How Much Paint Do I Need? A Room-by-Room Guide
Buy too little paint and you're back at the store mid-project, hoping the new tin matches. Buy too much and you've wasted money on cans that dry out in the garage. Working out the right amount is simple once you know the formula — and a calculator makes it effortless.
This guide shows you how to estimate paint for any room, what affects coverage, and how to avoid the most common mistakes.
The basic paint formula
Estimating paint comes down to four numbers: your total wall area, the area of doors and windows to subtract, the number of coats, and your paint's coverage rate.
The formula is: (wall area − doors and windows) × number of coats ÷ coverage rate = paint needed. Measure each wall's width times its height, add them up, subtract the openings, then apply your coats and coverage. It sounds fiddly, but it's just multiplication — and the calculator below does all of it for you.
How much does a litre or gallon cover?
As a rough guide, one litre of paint covers about 10–12 square metres for a single coat on a smooth, sealed surface. In imperial terms, one US gallon covers roughly 350–400 square feet.
These are best-case numbers. Rough, porous or unpainted surfaces — bare plaster, new drywall, brick or render — soak up more paint, so use the lower end of the range. Always check the coverage figure printed on your specific paint tin, since it varies by brand and product.
Don't forget the second coat
Most walls need two coats for an even, durable finish — especially when you're painting over a darker colour, a patched surface, or bare plaster. That effectively doubles the paint you need, so it's the number people most often forget.
If you're applying a primer or undercoat first, factor that in as an additional coat with its own coverage rate. Skipping the second coat to save money usually backfires: patchy walls need touch-ups that use more paint (and time) in the end.
Tips for an accurate estimate
Measure twice and round up. It's always better to have a little paint left over for future touch-ups than to run short and risk a mismatched batch. A good rule is to add about 10% to your estimate as a buffer.
Also consider the surface texture: textured or heavily-patterned walls have more surface area than their flat dimensions suggest and use more paint. And if you're changing colour dramatically — say, dark to light — plan for an extra coat.
Use the free paint calculator
Instead of doing the maths by hand, enter your wall area, the number of doors and windows, your coats and coverage rate into our free paint calculator. It instantly tells you how many litres or gallons to buy, in metric or imperial.
It runs entirely in your browser, works on your phone at the hardware store, and takes the guesswork out of your next painting project.