^2\). There is 1 door (\(0.9 \times 2.1 = 1.89 \text{ m}^2\)) and 2 windows (each \(1.2 \times 1.0 = 1.2 \text{ m}^2\), so \(2.4 \text{ m}^2\)). Net paintable area = $$28.8 - 1.89 - 2.4 = \mathbf{24.51 \text{ m}^2}$$
FAQ
Should I deduct small windows? For accuracy you can, but many decorators ignore openings under about 0.5 m² and round up to allow for waste and trim.
How much paint will I need? Divide the net area by your paint's coverage rate (often 10–12 m² per litre per coat), then multiply by the number of coats.
Can I use it for ceilings? This tool is designed for walls. For ceilings, measure room length × width and use that as a single rectangle with no deductions.
