Paint is estimated by wall area and the number of coats. One gallon covers a known area — the rest is measuring and rounding up.
The rule of thumb
A gallon of wall paint covers about 350–400 sq ft per coat. Measure each wall (length × height), add them up, then subtract doors and windows.
Worked example
A 12 × 12 ft room with 8-ft walls has about 384 sq ft of wall. Two coats need roughly 2 gallons — a bit less after you subtract a door and a window.
Don’t forget
- Primer if you’re going over bare drywall or a big colour change.
- Ceilings and trim are separate — estimate them on their own.
- Textured or porous surfaces drink more paint; round up.
Open the paint calculator → to get gallons by coats, coverage and openings.