Calculate paintable wall area and whole gallons for a rectangular room without pretending every paint covers equally.
Results use your measurements and editable assumptions. Verify product coverage, density, waste and order rounding with the supplier before purchasing.
Paintable area = wall perimeter × height − openings. Gallons = area × coats ÷ coverage.
BuildCalc converts measurements to a consistent internal unit, applies your project geometry and only then adds the selected waste allowance. Packaging quantities are rounded up because partial bags, blocks or boxes usually cannot be ordered.
Paint quantity depends on paintable wall area, number of coats and the coverage of the selected product. Enter total door and window area separately, then replace the default coverage with the value printed on the paint can. Porous surfaces, dramatic color changes and application method can reduce real coverage.
With 8 ft walls, the four walls total 352 square feet. Subtracting 42 square feet of doors and windows leaves 310 square feet. Two coats plus 8% waste require about 670 square feet of coverage, which rounds up to 2 gallons at 350 square feet per gallon.
Large openings can be subtracted from gross wall area. Small deductions may be offset by touch-ups and surface variation, so measure them consistently.
No. Primer coverage and coat count can differ from finish paint and should be estimated as a separate product.
Published and maintained by EasyTool.me. Review the calculation methodology, verify product-specific values with the supplier, and report reproducible corrections. Last materially reviewed August 16, 2026.