HowMuchIt / Paint
How much paint do I need?
Three measurements and a coverage figure answer it: the walls' length, width and height, and how many square feet one gallon covers on your kind of wall. We give the coverage; you bring a tape measure.
Work out your paint
| Step | How |
|---|---|
| Wall area | (length + width) x 2 x wall height |
| Take off | 20 sq ft per door, 10 to 15 sq ft per window |
| Gallons | area x coats / 350 (smooth) or / 250 (rough) |
| Small jobs | one quart covers about 100 sq ft |
We take off 10 sq ft per window, the low end of the range, so you do not come up short.
Measure the room in five minutes
Walk around the room with a tape measure and write down each wall's length, then the height from the floor to the ceiling. For a plain rectangle, length and width are enough: the calculator goes around the room for you. Count the doors and windows you will not paint, because they come off the total. An odd-shaped room is easiest done wall by wall: add the lengths of all the walls and use that total as the length, with a width of zero.
Why the answer changes from wall to wall
Paint spreads further on a smooth wall that was painted before than on rough plaster, bare wood or new drywall, which drink the first coat. Two coats are the normal plan for walls, and a strong color change or a patched wall almost always needs them. New drywall and a lighter color over a dark one call for a primer first, which is a separate can. A textured ceiling is its own small job: tick the box to add the floor area.
Before you open the cans
- Buy the whole job from one batch, or mix the cans in a bucket, so the color matches from wall to wall.
- Round up: a leftover quart is handy for touch-ups later.
- Read the label of your own paint: makers print their coverage there.
Good to know
- One US gallon of interior wall paint covers about 350 to 400 square feet in one coat on a smooth surface.1,2,3
- Rough, textured or never-painted surfaces can cut coverage to about 250 square feet per gallon.4,5
- A quart of paint covers about 100 square feet.1,2,6
- Plan on two coats of paint on walls; unfinished, heavily patched or dark walls especially need two.2,4,7
- Prime first over new drywall, and when you paint a lighter color over a darker one.2,4,8
- Wall area = room perimeter (sum of all wall lengths) x wall height from floor to ceiling.2,6,7
- Subtract about 20 square feet for each standard door, and about 10 to 15 square feet for each window.2,6,7
- Ceiling area = room length x room width (the same as the floor area).2,7
How we checked this page
Checked on October 26, 2026. We kept a fact only when two independent publishers agree, and each one is numbered to its sources below. Where sources give different numbers, we keep only what they share. We never write a price. This page was written with an AI assistant and checked against the sources below.
Sources
- Paint Calculator, Sherwin-Williams
- How Much Paint Do I Need?, This Old House
- Paint Calculator, Glidden
- How Much Paint Do I Need?, Bob Vila
- BEHR PREMIUM PLUS Interior Ceiling Paint and Primer, Behr
- How Much Paint Do I Need?, The Old Farmer's Almanac
- Estimating How Much Paint to Buy, Dummies (John Wiley and Sons)
- Paint Coverage 101: How Much Does a Gallon of Paint Cover?, Clare