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How to Use the Paint Calculator
- Add the widths of the walls you plan to paint and enter that total.
- Enter the wall height.
- Confirm the number of coats.
- Use the coverage printed on the paint can or keep the 350-square-foot default.
- Review exact gallons and round up to the whole gallons to buy.
How the Paint Estimate Works
Painted area equals total wall width × wall height × number of coats. Exact gallons equal painted area ÷ coverage per gallon. The purchase estimate rounds that result up because partial gallons may not be available in the product you choose.
Many interior wall paints advertise roughly 350 to 400 square feet of coverage per gallon, but the manufacturer’s label is the best input. Rough, porous, unprimed, or repaired surfaces often use more paint.
Paint Calculator Example
A room with 40 feet of total wall width
At 8 feet high, the walls have 320 square feet per coat. Two coats create 640 square feet of painted area. At 350 square feet per gallon, the exact estimate is 1.83 gallons, so buy at least 2 whole gallons before considering waste.
Doors, Windows, and Waste
This fast estimate does not subtract openings. For a more detailed order, subtract the area of large doors and windows, then add a practical allowance for spills, roller and brush loading, touch-ups, and future repairs. Keeping a little paint from the same batch can make later touch-ups easier.
Common Paint Estimating Mistakes
Common mistakes include measuring only one wall, forgetting the second coat, using floor area instead of wall area, assuming every product has the same coverage, and rounding exact gallons down. Confirm whether primer, ceilings, trim, and cabinets require separate products.