How to use this calculator
- Add each interior wall across the rooms being painted.
- Enter optional combined ceiling area if ceilings are included.
- Enter door and window counts plus the average opening areas used for deduction.
- Set coats and product coverage, then calculate.
How the calculation works
Wall areas and optional ceiling area form the gross interior surface. Entered door and window areas are deducted once, then the paintable area is multiplied by coats and divided by the editable coverage rate.
Formula
Gross area = Σ(wall length × height) + optional ceiling area Opening area = doors × entered door area + windows × entered window area Gallons = (gross area − openings) × coats ÷ coverage
Worked example
Four walls totaling 352 square feet plus a 120 square foot ceiling give 472 square feet. Deducting one 20 square foot door and two 15 square foot windows leaves 422 square feet. Two coats at 350 square feet per gallon require about 2.41 gallons.
Project considerations
- Door, window, ceiling, coat, and coverage values are all explicit inputs.
- Coverage varies with product, color change, texture, application, and surface condition.
- Trim, cabinets, closets, and non-rectangular surfaces require separate measurement.
Frequently asked questions
Can walls from several rooms be combined?
Yes. Add every wall as a separate section and enter one combined optional ceiling area if needed.
Are door and window sizes fixed?
No. The average areas are visible and editable so the deduction can match the project.