Paint Calculator

How to use this paint calculator

1. Measure the room’s length and width along the floor, and the wall height from the floor to the ceiling. Feet and inches are fine. Switch to metric if your tape measure reads in meters.
2. Count the doors and windows. The calculator subtracts 20 square feet for each door and 15 for each window. If yours are much bigger, such as a patio door or a picture window, change those sizes in the settings.
3. Choose the number of coats and the wall surface. Two coats is the safe default. Pick “new drywall or light texture” for fresh walls, or enter the coverage printed on your can.
4. Tick the ceiling and trim boxes if you’re painting them too. They show up as separate lines because ceiling paint and trim enamel are different products from wall paint.
5. Add more rooms if the same color goes in several of them. The calculator adds them together before choosing can sizes, which usually costs less than buying for each room on its own.

The result rounds up to real containers: quarts, gallons, and 5-gallon buckets. Enter the prices at your store to get a cost estimate. The defaults are only examples.

How the calculator works

The math is the same one a painter does on the back of an envelope, applied to each room and then added up:

wall area   = 2 × (length + width) × height
paintable   = wall area − (doors × 20 sq ft) − (windows × 15 sq ft)
gallons     = paintable area × coats ÷ coverage per gallon
to buy      = gallons × 1.10, rounded up to whole cans

Coverage is the number that matters most. Benjamin Moore’s data sheet for Regal Select gives a spread rate of 400 to 450 square feet per gallon, and This Old House uses 350 to 400 for typical interior paint. We default to 400 for smooth, previously painted walls and 350 for new drywall or light texture. For heavy texture, brick, or block we use 300. That last figure is our own cautious estimate, so check your label if you can.

Primer covers less, about 200 to 300 square feet per gallon. We use 250 and assume one coat over the walls and ceiling.

The ceiling is simply length × width. Trim is an estimate: a 4-inch baseboard around the room, minus the door openings, plus 2.5-inch casing (about 17 feet of casing per door and 14 per window). For a typical bedroom that comes to roughly 25 square feet, which is why trim almost always fits in one quart.

The extra 10% covers paint left in the roller and tray, a slightly rough wall, and a little for touch-ups later. You can change it in the settings.

A worked example

Say you’re painting a 12 × 14 bedroom with 8-foot walls, one door, and two windows, and you want two coats on the walls and the ceiling.

• Wall area: 2 × (12 + 14) × 8 = 416 square feet
• Minus one door (20) and two windows (30): 366 square feet
• Wall paint: 366 × 2 ÷ 400 = 1.83 gallons, or 2.01 gallons with 10% extra
• Ceiling: 12 × 14 = 168 square feet, × 2 ÷ 400 = 0.84 gallons, or 0.92 with extra

So you’d buy 2 gallons plus 1 quart of wall paint and 1 gallon of ceiling paint. The 10% allowance tips the walls just past 2 gallons. If you roll carefully and don’t need touch-up paint, 2 gallons will likely do. The quart is cheap insurance against running out halfway through the last wall.

Paint for common room sizes

Wall paint only, with 8-foot walls, one door, two windows, two coats, 400 square feet per gallon, and 10% extra:

The last column uses our example prices. Rooms in the middle of the table land close to a can boundary, so your own prices can change the answer by a quart.

Tips for buying the right amount

• Buy all the paint for one color at once. Custom-tinted paint can vary slightly from batch to batch, so pour all your cans of one color into a large bucket and mix them before you start. The last wall will match the first.
• Read the coverage on the label. The spread rate is printed on the can or the product’s data sheet. Use the low end of the range in the calculator.
• Prime bare drywall. Many paint-and-primer products are rated as self-priming on drywall, but makers still recommend two coats on bare surfaces. A coat of primer evens out the absorption so the finish coats look uniform.
• Plan an extra coat for dark and bold colors. Deep reds, blues, and greens often need a third coat or a tinted primer underneath. Set coats to 3 if you’re covering a dark color with a light one.
• If you’re painting inside a closet, add it as a small room. Otherwise count its door like any other door.
• Keep the leftovers. Label the can with the room and the date. A quart of matching paint makes small repairs easy years later.