Paint Coverage Calculator
Estimate gallons of wall paint for a room with door and window subtraction, multiple coats, and an adjustable coverage rate for porous surfaces.
Each door subtracts 21 sq ft (standard 3×7 slab).
Each window subtracts 15 sq ft (average 3×5).
350 for smooth-drywall latex is the median manufacturer claim. Drop toward 250 for porous surfaces (raw drywall, plaster, textured walls).
About this tool
Paint take-offs are usually wrong in the same direction. People measure the wall area, divide by the coverage number on the can, and end up short because they forgot the second coat or did not subtract anything for doors and windows. This calculator does both subtractions and the coat multiplication so the gallon number you take to the paint counter actually matches what you need.
Enter the room length, width, and ceiling height in feet. Door and window subtractions use industry-standard footprints: each door takes 21 sq ft off the wall area (a typical 3 ft × 7 ft slab), each window takes 15 sq ft (a 3 ft × 5 ft average). If your openings are very different, count two doors for one extra-wide door, or zero for a very small one.
The coverage default is 350 sq ft per gallon, which matches most premium latex wall paint on smooth drywall. Drop it toward 250 for porous surfaces (raw drywall, plaster, masonry primer). The result rounds up to the nearest quart since paint stores typically sell in quart, gallon, and 5-gallon containers. See the editorial standards page for how we set these defaults.
How it works
Wall area is the perimeter (2 × (length + width)) times ceiling height. Door and window areas subtract from the wall area. The remainder is multiplied by the number of coats to get total surface area to cover. Total area divided by gallon coverage gives raw gallons; the result rounds up to the nearest quart so you actually have enough at the can.
The 350 sq ft per gallon coverage figure is the median manufacturer claim across major US wall-paint brands (Sherwin-Williams, Benjamin Moore, Behr) for smooth-drywall latex applied at the recommended film thickness. Manufacturer technical data sheets are the source of record for any specific product. Film-thickness assumptions matter: the published coverage assumes the recommended wet-film build, which depends on the roller nap and the painter's pace.
Examples
A 14×12 bedroom or small living room. Wall area 416 sq ft, minus 51 sq ft of openings, yields 365 paintable sq ft. At two coats and 350 sq ft/gal coverage, 2.25 gallons covers it with a small cushion.
A 10×10 bedroom with standard door and window. 284 paintable sq ft × 2 coats = 568 sq ft, which rounds up to 1.75 gal at the next quart. This example replaces the prior tool's incorrect 1.50 gal stated output.
A 20×16 living room with 9-foot ceilings, plus lower coverage rate for textured walls. 546 paintable × 2 coats = 1,092 sq ft / 250 = 4.37 gallons raw, rounded up to 4.50 gal. Textured walls (orange peel, knockdown) reliably drink paint at about 25-30% higher rate than smooth drywall.
When to use
Use this for wall paint on a single room or a single same-color suite of rooms. Run it again per color if you have an accent wall. The tool does not include ceiling area; if you are painting the ceiling, add the floor footprint (length × width) as a separate calculation. For exterior siding or trim paint, the coverage rate drops further; check the manufacturer data sheet. Pair with the lumber board feet calculator when scoping a remodel.
Related concepts
- Latex paint coverage : Why we default to 350 sq ft per gallon
Frequently asked questions
Should I include the ceiling in the wall area?
No. The wall-area formula here is perimeter × height. If you are painting the ceiling, add the floor footprint (length × width) and a second gallon estimate, since ceiling paint is often a different product.
What if my doors or windows are very different from the defaults?
Count proportionally. A double-wide door counts as two doors. A small kitchen window counts as half. The exact subtraction matters less than catching the fact that openings exist at all.
Why does it round up to a quart?
Paint stores sell in quart, gallon, and 5-gallon increments. Rounding to the next quart up gives you actual orderable quantities and a small cushion. Running out mid-coat is worse than buying a small extra.
Sources
- Sherwin-Williams Paint Product Data Sheets (manufacturer coverage specifications)
(primary, accessed Apr 15, 2026)
Manufacturer coverage specifications are one of the three median anchors (Sherwin-Williams, Benjamin Moore, Behr) for the 350 sq ft per gallon default.
- Benjamin Moore Technical Data Sheets
(primary, accessed Apr 15, 2026)
Second manufacturer reference anchoring the coverage range; typical interior paint published at 350 to 400 sq ft per gallon on previously painted, non-porous surfaces.
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