Paint Calculator
Estimate interior wall paint from room dimensions, doors, windows, coats, coverage per gallon, waste allowance, and optional price.
What the paint calculator estimates
This paint calculator estimates interior wall paint for a rectangular room. It uses room length, room width, wall height, doors, windows, coverage per gallon, number of coats, waste allowance, and optional price per gallon.
- Enter the room dimensions in feet.
- Set doors and windows with their sizes in feet.
- Choose a paint type or enter the coverage rate from the can in square feet per gallon.
- Set coats, waste allowance, and price per gallon to estimate purchase amount and cost.
Paint calculator formulas
The estimate starts with wall area after openings, then divides by paint coverage and applies coats and waste allowance. The result is rounded up to whole gallons for purchase.
L is room length, W is room width, H is wall height, n d and n w are door and window counts, and w and h are opening width and height.
G base is gallons before allowance, S paint is net paint area, C gal is coverage in sq ft per gallon, n coats is the number of coats.
G buy is gallons to purchase and r is waste allowance percent.
Choosing coverage and allowance
| Input | How to use it |
|---|---|
| Coverage | Enter the manufacturer value when available; many interior paints are roughly 350 to 400 sq ft per gallon per coat. |
| Coats | Two coats are common for repainting walls, while primer or strong color changes can need more planning. |
| Waste allowance | Smooth primed walls need less reserve than textured, porous, patched, or heavily repaired walls. |
| Price per gallon | Optional. It turns the gallons-to-buy result into a quick material cost estimate. |
Typical interior paint coverage
| Paint type | Typical coverage | Notes |
|---|---|---|
| Interior latex | 350-400 sq ft/gal | Common wall paint on smooth primed surfaces |
| Primer | 250-350 sq ft/gal | Varies strongly by substrate and product |
| Ceiling paint | 300-400 sq ft/gal | Often used on broad flat surfaces |
| Textured surface | 200-300 sq ft/gal | Rough surfaces can use significantly more paint |
| Trim enamel | 300-400 sq ft/gal | Use separate trim dimensions for detailed estimates |
Estimate limits
- The calculator is for interior walls in a rectangular room, not exterior siding, metal, floors, cabinets, or detailed trim takeoffs.
- The result is shown in gallons and a quart equivalent, then rounded up to whole gallons for purchase.
- Ceilings, trim, closets, stairwells, and accent walls should be measured separately if they matter.
- For multiple rooms, calculate each room separately or combine wall areas from your project plan.
Frequently Asked Questions
Sources and References
- Paint CalculatorSherwin-Williams
- How Much Paint Do I Need?Behr
- Paint Coverage CalculatorThe Paint Calculator
Calculations are based on the listed reference sources. Links open in a new tab.