Paint Calculator

How to measure a room for paint

Paint is figured from wall area. Measure the length and width of the room in feet, and the wall height from floor to ceiling. The calculator finds the perimeter (2 × length + 2 × width), multiplies by height, and subtracts an allowance for doors and windows.

• Count every door and window on the walls you are painting.
• If you are painting only one accent wall, enter that wall's length as the room length and 0 as the width. The perimeter becomes twice the length, so halve the result, or measure the wall area yourself.
• Ceilings are separate. Find the ceiling area (length × width) and divide by the coverage of your ceiling paint.

The formula

Wall area = perimeter × height − (doors × 20 sq ft) − (windows × 15 sq ft). Those openings are typical averages; large patio doors or picture windows take away more. Gallons = wall area × coats ÷ coverage per gallon. Many paint labels list about 350–400 sq ft per gallon on smooth, primed surfaces, which is why 350 is the default.

Worked example

A 14 ft × 12 ft bedroom with 8 ft walls, one door, two windows and two coats:

• Perimeter: 2 × (14 + 12) = 52 ft
• Wall area: 52 × 8 = 416 sq ft, minus 20 for the door and 30 for the windows = 366 sq ft
• Paint: 366 × 2 ÷ 350 = 2.09 gallons

The calculator rounds up to 3 gallons. In practice, many people buy 2 gallons plus a quart instead, since that covers the estimate with a little left for touch-ups.

What changes coverage

• Surface texture. Textured walls, popcorn ceilings and rough siding have more surface area and soak up more paint.
• Porous surfaces. New drywall, bare wood and patched areas absorb the first coat. A primer is often recommended.
• Big color changes. Going from dark to light, or to bright reds and yellows, can take an extra coat. Tinted primer can help.
• Application. Sprayers use more paint than rollers because of overspray.

Common mistakes

• Planning one coat. Most jobs need two coats for even color, even with "paint and primer" products.
• Ignoring trim. Doors, baseboards and trim are usually a different sheen and need their own can, often a quart is enough for a single room's trim.
• Not boxing paint. If you buy several gallons, mix them together in a larger bucket so the color is uniform across walls.
• Skipping prep. Cleaning, patching and sanding take longer than painting but decide how the finish looks.

Buying tips

Paint in the US is commonly sold in quarts, 1-gallon cans and 5-gallon buckets. A 5-gallon bucket is often cheaper per gallon if you are painting several rooms in the same color. Check the coverage number on the actual can you are buying and enter it in the calculator. Write the color formula and sheen on the lid or keep the receipt, so you can match it later. Keep a little leftover paint for touch-ups, sealed tightly and stored where it will not freeze.