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Paint Calculator

Tell the calculator your room dimensions, ceiling height and how many coats you plan. It subtracts doors and windows and returns the gallons to buy — including the half gallon you would have forgotten.

How the paint calculation works

  1. Add the four walls: multiply (length + width) by 2, then by the ceiling height.
  2. Subtract 21 ft² for each door and 15 ft² for each window.
  3. Multiply by the number of coats — most repaints need two.
  4. Divide by the coverage on the can. 350 ft² per gallon is the safe default; some paints claim 400.
  5. Round up. Paint is sold in whole gallons and leftover paint stores for years.

The formula

Wall area = 2 × (Length + Width) × Height. Deduct 21 ft² per door and 15 ft² per window. Gallons = Wall area × Coats ÷ Coverage.

Buying materials?

Compare prices before you order:

Common questions

How much area does one gallon of paint cover?

About 350 ft² per coat on a typical interior wall. Smooth, primed drywall can reach 400 ft² while rough or unpainted surfaces may only get 300 ft².

Do I need one coat or two?

Two in almost every case. One coat over an existing colour rarely covers fully, and a second coat is what gives durability and an even finish. Budget for two unless you are touching up.

Should I subtract doors and windows?

Yes. Deducting 21 ft² per door and 15 ft² per window keeps you from buying a whole extra gallon. The calculator does this automatically.

Should I paint the ceiling too?

This calculator covers walls only. For a ceiling, add length × width as another flat surface. A 12 × 12 room adds 144 ft², about half a gallon per coat.

How long should I wait between coats?

Typically two to four hours for water-based paint at room temperature. Humidity and thick application both extend it. Check the can — recoating too early lifts the first coat.

Sizes people search for

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