TheCalculatory
Construction & Home · Area & Materials Formula verified

Paint Calculator

Enter the combined length of the walls, the ceiling height, how many doors and windows to skip, and how many coats — this works out the paintable area and how many gallons to buy.

Room
ft
ft
sq ft
$
On-screen number pad
Tap to type into the highlighted field
Paint needed
2
gallons
Wall area
368
Paintable area
281

Step-by-step proofCheck by hand
This is a planning estimate, not a guarantee of coverage. The default 350 sq ft per gallon is a typical figure for flat-to-eggshell interior paint on a smooth wall; darker colors, textured surfaces, a color change, or a different paint (semi-gloss, exterior, primer) can cover noticeably less. Check the coverage rate printed on your specific can and adjust the field above if it differs.

How the gallon count is calculated

Combined wall length times ceiling height gives the total wall area. Standard deduction sizes for doors (21 sq ft each) and windows (15 sq ft each) are subtracted to get the actual paintable area, since those spots don't get painted. Multiplying by the number of coats and dividing by the coverage rate gives the gallons needed, rounded up to the next whole can.

The formula

paintable area = (length × height) − (doors × 21) − (windows × 15) · gallons = ⌈(paintable area × coats) ÷ coverage⌉

Worked example

A room with 46 ft of combined wall length, an 8 ft ceiling, 2 doors and 3 windows: wall area = 46 × 8 = 368 sq ft, minus (2 × 21) + (3 × 15) = 87 sq ft, leaves 281 sq ft paintable. At 2 coats and 350 sq ft per gallon, that's (281 × 2) ÷ 350 = 1.61, rounded up to 2 gallons.

Frequently asked questions

Why does the gallon count round up to a whole number?
Paint is sold by the can — a partial gallon still has to be bought — so the gallon count is always rounded up to the next whole gallon, the same way a bag of concrete or a box of tile is rounded up.
Where do the door and window deduction sizes come from?
21 square feet for a standard interior door and 15 square feet for an average window are commonly published rule-of-thumb figures. Oversized doors, sliding glass doors, or picture windows cover noticeably more wall space, so treat these as a reasonable default rather than a measurement of your specific doors and windows.
Should I always buy extra beyond the calculated amount?
It's common practice to round up to the next full gallon or can (which this calculator already does) and, for touch-ups or an uneven surface, to buy a little more than the bare minimum — running out mid-wall means a delay waiting for the same batch or color to match.
Sources and method
  • Wall area and paint-coverage arithmetic — general published mathematics.
  • Standard door/window deduction sizes and typical interior-paint coverage rate — commonly published rule-of-thumb figures used by paint retailers and contractors; check your specific can for its actual coverage.
Last reviewed: 20 Sep 2026 Sources last verified: 20 Sep 2026 Results use the assumptions explained on this page. Report an error How we check calculations

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