Paint Calculator: How Much Paint Do I Need for a Room?
Estimate gallons of wall paint from your room's wall length, height, doors, windows, number of coats and the coverage printed on your paint can.
How the calculation works
The calculator finds the paintable wall area and divides it by the coverage of your paint.
- Wall area = total wall length × wall height. Total wall length is the distance around the room: for a 12 × 12 ft room, that is 12 + 12 + 12 + 12 = 48 ft.
- Subtract openings. The calculator subtracts 20 sq ft for each door and 15 sq ft for each window as a planning approximation. If your openings are much larger, such as patio doors or picture windows, reduce the window count or measure the openings and adjust the wall length instead.
- Multiply by coats. Two coats are typical for a color change or new drywall.
- Divide by coverage. Coverage is printed on the paint can or the manufacturer’s data sheet in square feet per gallon. The field starts at 350 only as an example; replace it with your product’s figure.
Worked example
A 12 × 12 ft bedroom with 8 ft walls, one door and two windows:
| Step | Value |
|---|---|
| Wall length × height | 48 × 8 = 384 sq ft |
| Minus 1 door (20) and 2 windows (30) | 384 − 50 = 334 sq ft |
| Two coats | 334 × 2 = 668 sq ft |
| Divide by coverage of 350 sq ft per gallon | 668 ÷ 350 ≈ 1.9 gallons |
Buy 2 gallons. If you are also painting the ceiling, measure it separately (length × width) and add a separate estimate for ceiling paint.
What changes how much paint you use
- Surface texture. Rough, textured or porous surfaces such as new drywall, plaster or masonry absorb more paint than a smooth previously painted wall, so actual coverage is lower than the label figure.
- Primer. Priming new drywall, patches or stains first can reduce how much finish paint you need and gives a more even result.
- Color change. Covering a dark color with a light one may need an extra coat, or a tinted primer.
- Application. Rollers with a thick nap hold and apply more paint; sprayers can use more paint through overspray.
Before you open the can
If your home was built before 1978, the EPA notes it is more likely to have lead-based paint, and that renovation, repair and painting that disturbs old paint can create lead dust. Painting over intact paint is low risk, but test before scraping or sanding. Paints also release volatile organic compounds as they dry; the EPA lists paints and paint strippers among household sources of VOCs, so ventilate the room well and consider low-VOC products.
Tips for buying
Buy all the paint for one room from the same batch, or mix the cans together in a bucket (called boxing) so the color is consistent from wall to wall. Keep a small amount of leftover paint sealed and labeled with the room name for touch-ups. For trim, doors and cabinets, estimate separately, since they usually take a different sheen and much less paint; measure the total length of trim, multiply by its width, and use the same coverage figure from the can.
Limits of this estimate
This is a planning estimate, not a guarantee. The coverage on the label assumes good conditions on a suitable surface. Round up rather than down, and check the label for the recommended number of coats and recoat time.