How to calculate it yourself
For two coats: gallons = ceil((wall_area × 2) ÷ ft²_per_gallon). Always round up — paint is sold in whole gallons.
Common scenarios
Bedroom (350 ft² walls)
350 ft² × 2 coats ÷ 350 = 2 gallons exactly. Always round up — buy 2.
Stucco facade (1,200 ft²)
Stucco eats paint. 1,200 × 2 ÷ 175 = 14 gallons. The 350 ft² rule does NOT apply outdoors on rough surfaces.