How to use this calculator
- Choose US customary or metric measurements.
- Enter each wall’s length and height.
- Enter door and window counts, coats, and the coverage from the paint label.
- Calculate and round purchasing quantities based on available container sizes.
How the calculation works
The calculator adds wall areas, subtracts 20 square feet per door and 15 square feet per window, multiplies the paintable area by the number of coats, and divides by the chosen coverage rate.
Formula
Paintable area = total wall area − door area − window area Paint = paintable area × coats ÷ coverage rate
Worked example
Four 12 ft × 8 ft walls total 384 square feet. After one 20 sq ft door and two 15 sq ft windows, 334 square feet remain. Two coats at 350 sq ft per gallon require about 1.91 gallons.
Project considerations
- Coverage varies with surface texture, color change, application method, and product.
- Use the manufacturer’s coverage rate instead of relying on the default.
- Trim, ceilings, repairs, and primer are not included.
Frequently asked questions
Why is the default 350 square feet per gallon?
It is a planning assumption, not a guarantee. Replace it with the coverage shown on the paint you plan to use.
Should I subtract doors and windows?
For a closer wall estimate, yes. This tool uses 20 square feet per door and 15 square feet per window; unusual openings may need a separate area calculation.