Paint calculator
Estimate gallons before you buy paint.
Enter the room size, subtract doors and windows, then choose coats and paint coverage. The result rounds up to purchasable buckets.
Room and paint inputs
Tip: textured walls, strong color changes, and primer needs can increase paint usage. Round up when the room has patched or porous surfaces.
How this paint estimate is calculated
The calculator starts with wall area instead of floor area. For a rectangular room, wall area is the room perimeter multiplied by wall height. Door and window openings are subtracted, then the remaining paintable area is multiplied by the number of coats. The final gallon estimate is rounded up to whole buckets because most stores do not sell partial buckets.
Formula: paintable wall area = 2 x (length + width) x wall height - doors and windows. Coverage to buy for = paintable wall area x coats. Gallons = coverage to buy for / coverage per gallon.
| Input | Why it matters | Practical note |
|---|---|---|
| Room length and width | These create the perimeter used for wall area. | Measure along the wall, not across furniture or trim. |
| Wall height | Tall rooms can use far more paint than floor size suggests. | Measure to the ceiling or crown molding line you will paint to. |
| Doors and windows | Openings reduce paintable surface. | Use a rough area total if you only need a buying estimate. |
| Coverage per gallon | Product labels vary by paint type and surface condition. | Use the lower end of the label range for rough or patched walls. |
Example: 12 ft by 10 ft room
A 12 ft by 10 ft room with 8 ft walls has a perimeter of 44 ft. Multiplying by 8 ft gives 352 sq ft of wall surface. If the doors and windows total 45 sq ft, the paintable wall area is 307 sq ft. With two coats, the room needs coverage for 614 sq ft.
If the chosen paint covers 350 sq ft per gallon, the estimate is 1.75 gallons. A cautious purchase would be two 1-gallon buckets, or one larger container if the store sells the exact size you need.
When to round up
- Fresh drywall, skim-coated areas, or many wall repairs.
- Textured walls, brick, rough plaster, or porous surfaces.
- Strong color changes, especially light over dark or dark over light.
- Rooms where future touch-ups need an exact color match.
- Projects where primer is skipped or rolled thinly.