Estimate paint and primer for up to three rooms, or enter a measured wall area for an irregular space. Include ceilings if needed, subtract doors and windows, set separate coverages and coat counts, and round purchases to the container sizes and prices you enter.
Formula
For each rectangular room:
wall area = 2 × (length + width) × wall height
paintable area = walls + selected ceilings − doors − windows
paint volume = paintable area × paint coats ÷ paint coverage
primer volume = paintable area × primer coats ÷ primer coverage
containers = ceil(required volume ÷ entered container size)
US and metric room dimensions convert internally. Door and window area inputs are square feet so their unit is explicit even when room dimensions use metres.
Worked example
A 12 ft × 10 ft room with 8 ft walls has 352 ft² of wall area. Subtract one 20 ft² door and two 15 ft² windows to get 302 ft². Two paint coats create 604 ft² of coverage demand. At 400 ft² per gallon, mathematical paint need is 1.51 gallons, which rounds to 2 one-gallon containers. One primer coat at 300 ft² per gallon needs 1.01 gallons, which rounds to 2 one-gallon primer containers.
Coverage and purchasing
Use the actual product label or technical data sheet. Sherwin-Williams states that a gallon of paint commonly covers about 350–400 ft² and primer about 200–300 ft², while also warning that application method and surface condition change requirements. Its paint calculator guidance is the source for those example ranges. The defaults are editable assumptions, not guarantees for a particular coating.
Texture, porosity, repairs, dramatic color changes, application loss, and an uneven substrate can increase use. Common mistakes include using floor area as wall area, forgetting coats, subtracting openings twice, combining primer and paint coverage, or rounding gallons before multiplying by coats.
Examples
Manual wall area
If careful measurement gives 750 ft² and the project needs two coats, enter 750 ft² in Manual mode and two coats. At 375 ft²/gal, required paint is 4 gal.