Interiors calculator
Paint Calculator
Estimate finish paint by surface area, coat count, label coverage, and the can size you plan to buy.
Planning estimate, not a contractor quote. Review the assumptions and replace presets with local prices when available.
Method
How this calculator works
A floor plan alone does not describe the wall area to paint. Enter room length, width, wall height, and the combined door and window area. Include the ceiling only if it uses the same paint and coverage rate; otherwise calculate it separately so different products do not get merged into one purchase.
Wall area = 2 × (length + width) × wall height − openings. Add ceiling area if selected. US gallons = surface × coats × allowance factor ÷ coverage per gallon. Cans = ceil(gallons ÷ can volume).
Worked example
A realistic starting point
A 20 × 10 ft room with 8 ft walls and 20 ft² of openings has 460 ft² of wall surface. Two coats at an illustrative 350 ft² per US gallon with 10% allowance need 2.89 gallons, rounded to three 1-gallon cans.
What changes the answer
Factors worth checking
Use product coverage
Enter the coverage stated for the particular finish and substrate. A previously painted smooth wall and a porous surface may require different allowances.
Separate products
Primer, wall color, ceiling paint, and trim paint should be separate estimates unless they truly share the same product and coverage.
Count complete coats
Coverage is entered per coat. The calculator multiplies by the number of coats once; do not halve label coverage to represent two coats and then also enter two coats.
Common questions
Paint FAQ
How much paint does one room need?
It depends on wall height, openings, coats, and label coverage. The worked example explains the calculation instead of assuming every room takes the same number of cans.
Should I subtract doors and windows?
Enter their combined area for the wall estimate. If a surface must still be painted, leave it in the estimate or calculate its different paint separately.
Why are cans rounded up?
Paint is purchased in complete containers. Use the physical volume of the selected can to compare package options.
Evidence
Sources & review
- BEHR — paint calculatorManufacturer estimating context. Enter coverage from the specific paint label.
- NIST — SI units: volumeUnit relationships; supplier product coverage and prices remain separate inputs.
Published by Calcstead. Report a correction.
Method and source links reviewed September 6, 2026. Price presets are editable planning inputs and should be replaced with current local information.