How to calculate it
Paintable wall area is the room perimeter multiplied by wall height, minus door and window openings. Multiply by coats and any waste allowance, then divide by the coverage printed on the can.
Worked example
A 12 × 12 ft room with 8 ft walls has 384 sq ft of wall before openings. With one 20 sq ft door and two 15 sq ft windows, 334 sq ft remain. At two coats and 350 sq ft per US gallon, that is about 1.91 gallons before extra allowance.
Before you order
- Different finishes for ceilings and walls should be purchased separately.
- Textured walls, bare plaster, new drywall and dramatic colour changes may require more coating.
- Coverage on the product label overrides any online average.
Frequently asked questions
Does the calculator subtract doors and windows?
Yes. It subtracts standard planning allowances of 20 sq ft per door and 15 sq ft per window. For unusually large openings, calculate the paintable wall area separately.
Is a US gallon the same as a Canadian imperial gallon?
No. This tool explicitly uses the US gallon (3.785 litres). Compare the litre volume on the can before purchasing.
Does this include primer?
No. Primer is a separate coating with its own coverage and cost.
Sources & assumptions
Calculator formulas and explanations prepared for RenovationBinder, 2026-10-15.