Paint Calculator: How Much Paint Do I Need?
Estimate wall and ceiling paint from room dimensions, subtract common door and window areas, choose the number of coats, and optionally calculate material cost.
Room measurements
Coverage baseline: 350 sq ft per gallon per coat.
This is a planning estimate. Texture, porosity, dramatic colour changes, application method and the manufacturer’s actual coverage all change the real amount needed.
How many gallons of paint do I need?
Start with the wall area, subtract the openings, multiply by the number of coats, then divide by the paint’s coverage rate. This calculator uses 350 square feet per gallon as a neutral planning baseline and shows it plainly, so you can replace it with the coverage printed on the paint you actually intend to buy.
The ceiling is treated as a separate area. Doors and windows come off the walls only — they never reduce the ceiling.
Coverage assumptions
| Situation | What to expect |
|---|---|
| Smooth, previously painted wall | Often close to labeled coverage |
| New drywall or other porous surface | May need primer and more paint |
| Strong colour change | An additional coat may be needed |
| Heavy texture | Coverage per gallon can drop noticeably |
How to measure a room for paint
Length and width are enough — the calculator works out the perimeter for you.
Use the wall height, not the ceiling height of the tallest point in a sloped room.
Standard deductions are used: 21 sq ft per door and 15 sq ft per window. Large picture windows will need a manual adjustment.
Common mistakes
- Buying for one coat when the colour change needs two.
- Using the paint can’s best-case coverage on new drywall or heavy texture.
- Forgetting the ceiling is a separate area that openings do not reduce.
- Measuring the room’s floor area instead of its wall area.
Replacing trim while the room is empty?
Measure baseboard linear footage before buying stock lengths.
Open the Baseboard Calculator →