How this calculator works
The calculator finds the wall area (perimeter × height), subtracts standard door and window sizes, optionally adds the ceiling, then multiplies by the number of coats and divides by the coverage printed on the can.
A primer coat is calculated separately at its own coverage. On new drywall or bare, porous surfaces painted without primer, the first coat soaks in, so the calculator adds about 25% to that coat. Enter prices to get the whole job’s cost, including rollers, tape and drop cloths.
Assumptions and tips
- When the leftover is three quarts or more, buying another gallon is usually cheaper, so the calculator rounds up.
- Trim, doors and cabinets are not included.
Worked example
Using the default values in the calculator above:
| Input | Value |
|---|---|
| Room length | 12 ft |
| Room width | 14 ft |
| Wall height | 8 ft |
| Coats | 2 |
| Doors | 1 |
| Windows | 2 |
| Paint the ceiling too? | No |
| Coverage per gallon | 350 sq ft |
| Wall surface | Painted, similar color |
| Primer coat | No primer |
| Paint price (optional) | 0 $/gal |
| Supplies: rollers, tape, drop cloths (optional) | $0 |
| Result | Value |
|---|---|
| Paint needed | 2.09 gallons |
| Buy | 2 gal + 1 qt |
| Paintable area (one coat) | 365 sq ft |
| Or buy whole gallons | 3 gallons |
Step by step:
- Wall area = 2 × (12 + 14) × 8 − openings = 365 sq ft
- Paintable area = 365 sq ft
- Gallons = 365 × 2 coats ÷ 350 = 2.09
Paint for common room sizes
8 ft walls, 2 coats, one door, two windows, 350 sq ft per gallon, walls only.
| Room | Paint needed | Buy |
|---|---|---|
| 10 × 10 ft | 1.54 gallons | 2 gal |
| 12 × 12 ft | 1.9 gallons | 2 gal |
| 12 × 14 ft | 2.09 gallons | 2 gal + 1 qt |
| 14 × 16 ft | 2.45 gallons | 2 gal + 2 qt |
| 16 × 20 ft | 3 gallons | 3 gal |
Common mistakes to avoid
- Planning one coat over a strong color change — usually you need primer or a second coat.
- Using the “up to” coverage on the label; textured or porous walls get much less.
Frequently asked questions
How much paint for a 12×14 room?
With 8 ft walls, two coats, one door and two windows: about 2.1 gallons — buy 2 gallons plus 1 quart.
How many square feet does a gallon of paint cover?
Most interior paints list roughly 350–400 sq ft per gallon per coat on smooth, primed walls. Rough or porous surfaces get less.
Do I need primer?
Yes on new drywall, bare wood, patched areas and big color changes. Over sound paint of a similar color, a quality paint usually covers in two coats without primer.