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Home / Square footage calculator

Square Footage Calculator

Measure one room, a whole floor, or an awkward shape. You'll get the square footage, the perimeter it implies, and a way to carry the number straight into whichever material you're pricing.

  • Rooms added up together
  • Perimeter as well as area
  • The number carried to any material

Project measurements

What are you measuring? Rooms first, because it is what most people want

Add each room as its own row. This is the one most people want, and the only one that gives a perimeter.

Measurements Tape measure or decimals

How this number is built

Any area calculator can do the math. The harder part is that square footage means different things to a shopper, a supplier, and an appraiser, and appraisers follow a published standard.

Measure the room at its widest points

Take each dimension at the widest place, not the average and not where the furniture is. For an irregular room, break it into rectangles at a doorway or where the wall steps, measure each, and add them. The calculator's own Rooms mode does exactly this, so you can keep the pieces separate rather than doing the arithmetic on paper.

Feet and inches, not decimal feet

A tape measure reads 12 ft 6 in, not 12.5 ft. Switch any dimension to the tape unit and enter both halves rather than converting in your head. Rounding 6 inches to half a foot is how a 12 × 12 room becomes 144 ft² when it is really 148.

Round the area at the end, not the sides

Carry the exact figures through the multiplication and round once. Rounding each measurement to the nearest foot first can move a room's area by several square feet, and on a whole floor that compounds.

For a house, appraisers use a standard, and it is not the sum of the rooms

ANSI Z765 Square Footage — Method for Calculating (Home Innovation Research Labs) is the residential standard that Fannie Mae and Freddie Mac require appraisers to use on single-family homes that get an inside inspection. It measures to the exterior face of the walls rather than room by room, reports above-grade and below-grade area separately, and counts only finished space. Finished space has to be enclosed, usable year-round, and connected to the rest of the house through stairs or hallways. It also needs a ceiling at least 7 ft high, with three exceptions: 6 ft 4 in is enough under beams and ducts, there is no minimum under stairs, and under a sloped ceiling at least half the room must reach 7 ft and nothing lower than 5 ft counts. Garages and unfinished space never count. Its own scope is single-family detached and attached homes, not apartments, and not room dimensions.

Two numbers, for two different purposes

A room-by-room sum is what you want for buying material, because it is the surface you will cover. A Z765 figure is what you want for pricing a house, and the two differ: a finished basement counts in the first and never in the second. Use the one that fits what you are doing.

Area conversions

From To ft² Exact?
1 m²10.7639104167yes, derived from 1 ft = 0.3048 m
1 yd²9yes
1 in²0.00064516yes
1 acre43,560yes
1 mi²27,878,400yes
1 ft²0.09290304 m²yes, the reverse of the first row

Every factor here is exact: NIST SP 811 defines 1 ft = 0.3048 m exactly, so 1 ft² is exactly 0.09290304 m². If your figure differs from a surveyor's, the measurements differ, not the math. ANSI Z765 reports a home's area to the nearest whole square foot, so a listing and a room-by-room sum rarely match to the digit. The standard is sold by Home Innovation Research Labs, and the rules above are summarized in our own words from the 2021 edition.

Measuring for a specific job? The number here carries straight into the concrete, paint, and roofing calculators, and into the other material pages.

Where these numbers come from

  • NIST SP 811, Appendix B.8 (Factors for Units Listed Alphabetically), via nvlpubs.nist.gov.
  • ANSI Z765-2021 Square Footage — Method for Calculating, published by Home Innovation Research Labs (a subsidiary of the National Association of Home Builders). The rules described above were checked against the 2021 edition.
  • The standard is sold by Home Innovation Research Labs, and this page summarizes it in plain words. If a lender, an appraiser, or a contract depends on a figure, use the published standard, not this summary.
  • Fannie Mae's Selling Guide (B4-1.3-05, Improvements Section of the Appraisal Report) requires appraisers to follow the standard. Freddie Mac's Seller/Servicer Guide (Bulletin 2023-15) requires it for appraisals effective on or after November 2, 2023.