Paint Calculator
Measure your walls and find out exactly how much paint to buy — no half-empty tins, no second trip to the store.
Last reviewed 2026-06-06
Buying paint is a guessing game for most people: too little means an extra trip mid-project (and a slightly different batch), too much means money sitting in the garage. This calculator removes the guesswork.
Enter the total length of the walls you are painting, the ceiling height, and how many doors and windows are in the room. We subtract standard door and window sizes automatically and multiply by the number of coats to give you the exact amount of paint to buy.
How to use the Paint Calculator
- Measure the length of each wall you plan to paint and add them together for “total wall length”.
- Measure floor-to-ceiling for the wall height.
- Count the doors and windows in the room — we subtract their area automatically.
- Choose your number of coats (two is standard; use three over bold colours).
- Read off the litres or gallons, and the number of tins to buy.
Frequently asked questions
How much does one gallon of paint cover?
A US gallon of wall paint covers roughly 350 square feet in a single coat on a smooth, primed surface. Textured, porous or previously unpainted walls absorb more, so you may get closer to 250–300 sq ft. One litre covers about 11 m².
How many coats of paint do I need?
Two coats is standard for an even, durable finish. Use one coat only when repainting a similar colour, and three coats when going from a dark colour to a light one or painting over bold colours like red.
Should I subtract doors and windows?
Yes — for a normal room it makes a real difference. This calculator subtracts a standard door (about 21 sq ft) and window (about 15 sq ft) for each one you enter, so you are not paying for paint you will not use.
Do I need primer as well?
Use primer on bare drywall, patched areas, glossy surfaces, or when covering a dark colour. Primer is calculated the same way as paint — measure the same area and apply one coat.