How to use this calculator
Add the areas of the walls you will paint. Subtract the area of doors and windows you will not paint. Enter coats, coverage per coat from the product label, and container size.
The math behind it
Paintable area = total wall area − openings Paint amount = paintable area × coats ÷ coverage Containers = round up(paint amount ÷ container size)
A worked example
For 500 ft² of walls minus 50 ft² of openings, 2 coats at 350 ft² per US gallon need about 2.571429 gallons: 3 one-gallon containers.
Assumptions & precision
Choose ft² per US gallon or m² per liter. Changing the selector changes labels only; re-enter all values in the selected units. Coverage is product-specific, not guaranteed. Coats must be whole numbers from 1 to 100. Zero paintable area needs zero paint. Rough or porous surfaces, application method and color changes may need more. No separate primer or waste calculation. Unrounded intermediate values; results display up to 6 decimal places. Magnitudes above 10¹⁵ are rejected.
A common question
Why does the estimate differ from the paint can?
Use the actual coverage printed on your product. Surface texture, porosity and application thickness can reduce that coverage.