How the Paint Calculator Works
The paint calculator estimates interior room paint from the four wall surfaces. It computes wall area = 2 x (length + width) x height, subtracts standard door and window allowances, adds ceiling area only when selected, multiplies by the number of coats, then divides by the coverage rate per gallon. The final gallons result rounds up to whole cans.
The default room is 12 ft by 10 ft with 8 ft walls. That is 352 square feet of wall area. One door and two windows subtract 45 square feet, leaving 307 square feet. Two coats at 350 square feet per gallon require 1.75 gallons, so the calculator rounds up to 2 cans.
Coverage Rate Is the Critical Input
Coverage rates vary by product, color, surface, application method, and primer. Smooth previously painted drywall may be close to the label coverage. Bare drywall, patched walls, masonry, textured ceilings, dark-to-light color changes, and sprayer application can use more paint. When in doubt, use the lower end of the label range and keep the final color batch consistent.
Project Notes
| Input or output | How to use it | Common adjustment |
|---|---|---|
| Room dimensions | Measure length, width, and wall height. | For vaulted or irregular rooms, split surfaces into rectangles. |
| Doors/windows | Uses standard area deductions. | Large openings, built-ins, and glass walls need manual adjustment. |
| Coats | Multiplies paintable area by 1, 2, or 3. | Primer is separate unless your product and surface support one-step coverage. |
| Coverage | Square feet per gallon from the paint label. | Lower it for texture, porosity, sprayers, and dramatic color changes. |
Safety and Prep Notes
- Pre-1978 homes: sanding, scraping, or demolition can disturb lead-based paint; review EPA RRP guidance.
- Surface repair: patching, cleaning, caulking, and primer often matter more than the paint quantity estimate.
- Ceilings and trim: this page models ceiling paint as the room footprint; trim and doors need separate estimating.
- Leftover paint: keep a labeled amount for touch-ups, but avoid buying excessive gallons that will age unused.