Paint Calculator
Calculate how many gallons of paint you need for a room, accounting for coats, doors, and windows.
About the Paint Calculator
Buying the right amount of paint avoids both an inconvenient mid-project store run and unnecessary leftover cans. This calculator finds the wall area to be painted, subtracts doors and windows that don't need paint, and divides by your paint's coverage rate to find the gallons needed.
How It Works
The calculator multiplies total wall width (the room's perimeter) by wall height and by the number of coats to get total paintable area, then subtracts a standard area allowance for each door and window, and finally divides the net area by the paint's coverage rate per gallon.
Gross Area = Perimeter × Height × Coats; Net Area = Gross Area − (Doors × 20 + Windows × 15); Gallons = Net Area / Coverage per Gallon.
Example
Scenario: A room with a 40ft perimeter, 9ft walls, 2 coats, 2 doors, and 3 windows, using paint rated for 350 sq ft per gallon.
Result: Gross area of 720 sq ft, minus 85 sq ft for doors/windows, netting about 635 sq ft, requiring roughly 1.8 gallons.
Assumptions & Limitations
- Uses standard door and window area allowances (about 20 sq ft per door, 15 sq ft per window) which are reasonable averages, though your actual openings may differ.
- Assumes even coverage and no significant paint loss from spills or absorption into unprimed surfaces.
- Textured or unprimed walls may absorb more paint than the rated coverage suggests — consider buying slightly extra for those surfaces.
- Does not separately account for ceilings, trim, or accent walls painted a different color.