Calculatoo

Paint Calculator: How Much Paint Do I Need for a Room?

Enter the room size, the number of doors and windows, and your paint's coverage. The calculator deducts the openings, adds your coats and spare paint, and rounds up to whole cans. After switching units, re-enter your measurements.

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Formula

wall area  = 2 × (length + width) × height − doors × 20 − windows × 15   (sq ft)
ceiling    = length × width                                              (optional)
paint      = (wall area + ceiling) × coats × (1 + extra%) ÷ coverage
cans       = round up (paint ÷ can size)

Worked example

A 12 ft by 10 ft room with 8 ft walls, one door and two windows, two coats, 10% extra, 350 sq ft per gallon, 1-gallon cans:

  • Walls: 2 × (12 + 10) × 8 = 352 sq ft, minus 20 for the door and 30 for the windows = 302 sq ft.
  • Paint: 302 × 2 × 1.1 ÷ 350 = 1.90 gallons.
  • Cans: round up to 2 one-gallon cans.

Adding the 12 × 10 ceiling gives 422 sq ft, 2.65 gallons and 3 cans.

Assumptions

  • A standard door is 20 sq ft and a window is 15 sq ft. Trim, skirting and built-ins are ignored.
  • Coverage is in square feet per US gallon (1 US gallon = 3.785411784 L). Metric lengths are converted exactly (1 ft = 0.3048 m).
  • Every coat uses the same coverage. A first coat on bare plaster often needs more, so a primer coat is not included.
  • The extra percentage is added on top of the total, and cans are always rounded up.

Frequently asked questions

How much paint do I need for a 12 by 10 ft room?

With 8 ft walls, one door, two windows and two coats, the walls are 302 sq ft. Add 10% extra and divide by 350 sq ft per gallon: 302 × 2 × 1.1 ÷ 350 = 1.9 gallons, so you buy two 1-gallon cans.

How many square feet does a gallon of paint cover?

Most interior paints are labelled at about 350 to 400 sq ft per US gallon on a smooth, primed wall. Rough or dark surfaces cover less, so enter a lower figure. 350 sq ft per gallon is about 8.6 m² per litre.

How are doors and windows handled?

The calculator removes 20 sq ft for every standard door (80 × 36 in) and 15 sq ft for every window (3 × 5 ft) from the wall area. If your openings are much larger or smaller, adjust the counts to match the real area.

Should I buy extra paint?

Yes. Ten percent extra covers spills, touch-ups and uneven walls, and one tin from the same batch avoids colour differences later. Set the extra to 0 for an exact figure.

How we calculate

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