Square Footage Calculator

Measure each rectangular space in feet and inches, add up to three of them, and the calculator returns the total square footage, the same area in square meters, and, once you enter a price per square foot, the cost at that rate. The feet-and-inches fields take measurements the way a tape gives them, so 12 ft 6 in goes in as 12 and 6 rather than being converted to 12.5 in your head. Read the primary figure as a plain geometric total of the rectangles you entered. Whether it matches the number in a listing, an appraisal or a county record depends on which measuring convention produced those figures, and the difference between conventions is what most of this page covers.

How the total is worked out

Each side is first converted to decimal feet, because an inch is a twelfth of a foot by definition:

decimal feet = feet + inches ÷ 12

Each area is then its length times its width, and the total is the sum of every area you switched on:

total = length₁ × width₁ + length₂ × width₂ + length₃ × width₃

Adding rectangles is not a shortcut peculiar to this page. The ANSI Z765 standard computes the finished square footage of a house the same way, level by level, as the sum of the finished areas on each level measured at floor level. The metric figure multiplies the total by 0.09290304, the exact number of square meters in one square foot, and the cost output multiplies the total by whatever rate you enter. On rounding, ANSI Z765 reports square footage to the nearest whole square foot and metric results, for which the standard's term is floor area, to the nearest 0.1 square meter; the calculator keeps one decimal on the square-foot total so small changes stay visible.

Two worked examples

Take a single room measuring 12 ft 6 in by 10 ft 9 in. The sides convert to 12 + 6 ÷ 12 = 12.5 ft and 10 + 9 ÷ 12 = 10.75 ft, and the area is 12.5 × 10.75 = 134.375 square feet, which reports as 134 sq ft under the ANSI rounding convention. In metric it is 134.375 × 0.09290304 = 12.484, or 12.5 m² to the nearest tenth.

An L-shaped space shows why the calculator takes several areas. Split the L into a 20 ft by 14 ft rectangle and an 8 ft by 6 ft one: 20 × 14 = 280 and 8 × 6 = 48, so the total is 328 square feet, or 30.47 m². At $6 per square foot for flooring, that L-shaped job prices out at 328 × 6 = $1,968, which is the arithmetic the cost field performs.

A foot that took until 1959 to settle

The foot began as a body-based unit. The Roman pes ran to roughly 296 mm by modern estimates and was divided into 12 unciae, the ancestors of inches. Medieval England anchored the chain in seed corn instead: an English statute of around 1300, whose exact year and royal sponsor the surviving sources disagree on, ordained that three grains of barley, dry and round, make an inch, that twelve inches make a foot, and that three feet make the unit that became the yard. The modern American foot, by contrast, has been defined in metric terms for longer than most people would guess. Since 1893 the National Bureau of Standards and its predecessor derived the yard from the international meter, with the pre-1959 American yard set at 3600/3937 of a meter. Other English-speaking countries carried slightly different values, and per the 1959 Federal Register notice those minor discrepancies had grown troublesome for groups doing very accurate measurement. On July 1, 1959, the national standards laboratories of the United States, the United Kingdom, Canada, Australia, New Zealand and South Africa adopted the international yard of exactly 0.9144 meters, which makes the foot exactly 0.3048 meters. The change shrank the American yard by just two parts per million, and it fixed the inch at exactly 25.4 millimeters, a value American industry had already worked to since a 1933 standard. Squaring the 1959 foot produces this page's conversion factor, 0.3048 × 0.3048 = 0.09290304, listed as exact in NIST Special Publication 811. One relic survived: the old 1200/3937 definition lived on as the U.S. survey foot for geodetic survey data until NIST and NOAA retired it at the end of 2022, and since January 1, 2023 the international foot is the only foot.

How American houses came to be measured

Commercial landlords standardized first: the foreword to ANSI Z765 records that in 1915 the Building Owners and Managers Association developed a standard method for measuring floor area in office buildings, and that before Z765 was first adopted in 1996 no national standard existed in the United States for measuring the square footage of single-family houses. The same foreword preserves the origin story: in April 1994 the National Association of Home Builders, at the request of its Greater New Orleans association and other builder members, commissioned the NAHB Research Center, renamed Home Innovation Research Labs in 2013, as secretariat, and the drafting committee first met on November 22, 1994. The method measures a detached house to the exterior finished surface of the outside walls, or to the centerlines between attached houses, so the walls themselves sit inside the number. Finished area is split into above-grade and below-grade figures, and a level counts as above-grade only when it is entirely above grade, which places a walk-out basement on a sloped lot in the below-grade figure however much daylight it gets. Garages and unfinished areas are never included, openings to the floor below, such as a two-story foyer, are excluded beyond the stair area, and ceiling height gates everything: finished area needs at least 7 ft of headroom, 6 ft 4 in under beams and ducts, and where the ceiling slopes at least half of the room's finished area must reach 7 ft, with nothing under 5 ft counted at all. The standard is voluntary, and it does not apply to apartment or multifamily buildings. Its enforcement is contractual rather than legal: Fannie Mae announced in December 2021 that appraisals with effective dates of April 1, 2022 or later on loans sold to it must follow ANSI Z765-2021, standardizing how appraisers report gross living area.

