How it works
How to calculate it
The calculator applies the values you enter to this formula:
Gallons = ((wall area − openings) × coats × (1 + waste rate)) ÷ coverage per gallon.
How to use this calculator
- Measure room length, width, and wall height.
- Subtract major doors and windows.
- Enter coats, product coverage, waste, and local price.
Worked example
A 20 × 15 ft room with 8 ft walls, 42 sq ft of openings, two coats, and 350 sq ft/gal coverage needs about 3.3 gallons before rounding to purchasable cans.
Important assumptions
- The room is rectangular.
- Ceilings and trim are excluded.
- Coverage varies with surface texture, color change, and application method.
Common mistakes to avoid
- Using floor area instead of wall area.
- Forgetting a second coat or primer.
- Assuming every surface achieves label coverage.
Method reviewed June 10, 2026. Prices and rates are user-provided defaults and should be replaced with local values.