How to calculate it yourself
Total gallons = (wall area in ft² ÷ coverage rate) × number of coats. Round up — most paint cans cost the same per gallon at 1 vs 5, and you will use the leftover for touch-ups.
Common scenarios
Single bedroom (12×12, 8-ft ceilings)
384 ft² of wall = ~1.1 gallons per coat. Two coats = 2.2 gallons. Buy 3 gallons to cover the ceiling and have touch-up paint.
Whole interior repaint (1,800 ft² home)
Walls roughly equal floor area × 3 = 5,400 ft². Two coats = 31 gallons. Buy 5-gallon buckets for the savings (~$30/bucket vs 5 gallons).
Exterior siding (2,000 ft² of wood)
2,000 ÷ 250 = 8 gallons per coat. With prime + 2 coats: 24 gallons. Plan for 30 gallons including trim and corner work.