Paint Calculator — How Many Gallons Do You Need?
A 12×12 ft room with 8 ft ceilings needs about 2 gallons of paint for two coats on the walls. One gallon covers roughly 350–400 square feet per coat, so most average rooms take 1–2 gallons for the walls, plus extra for the ceiling.
Enter your room dimensions and the number of doors and windows. This calculator subtracts the openings and tells you how many gallons of paint you need for the walls at your chosen number of coats.
How to use this calculator
- Enter the room length, width, and ceiling height in feet.
- Count the doors and standard windows — they are subtracted automatically (≈21 sq ft per door, ≈15 sq ft per window).
- Choose how many coats; two coats is standard, especially over a new or contrasting color.
- Adjust coverage per gallon if your paint can lists a different figure.
The formula
Start with the total wall area, remove the openings, then divide by how far a gallon goes.
Wall area = 2 × (length + width) × height. This is the perimeter times the height.
Net area = wall area − (doors × 21 sq ft) − (windows × 15 sq ft).
Gallons = net area × number of coats ÷ coverage per gallon (about 350 sq ft), rounded up to whole cans.
Worked example
A 12 ft × 10 ft room with 8 ft ceilings, 1 door, 2 windows, 2 coats, 350 sq ft/gal.
Wall area = 2 × (12 + 10) × 8 = 352 sq ftOpenings = 1 × 21 + 2 × 15 = 51 sq ftNet area = 352 − 51 = 301 sq ftTotal = 301 × 2 coats = 602 sq ftGallons = 602 ÷ 350 = 1.72 → 2 gallons
You need about 2 gallons of paint for two coats on this room.
Reference tables
| Room size | Wall area | Gallons (2 coats) |
|---|---|---|
| 10 × 10 ft | 320 sq ft | 2 |
| 12 × 12 ft | 384 sq ft | 2 |
| 15 × 15 ft | 480 sq ft | 3 |
| 20 × 20 ft | 640 sq ft | 4 |
Frequently asked questions
How much does a gallon of paint cover?
A gallon of interior paint typically covers 350–400 square feet in one coat. Rough or porous surfaces and dark-to-light color changes reduce coverage and may need an extra coat.
How many gallons of paint do I need for a 12x12 room?
A 12×12 room with 8 ft ceilings has about 384 sq ft of wall. After typical openings and two coats (~730 sq ft) you need roughly 2 gallons.
Do I need primer too?
Use primer on bare drywall, patched areas, stains, or when going from a dark to a light color. A gallon of primer covers about the same area as paint.
Should I include the ceiling?
This calculator covers walls only. For the ceiling, add length × width (e.g. 12 × 10 = 120 sq ft) and divide by your coverage for the extra paint.