How to calculate it yourself
Wall area in ft² ÷ 350 = gallons per coat. Multiply by number of coats. A single gallon at one coat handles roughly 350 ft²; the same gallon stretched to two coats handles only 175 ft².
Common scenarios
Small bedroom (12×12, 8-ft ceiling)
384 ft² of wall. One gallon = one coat with 35 ft² to spare. Two coats need 2 gallons.
Hallway (3 ft × 20 ft × 8 ft)
Wall area 368 ft². One gallon for one coat; 2 gallons for two coats.
Single accent wall (15×8 ft)
120 ft². One quart (¼ gallon) covers it for one coat with extra. Two coats: ½ gallon.