Paint calculator
Measure the room, count the doors and windows, and choose how many coats. The calculator subtracts openings, applies the coverage printed on the can, and tells you how many gallons to buy.
How the paint calculator works
The calculator finds the perimeter of the room, multiplies by wall height for gross wall area, then subtracts a standard 21 square feet per door and 15 square feet per window. That net area is multiplied by the number of coats and divided by the coverage rate on the can, usually 350 to 400 square feet per gallon.
Ceilings are figured separately from length × width, and primer is calculated as a single coat over everything you are painting. Results round up to whole gallons because that is how paint is sold, though the exact figure is shown so you can decide whether a quart will cover the remainder.
Formula
wall area = 2 × (length + width) × height − doors × 21 − windows × 15
gallons = wall area × coats ÷ coverage per gallon
Example: a 12 × 14 ft room with 8 ft walls has 416 square feet of gross wall. Subtract one door and two windows (51 sq ft) for 365 net. Two coats at 375 sq ft per gallon is 730 ÷ 375 = 1.95 gallons, so buy 2 gallons.
Getting the coverage number right
- New drywall soaks up paint. Prime first or expect 25 percent less coverage on the first coat.
- Textured walls such as orange peel or knockdown can cut coverage by 20 to 50 percent. Use 250 to 300 sq ft per gallon.
- Dark to light color changes usually need a tinted primer plus two coats. Deep reds and yellows often need three.
- Paint-and-primer-in-one products still need two coats on most surfaces; the "primer" claim refers to adhesion, not hiding.
- Trim and doors are usually painted with a separate semi-gloss. A quart covers about 100 square feet, enough for the trim in an average bedroom.
Painting new walls? Start with the drywall calculator for sheets, mud, and tape.
Common questions
How much paint do I need for a 12x12 room?
About 2 gallons for two coats on the walls with 8 ft ceilings, after subtracting a door and a window. Add 1 gallon for the ceiling if you are painting it too.
How many square feet does a gallon of paint cover?
350 to 400 square feet per coat on smooth, previously painted walls. Textured, porous, or unprimed surfaces cover less, sometimes as little as 250 square feet.
Do I need two coats of paint?
Almost always. One coat rarely hides the old color evenly, and a second coat adds durability and washability. The main exception is repainting with the exact same color and sheen.
How much paint do I need for a ceiling?
Divide the ceiling area by the coverage rate. A 12 × 14 ft ceiling is 168 square feet, so one gallon of flat ceiling paint handles two coats with room to spare.