Paint Calculator
Work out how many gallons of paint you need for a room. Enter the room size, doors, windows and number of coats to get wall, ceiling and total paint quantities.
By CalcBuilt Editorial TeamUpdated September 9, 2026Formulas checked against manufacturer specs, see how we calculate.
Results
- Total paint needed
- 1.91 gal
- Buy 2 gallons for 2 coats
- Wall paint
- 1.91 gal
- 334 sq ft of wall after deducting openings
- Paintable area per coat
- 334 sq ft
Paint formula
Wall area = 2 x (Length + Width) x Height - (Doors x 20) - (Windows x 15)
Ceiling area = Length x Width
Gallons = (Area x Coats) / Coverage per gallon (typically 350 to 400 sq ft)Worked example
A 14 x 16 ft bedroom with 9 ft ceilings, two doors and three windows: 2 x 30 x 9 = 540 sq ft, minus 40 for doors and 45 for windows = 455 sq ft of wall. Two coats at 350 sq ft per gallon: 455 x 2 / 350 = 2.6 gallons, so buy 3 gallons.
Adding the 224 sq ft ceiling needs another 1.3 gallons for two coats, so buy 2 gallons of ceiling paint.
Quick reference
| Room size | Wall area | Gallons needed | Gallons to buy |
|---|---|---|---|
| 10 x 10 ft | 270 sq ft | 1.54 | 2 |
| 12 x 12 ft | 334 sq ft | 1.91 | 2 |
| 12 x 15 ft | 382 sq ft | 2.18 | 3 |
| 15 x 20 ft | 510 sq ft | 2.91 | 3 |
| 20 x 20 ft | 590 sq ft | 3.37 | 4 |
How to calculate paint for a room
Add the lengths of all walls to get the perimeter, then multiply by ceiling height for gross wall area. A 12 by 12 room with 8 ft ceilings has 48 ft of wall by 8 ft, or 384 square feet. Subtract about 20 square feet per door and 15 per window.
Divide the paintable area by the coverage printed on the can (usually 350 to 400 square feet per gallon) and multiply by the number of coats. Two coats is standard for a color change or new drywall; one coat may be enough when repainting the same color with a premium paint.
Tips for buying the right amount
Textured walls, bare drywall and dark-to-light color changes soak up more paint, so use the low end of the coverage range. Buy all the paint at once so it comes from the same batch, and keep a quart for touch-ups.
Key facts
- One gallon of interior paint covers about 350 to 400 sq ft per coat on smooth walls.
- A standard interior door is deducted as 20 sq ft and an average window as 15 sq ft.
- Two coats are recommended for color changes and for new, unpainted drywall.
- A quart covers about 90 to 100 sq ft; use quarts for trim, doors and touch-ups.
Frequently asked questions
How much paint do I need for a 12x12 room?
For walls only with 8 ft ceilings, one door and two windows, you need about 2 gallons for two coats. Add about 0.85 gallons for the ceiling.
How many square feet does a gallon of paint cover?
Roughly 350 to 400 square feet per coat on smooth, previously painted walls. Rough or porous surfaces cover less.
Do I need two coats of paint?
Usually yes. Two coats give even color and full coverage, especially when changing color or painting new drywall. Some premium paint-and-primer products cover in one coat when repainting a similar shade.
Sources and references
- How to use the paint calculator: coverage per gallon (Sherwin-Williams)
- How much interior paint to buy (Behr)
- Paint calculator for rooms, doors and windows (Benjamin Moore)
Next steps for this project
The usual order for a paint & walls project. See all paint & walls calculators.
Results are estimates based on standard formulas and typical product specifications. Confirm quantities with your supplier and local code before ordering.