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

Written by

RenoCalcs Team

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Enter the size of the room, the number of doors and windows and how many coats you plan to apply. The calculator shows the paintable wall area, the gallons you need and a suggested mix of cans to buy.

How the paint calculator works

  • Wall area = 2 × (length + width) × wall height
  • Each door subtracts 21 sq ft (3 × 7 ft) and each window 15 sq ft (3 × 5 ft)
  • Paint = paintable area × coats ÷ coverage per gallon, plus the waste allowance

Worked example

A 12 × 12 ft bedroom with 8 ft walls, one door and one window has 384 sq ft of wall. Minus 36 sq ft for the door and window leaves 348 sq ft. Two coats at 350 sq ft per gallon need 696 ÷ 350 = about 2 gallons. With 5% extra that is 2.09 gallons: two 1-gallon cans and a quart.

What coverage should you use?

Most interior wall paints list 350 to 400 sq ft per gallon for a smooth, primed surface. This calculator starts at 350 sq ft so the estimate errs on the safe side. Use a lower number for rough or porous surfaces, new drywall, or a big color change, and check the label on the paint you are buying.

Step by step: painting a room

Measure first, then paint. These four steps match the inputs in the calculator above.

Tape measure along the bottom of a wall with arrows showing wall length and height
Step 1. Measure the length of each wall along the floor, and the wall height from floor to ceiling.
Door and window outlined with painter's tape to show the areas subtracted from the wall
Step 2. Count the doors and windows. Each one is subtracted from the wall area, so you do not buy paint for them.
Cutting in along the ceiling line with an angled brush, with a roller and tray below
Step 3. Cut in along the ceiling, corners and trim with an angled brush before you roll.
Roller applying a second coat of paint next to a gallon and a quart can
Step 4. Roll the walls, let the first coat dry fully, then apply the second coat.

Buying tips

  • If you need four gallons or more, a 5-gallon bucket is usually cheaper than single gallons.
  • Buy all the paint for a room from the same batch, or mix the cans together in a larger bucket, so the color stays consistent.
  • Keep a small amount for touch-ups and write the color name and number on the lid.

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