How to use this calculator
- Enter room length, width, and ceiling height.
- Subtract counts and typical areas of doors and windows.
- Adjust coats, coverage, waste, paint container size, and your local quoted costs.
The calculation formula
Net walls = 2 × (length + width) × height − opening area. Volume = net walls × coats × (1 + waste%) ÷ coverage. Containers = round up(volume ÷ container size).
Worked example
For a 12 × 10 × 8 ft room with one 21 ft² door and two 15 ft² windows, net wall area is 301 ft². Two coats at 350 ft² per gallon plus 10% waste need about 1.89 gallons; buy two 1-gallon cans.
Assumptions and limitations
- Coverage depends on surface porosity, paint color, and application method.
- Ceilings, trim, and irregular walls are excluded; estimate separately.
Actual project costs may be affected by supplier rates, material grade, labor market, location, disposal, site condition, access, permits, taxes, and schedule. All entered prices are yours to revise. See our methodology and disclaimer.
Frequently asked questions
Why do I need more paint for a second coat?
The net surface receives paint twice, so required volume grows proportionally.
Are paint costs current local prices?
No. All prices are illustrative editable planning values.