How the calculator works
It adds up the four walls, subtracts the doors and windows, multiplies by the number of coats, and divides by the coverage rate for your kind of wall. Then it adds 10% for waste and touch-ups and rounds up to the cans you can actually buy: gallons and quarts in the United States, or 3.78 L and 946 mL cans in Canada.
The numbers it uses
Each door counts as 20 sq ft (about 1.9 m²) and each window as 15 sq ft (about 1.4 m²). An average wall gets 350 sq ft per gallon per coat. Smooth, previously painted walls get 400; textured walls 300; new drywall, brick and plaster 275. Primer is calculated at 300 sq ft per gallon, one coat. Most interior wall paints list between 350 and 400 sq ft per gallon on the can.
Why the surface changes the answer
Smooth walls that were painted before take the least paint. Texture adds surface area, and new drywall, brick and plaster soak up the first coat, which is why they get a lower rate and usually need primer.
Should I round up?
Yes. A second trip to the store costs more than a leftover quart, and the leftover is your touch-up paint. Label it with the room and the date.
When to check the can
Coverage varies between products and sheens. If your paint lists a different rate, use the rate on the can: it is the number that matters for your wall.
Results are estimates. Coverage depends on the product, the tools and the wall; the rate printed on your can wins.