Buying too much paint wastes money, and buying too little means a second trip with a half finished wall. This paint calculator works out your true wall area, subtracts doors and windows, and tells you exactly how much paint to buy for one or two coats, with primer included if you need it.
Paint Calculator
Measure once and buy the right amount of paint. Doors and windows are subtracted for you.
Assumptions: a standard door counts as 21 sq ft (1.95 sq m) and a standard window as 15 sq ft (1.39 sq m). Coverage varies by paint brand, surface texture and colour change, so check the tin before you buy. Deep colours and bare plaster often need the primer coat.
How it works
- Enter the room length, width and ceiling height, in feet or metres.
- Add how many doors and windows the room has, and they are subtracted at standard sizes.
- Choose one or two coats, set the coverage from your paint tin, and tick the primer box if you are painting bare plaster or a big colour change.
- The result shows the net wall area, the exact amount of paint, and a simple buy recommendation in gallons and quarts (or litres).
Frequently asked questions
How much paint does one gallon cover?
Most standard paints cover about 350 sq ft per gallon on a smooth wall, which is the default in this calculator. Rough, porous or heavily textured walls drink more paint, so check the coverage printed on your tin and adjust the number if it differs.
Do I really need two coats?
Two coats give an even colour and better durability, and they are usually needed when you change colour by more than a shade or two. Painting a similar colour over a sound wall can work with one good coat.
Should I buy primer as well?
Primer helps on bare plaster, patched areas, stains and strong colour changes, because it seals the surface so the top coat goes further and looks even. If your walls are already painted in a similar colour, you can usually skip it.