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.
Worked example
Scenario: A room with a 40 ft perimeter, 9 ft walls, 2 coats, 2 doors and 3 windows, using paint that covers 350 sq ft per gallon.
- Gross wall area: 40 × 9 = 360 sq ft.
- Openings: 2 doors × 20 + 3 windows × 15 = 85 sq ft, leaving 275 sq ft to paint.
- Two coats: 275 × 2 = 550 sq ft.
- Paint: 550 ÷ 350 = 1.57 gallons — buy 2 gallons.
Result: About 1.57 gallons, so 2 gallons to buy.
Tips and common mistakes
- Door and window allowances are typical sizes (20 and 15 sq ft); measure unusual openings and adjust.
- New drywall, dark colours and textured walls may need an extra coat or primer.
- Ceilings are separate: add length × width of the room if you're painting them.
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.
Estimating paint more accurately
Wall area is only the starting point. The number of coats, surface texture, primer, color change and paint coverage rating can all change the amount required. New drywall and porous surfaces often absorb more product, while a major light-to-dark or dark-to-light color change may require additional coats. Round up to practical container sizes and keep some matching paint for touch-ups.