Paint Calculator

Work out how much paint you need for a room, a single wall or a ceiling. Enter the dimensions, the number of doors and windows and how many coats you plan to apply, and get the gallons to buy.

Whole room
ft
ft
ft
Options
Two coats for a color change; one for a refresh in the same color.
sq ft
Check the can — most interior paint covers 350–400 sq ft per gallon.
Paint needed1.9 gallons
What to buy
2 gallons
Paintable area
333 sq ft
Total coverage (all coats)
666 sq ft

How this calculator works

The wall area of a room is its perimeter, 2 × (length + width), multiplied by the wall height. A standard door (about 21 sq ft) and window (about 15 sq ft) are subtracted for each one you enter.

If you include the ceiling, its area (length × width) is added.

The total area is multiplied by the number of coats and divided by the coverage per gallon printed on the can. Most interior paints cover 350 to 400 square feet per gallon on smooth walls.

Example: a 12 × 12 ft bedroom with 8 ft walls

  1. Wall area: 2 × (12 + 12) × 8 = 384 sq ft.
  2. Subtract 1 door and 2 windows: 384 − 21 − 30 = 333 sq ft.
  3. Two coats: 333 × 2 = 666 sq ft.
  4. Paint needed: 666 ÷ 350 = 1.9 gallons, so buy 2 gallons.

Quick reference

Approximate paint for the walls of a room (2 coats, 8 ft ceilings, 350 sq ft/gal)
Room sizeWall areaPaint needed
10 × 10 ft320 sq ft1.8 gal
12 × 12 ft384 sq ft2.2 gal
12 × 16 ft448 sq ft2.6 gal
15 × 20 ft560 sq ft3.2 gal

Tips before you buy

  • Rough, textured or porous surfaces such as new drywall or brick absorb more paint. Use a lower coverage figure or apply primer first.
  • A dark color over a light wall (or the reverse) may need a third coat.
  • Keep a little leftover paint from the same batch for touch-ups.

Common sizes

Frequently asked questions

How much paint do I need for a 12 × 12 room?

A 12 × 12 ft room with 8 ft walls has 384 square feet of wall. With two coats at 350 sq ft per gallon you need about 2.2 gallons before subtracting doors and windows — usually 2 gallons once they are taken out.

How many square feet does a gallon of paint cover?

Most interior wall paint covers 350 to 400 square feet per gallon for one coat. Check the label of the paint you are buying.

Should I subtract doors and windows?

Yes, for a more accurate estimate. The calculator subtracts about 21 sq ft per door and 15 sq ft per window. For very small rooms you can skip this to keep a margin.

Do I need primer?

Primer is recommended on new drywall, bare wood, stained surfaces or when covering a much darker color. Primer usually covers a similar area per gallon as paint.

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Last updated October 25, 2026