Paint Calculator

Tip: if you need more than one gallon of a custom color, mix all the cans together in a bucket before you start. Small differences between cans show up on a wall.

Worked example

A 12 ft by 12 ft bedroom with 8 ft walls has a perimeter of 48 ft, so the walls total 48 x 8 = 384 square feet. Taking out one door at 21 square feet and two windows at 15 each leaves 333 square feet to paint.

Two coats means covering 666 square feet. At 350 square feet per gallon that is 1.90 gallons, so you buy 2 gallons.

The ceiling is 144 square feet. Two coats would take another 0.82 gallons, which is one more gallon of ceiling paint.

How the paint calculator works

Wall area is the distance around the room times the wall height. The calculator subtracts about 21 square feet for each standard door and 15 square feet for each average window, then multiplies by the number of coats. Dividing by the paint's coverage gives gallons, which you round up to whole cans.

For rooms with an unusually large opening, like a wide archway or patio door, measure it and treat it as several doors or windows. For very large windows, subtract their actual width times height.

Coverage per gallon

Most interior paint lists 350 to 400 square feet per gallon on smooth, primed walls. That is the default here. Real coverage is often lower:

• Textured, rough or porous walls: 250 to 300 square feet per gallon.
• Bare drywall and new plaster soak up paint. Prime first or plan on an extra coat.
• Rolling with a thick nap roller uses more paint than a short nap.

Check the label of the paint you are buying and enter its coverage above.

How many coats do you need?

Two coats is standard. It gives even color and sheen and hides roller marks. One coat can work when you are repainting the same color with a good paint. Plan on three coats, or a tinted primer plus two coats, when making a big color change, especially dark to light or to bright reds and yellows, which hide poorly.

Paint-and-primer products help on previously painted walls in good condition, but they are not a substitute for a real primer over bare drywall, patches, stains or glossy surfaces.

Ceilings, trim and doors

Ceilings are usually painted with a flat ceiling paint in a separate color. The area is just length times width. The calculator shows the gallons for the ceiling separately so you can buy the right product.

Trim, doors and cabinets usually take a semi-gloss or satin enamel, bought in quarts. One quart covers about 100 square feet, which is enough for the baseboards and door casings in an average room. A standard door has about 20 square feet per side.

Exterior painting

The same math works for a house exterior. Measure each wall, length times height, and add the triangular gable ends as half of base times height. Siding and stucco absorb more paint than interior walls, so use 250 to 300 square feet per gallon. The square footage guide covers triangles and other shapes.

Prep that affects how much paint you use

Clean walls take paint evenly. Wash greasy kitchen walls and bathroom walls before painting, fill holes and sand patches smooth, then spot prime the patches so they do not show through as dull spots. Glossy walls need a light sanding or a bonding primer.

Primer is calculated the same way as paint, usually for one coat, and it often covers 200 to 300 square feet per gallon on bare drywall. Tinting the primer toward the finish color can save a coat of paint on a big color change.

More calculators

Results are estimates, not a contractor quote. Check quantities with your supplier before you order, and follow local building codes.