How the paint calculator works
It works out the paintable wall area (minus doors and windows), adds the ceiling if you’re doing it, multiplies by the number of coats, and divides by the paint’s coverage rate. Then it turns the litres into the cans actually sold in Canada: 3.78 L “gallons” topped up with 946 mL “quarts”. If you’d need three or more quarts, it rounds up to another gallon instead, since that’s usually cheaper.
The formula
paintable area = 2 × (length + width) × height − doors × 20 ft² − windows × 15 ft²
litres = paintable area × coats ÷ coverage
Coverage of 400 ft² per US gallon is about 9.8 m² per litre.
Tips
- Two coats is the norm, and essential when changing colour. New drywall needs a coat of primer first, so count it as an extra coat.
- Read the can. Coverage varies from about 300 to 400 ft²/gal. Rough or textured walls, and deep colours, are at the low end.
- Ceilings: flat ceiling paint covers a little less than wall paint. If you’re doing both, run the calculator twice with each product’s coverage.
- Buy from one batch. If you need several gallons, buy them together, or mix them in a bucket (“boxing”) so the colour stays consistent.