Paint Calculator

Find out how many gallons of paint to buy for a room. Enter the room size and wall height, and we subtract doors and windows for you.

Your measurements

Room length
Room width
Wall height

Most rooms are 8 ft. Measure floor to ceiling.

sq ft/gal

Check the label. 350–400 is typical.

* Required. Use feet, inches or both. Decimals and fractions like 6 1/2 are fine.

Results

Enter your measurements — results appear here instantly.

How the paint calculator works

Paint is sold by the gallon, and the can tells you how many square feet one gallon covers. So the job is to work out how many square feet you're painting, then divide.

  1. Wall area = perimeter × wall height, where perimeter = 2 × (length + width).
  2. Subtract openings: 20 sq ft for each door and 15 sq ft for each window.
  3. Add the ceiling (length × width), if you ticked that box.
  4. Gallons = paintable area × number of coats ÷ coverage per gallon.

Assumptions

  • The room is a rectangle with four full-height walls.
  • A standard interior door (about 3 ft × 6 ft 8 in) is counted as 20 sq ft. A typical window is counted as 15 sq ft.
  • Coverage defaults to 350 sq ft per gallon, the low end of most manufacturer ratings. Textured, porous or unprimed surfaces can use 10–30% more.
  • Trim, doors and cabinets are not included. They usually take a separate semi-gloss paint.

Worked example

A bedroom is 12 ft × 14 ft with 8 ft walls, one door and two windows, and you're painting two coats.

  • Wall area: 2 × (12 + 14) × 8 = 416 sq ft
  • Openings: 1 × 20 + 2 × 15 = 50 sq ft, so 416 − 50 = 366 sq ft
  • Paint: 366 × 2 coats ÷ 350 = 2.09 gallons

That's just over two gallons, so buy 3 gallons. If you're confident in your coverage, 2 gallons plus a quart may do the job.

Tips before you buy

  • Changing from a dark color to a light one? Use a tinted primer first, or plan on a third coat.
  • Buy all your gallons in one trip, or mix them together in a bucket ("boxing"), so the color matches exactly.
  • Keep a little leftover paint, labeled with the room and date, for touch-ups.

Frequently asked questions

How much does one gallon of paint cover?

Most interior wall paints cover 350–400 sq ft per gallon in one coat on smooth, primed drywall. Rough or porous surfaces such as new plaster, brick or textured walls can drop to around 250–300 sq ft.

Do I really need two coats?

Usually, yes. A second coat gives even color and better durability, especially when you change colors. Paint-and-primer products sometimes cover in one coat when the old and new colors are similar.

Should I subtract doors and windows?

Yes, for an accurate estimate. In a room with only a few small openings, some painters skip the subtraction and keep the difference as a buffer.