How this paint calculator works
To estimate paint, you first need the paintable wall area. Add up the length of every wall (the room's perimeter) and multiply by the wall height. For a rectangular room that is 2 × (length + width) × height. Then subtract the parts you will not paint — the doors and windows.
Once you know the area, divide it by your paint's coverage (the square feet one gallon covers) and multiply by the number of coats. Round up, because you can't buy a fraction of a can.
Worked example: a 12 × 12 room
Take a 12 ft × 12 ft room with 8 ft walls, two doors and two windows, painted in two coats:
- Perimeter = 2 × (12 + 12) = 48 ft
- Wall area = 48 × 8 = 384 sq ft
- Subtract doors and windows = 384 − (2 × 21) − (2 × 15) = 384 − 72 = 312 sq ft
- Two coats = 312 × 2 = 624 sq ft to cover
- At 350 sq ft per gallon → 624 ÷ 350 ≈ 1.78 → 2 gallons
| Container | Covers (1 coat) | Good for |
|---|---|---|
| 1 gallon | ≈ 350 sq ft | An average room's walls |
| 1 quart | ≈ 90 sq ft | Trim, doors, touch-ups, accent walls |
Frequently asked questions
- How much paint do I need for a room?
- Estimate the wall area (room perimeter × wall height), subtract the area of doors and windows, divide by the paint's coverage (about 350 sq ft per gallon), then multiply by the number of coats. The calculator above does this for you.
- How many square feet does a gallon of paint cover?
- One gallon of interior paint covers roughly 350 square feet in a single coat on a smooth, primed wall. Textured, porous or dark-to-light surfaces cover less, so check the can's stated coverage.
- How many coats of paint should I use?
- Two coats is standard for an even, durable finish and good colour coverage. You may get away with one coat when repainting a similar colour, and may need three when going from dark to light.
- Do I subtract windows and doors?
- Yes. You do not paint glass or door faces, so subtract them from the wall area. A typical interior door is about 21 sq ft and a standard window about 15 sq ft. The calculator subtracts these by default.
- How much paint for a 12x12 room?
- A 12 × 12 ft room with 8 ft walls has about 384 sq ft of wall. After subtracting two doors and two windows (about 72 sq ft) you have roughly 312 sq ft. At 350 sq ft per gallon that is about one gallon per coat, or two gallons for two coats.
- How do I calculate ceiling paint?
- The ceiling area equals the room length times its width. Add that to the wall area before dividing by coverage. Tick the include-ceiling box above to add length × width automatically.