How to calculate it yourself
Gallons = ceil((wall_area − doors − windows) × coats ÷ 350). Doors ≈ 21 ft². Windows ≈ 15 ft². For two coats, multiply by 2 before dividing.
Common scenarios
Bedroom refresh (12×12, 8' ceiling)
Walls: 384 ft² − 1 door − 1 window = ~348 ft². Two coats = 696 ft² ÷ 350 ≈ 2 gallons.
Whole-room makeover with primer
Add 1 gallon of primer to the 2 gallons of color. Use primer when going from dark to light or covering stains.
Exterior siding (1,500 ft² home)
Exterior typically needs 6–8 gallons because siding texture absorbs more paint. Always two coats.