Paint Calculator Guide: Gallons, Coats, Coverage, and Waste
Use room measurements, coverage per gallon, coat count, and waste margin to estimate paint before buying.
Last reviewed: July 5, 2026. Paint estimates fail when people use floor area instead of wall area. This guide shows how to enter the right numbers.
The wall area method
Wall area: 2 × (length + width) × wall height. Then multiply by coats and divide by coverage per gallon.
For a 12 × 12 ft room with 9 ft walls, the wall area before openings is 432 sq ft. Two coats at 350 sq ft per gallon needs about 2.47 gallons before waste.
Inputs that change the estimate
- Coats: two coats are common for repainting, but primer may change the plan.
- Coverage: use the number printed on the can for the exact product.
- Surface condition: patched, rough, or porous walls use more paint.
- Color change: dark-to-light and light-to-dark changes may need more coverage.
Should you subtract doors and windows?
For a quick small-room estimate, many people leave them in and treat the extra as touch-up margin. For rooms with large windows, subtracting the opening area gives a tighter estimate.
Buying advice
Round in a practical way. Running out halfway through a wall can create color and sheen differences, especially if the second can comes from a different batch.
Related calculator
Use the Paint Calculator to test your own numbers after reading the guide.