Estimate exactly how much paint to buy based on your wall area, number of coats, doors/windows to subtract, and your paint's coverage rate.
Most quality wall paints cover 300–400 sq ft per gallon per coat. Check the coverage rate printed on your specific paint can for accuracy.
Two coats is standard for a durable, even finish, especially on new drywall or when changing colours significantly. One coat may suffice for a touch-up in the same colour.
Yes — this calculator is for walls only. If you're also painting the ceiling, calculate that area separately using length × width.