Paint Calculator
Gallons and cost for walls from size, coats, coverage, and openings.
What it does
Paint math is area divided by coverage. Measure the walls, subtract doors and windows, multiply by coats, and divide by the square feet each gallon covers. The can coverage already assumes normal rolling on smooth primed drywall.
The tool above follows that path and then rounds up to full cans, since stores do not sell 0.93 gallons. It also prices the job at your per gallon figure.
Texture and thirst change the answer. Rough, repaired, or bare drywall can drink 10 to 20% more than the label, while a second coat over the same color stretches further than the first.
How to use
- Switch the unit system first: imperial for feet and gallons, metric for meters and liters.
- Add every wall length into one total. Measure twice around obstacles.
- Enter wall height and subtract doors plus windows as one area.
- Pick coats: one for refreshes, two for new drywall or dark to light changes.
- Read coverage off your exact can and enter it. Buy the rounded up cans.
Formula
- L x H
- Gross wall area in square feet.
- openings
- Doors plus windows area subtracted.
- coverage
- Square feet per gallon off your exact can.
- coats
- Multiply for second coats.
Ceilings need their own pass with ceiling paint: length times width divided by coverage. Trim is usually quoted in linear feet with a quart covering near 100 ft of trim.
Worked example
Example: 12x12 ft room, 8 ft walls, one coat
Walls total 48 ft long and 8 ft high, openings 60 ft2, coverage 350, price $45.
- Gross wall: 48 x 8 = 384 ft2.
- Paintable: 384 - 60 = 324 ft2.
- Exact paint: 324 x 1 / 350 = 0.93 gal.
- Buy: round up to 1 gallon can.
- Cost: 1 x $45 = $45.00 before primer and supplies.
Reference table
| Room | Wall area | Exact paint | Buy |
|---|---|---|---|
| 10x10 ft, 8 ft walls | 320 ft2 | 0.91 gal | 1 gallon |
| 12x12 ft, 8 ft walls | 384 ft2 | 1.10 gal | 2 gallons |
| 12x12 ft, 9 ft walls | 432 ft2 | 1.23 gal | 2 gallons |
| 20x15 ft, 8 ft walls | 560 ft2 | 1.60 gal | 2 gallons |
Limits
- Walls only. Ceilings and trim need separate passes.
- Label coverage assumes smooth primed drywall.
- Texture and repairs drink 10 to 20% more.
- Primer and supplies excluded from paint cost.
Common mistakes
- Forgetting the second coat on new drywall, which roughly doubles the paint.
- Using the label maximum coverage on rough or repaired walls.
- Skipping primer over patches, so topcoat soaks in and flashes.
- Buying exact gallons. One spill or touch up later needs the spare quart.
- Painting the ceiling with wall paint instead of flat ceiling paint.
Try it yourself
Quick checks. Answers sit tucked under each question.
Two coats on 324 ft2 at 350 coverage?
648 / 350 = 1.85 gal, so buy 2 gallon cans.
One coat on a 12x12 ceiling?
144 / 350 = 0.41 gal, so 1 gallon can with spare.
Questions
How far does a gallon of paint go?
About 350 to 400 ft2 on smooth primed walls. Rough or bare surfaces drop toward 300.
One coat or two?
One refreshes the same color. Two covers new drywall, patches, stains, and bold color jumps.
How much for a 12x12 room?
Walls run 384 ft2 before openings. At 350 coverage that is 1.10 gallons per coat, so buy 2 gallons for one safe coat with spare.
Do I subtract windows and doors?
Yes for large counts. Small rooms with several openings can drop a full quart from the need.
What about the ceiling?
Measure length times width separately and use flat ceiling paint. A 12x12 ceiling needs about half a gallon per coat.