How Much Paint Do You Need for a Room: Walls, Doors, Coats and Coverage

Buying too little paint means a second trip to the store and a risk that the color of the new can does not quite match. Buying too much means paying for paint that sits in the garage. The estimate takes five minutes with a tape measure and works the same way for any room.

Step by step

1. Measure the perimeter and the wall height. Add up the length of every wall, then measure from floor to ceiling.
2. Multiply them. Perimeter × height is the area of the walls, as if the doors and windows were not there.
3. Subtract the openings. Doors and windows will not get paint. The Paint Calculator counts a standard door as 21 sq ft (3 × 7 ft) and a standard window as 15 sq ft (3 × 5 ft); measure large or unusual ones yourself.
4. Decide how many coats. Two is common. A rough, porous or unpainted surface, or a dramatic color change, can call for more paint.
5. Find the coverage on the can. It is the square feet one gallon covers with one coat, and it is printed on the label.
6. Divide and round up: gallons = paintable area × coats ÷ coverage.

Behr's guide says one gallon of its Ultra or Premium Plus interior paint and primer covers 250 to 400 sq ft of surface with one coat. Other brands and products differ, so use the number on your can.

Worked example: a 14 × 12 ft room with 8 ft walls

The perimeter is 2 × (14 + 12) = 52 ft, so the walls are 52 × 8 = 416 sq ft. One door and two windows remove 21 + 2 × 15 = 51 sq ft, which leaves 365 sq ft of wall. Two coats is 730 sq ft of paint to apply.

The range is 2 to 3 gallons, so the surface and the paint decide it. Smooth, primed walls and a similar color point to the low end; rough walls or a big color change point to the high end. Behr also notes that the more doors and windows a wall has, the less paint it needs.

Ceilings and trim

• Ceiling. Its area is length × width: 14 × 12 = 168 sq ft. If it gets the same paint, the room needs 533 sq ft per coat, which is 3 to 5 gallons for two coats. Ceilings are often painted with a different paint, so calculate them separately in that case.
• Trim. Behr's method is to add up the length of all the trim (baseboard, door and window casing, molding) and multiply by 0.5 ft, because the average trim is about six inches wide. For 90 ft of trim that is 90 × 0.5 = 45 sq ft.

Common mistakes

• Forgetting the coats. The coverage on the can is for one coat. Two coats need twice the paint.
• Using the coverage of another product. Read your own label; specialty finishes can behave differently.
• Rounding down. Round up to whole gallons, then decide whether a quart is enough for the remainder.
• Skipping the openings or counting them twice. Subtract each one once.
• Leaving out the trim and ceiling, if you plan to paint them.