Why two square-footage numbers rarely match

Put a tape measure in the hands of an agent, an appraiser and a tax assessor and the same house can return three totals. Interior measurement versus the ANSI exterior-surface method shifts the figure by the wall thickness all the way around the floor plan. Some practitioners fold a finished basement into the headline number where ANSI reports it separately, and the standard's own commentary concedes that its treatment of garages allows practitioners to apply local customs. Since the Fannie Mae mandate the appraisal figure is the one most likely to follow ANSI Z765 to the letter, while listing and assessor figures may follow older habits. None of this makes the other numbers dishonest: a house with 2,320 sq ft above grade and a 600 sq ft finished basement is exactly that, not a 2,920 sq ft house and not 2,320 sq ft with the basement ignored.

Measuring a room in practice

For an ordinary room, run the tape along the floor, where ANSI Z765 takes its measurements, and record length and width in feet and inches straight into the fields. A stepped or L-shaped room splits into rectangles: measure each one, enter each as its own area, and let the calculator add them. When pricing a per-square-foot job, measure what the material must cover rather than trusting a listed figure, and keep basement space in a separate calculation if you want a total comparable to an ANSI-style above-grade number. This page deliberately stops at rectangles. For circles, triangles, trapezoids and other shapes the site's area calculator handles the geometry, and its square-foot output can be added to a rectangle total from here.

Assumptions and limits

Every area is treated as a true rectangle measured in international feet of exactly 0.3048 meters; the deprecated U.S. survey foot of 1200/3937 meters appears nowhere in the math, and inches convert exactly as twelfths. The metric conversion multiplies by 0.09290304 with no rounding until display, where the square-foot total shows one decimal and the square-meter figure two. The cost output is a plain multiplication of area by rate and knows nothing of waste allowances, minimum charges or tiered pricing. Above all, the result is a measuring aid, not an appraisal: the calculator cannot see grade lines, ceiling heights or wall finishes, so it cannot apply the ANSI rules described above, and a figure that carries legal or valuation weight needs to come from a professional following the applicable standard.

Frequently asked questions

Does a finished basement count in square footage?

Under the ANSI Z765 standard it is not erased but reported separately: a level counts as above-grade finished area only when it sits entirely above grade, so a finished basement, including a walk-out on a sloped lot, lands in the below-grade figure. Listings often fold the two together, which is one of the commonest reasons a listed figure exceeds an appraisal's gross living area.

Do garages count toward square footage?

Not under ANSI Z765, which excludes garages and unfinished areas from finished square footage entirely. The standard's commentary does let local custom decide whether a garage under the main body of the house counts as unfinished square footage, and a county record or an older listing measured outside the standard may fold one in, so that difference alone can put a wide gap between two published figures.

Is square footage measured from inside or outside the walls?

ANSI Z765 measures a detached house to the exterior finished surface of the outside walls, or to the centerlines between attached houses, so the wall thickness is inside the number. Measuring room by room from the inside gives a smaller total. The standard does not apply to apartment or multifamily buildings, so it does not direct a condo owner to measure to the exterior walls.

How do you convert square feet to square meters?

Multiply by 0.09290304, which is exact: the 1959 international agreement fixed the foot at 0.3048 meters, and squaring that gives 0.09290304 square meters per square foot. NIST lists the factor as exact in Special Publication 811. Going the other way, divide by the same number. A 120 square foot room is 11.15 square meters.

How is square footage counted under sloped ceilings?

ANSI Z765 counts finished area only where the ceiling reaches at least 7 feet, or 6 feet 4 inches under beams and ducts. In a room with a sloped ceiling at least half of the finished area must have a ceiling of 7 feet or more, and no part lower than 5 feet may be included at all. That is why a finished attic contributes less area than its floor suggests.

Why is the appraisal square footage different from the listing?

Fannie Mae began requiring appraisers to follow ANSI Z765-2021 for appraisals with effective dates of April 1, 2022 or later on loans it buys, while agents and tax assessors are free to measure differently. Interior versus exterior measurement, basements folded into the total and local customs on garages each push the numbers apart, so two careful measurements of the same house can disagree without either being wrong.