How to Calculate How Much Paint You Need for a Room

Work out gallons of paint from room size, doors, windows and coats, with a worked example for a 12 × 12 foot room.

Buying paint by guesswork leaves you with half-used cans or a second trip to the store with a color that no longer matches. Here is a quick and reliable way to work out how much you need.

Step 1: Find the wall area

Add the lengths of all the walls to get the room's perimeter, then multiply by the ceiling height. For a rectangular room that is 2 × (length + width) × height.

Step 2: Subtract openings

A standard door is about 21 square feet and a typical window about 15 square feet. Subtract them for each opening you will not paint.

Step 3: Multiply by coats and divide by coverage

A gallon of paint typically covers about 350 to 400 square feet per coat on a smooth, previously painted wall. Multiply the wall area by the number of coats, then divide by the coverage. Always check the coverage printed on your can, because it varies by product.

Worked example: a 12 × 12 foot room with 8 foot ceilings

• Perimeter: 2 × (12 + 12) = 48 feet
• Gross wall area: 48 × 8 = 384 square feet
• Minus 2 doors and 2 windows: 384 − 42 − 30 = 312 square feet
• Two coats: 312 × 2 = 624 square feet
• At 350 square feet per gallon: 624 ÷ 350 = 1.78 gallons, so buy 2 gallons

For the ceiling, 12 × 12 is 144 square feet. Two coats is 288 square feet, which is roughly 0.7 to 0.8 of a gallon at typical ceiling-paint coverage.

When you need more

• Rough, textured or porous walls soak up more paint.
• Changing from a dark color to a light one usually needs extra coats or a tinted primer.
• Bare drywall needs primer first.

Tips

• Buy slightly more than the minimum so touch-ups match.
• Mix all cans of the same color together before you start to even out small color differences.
• Label leftover paint with the room and date.