Square Feet

Which of these conversions are exact, and which are rounded decimals

Most conversion tables print everything to three decimals and let you assume they are all equally solid. They are not. A short chain of these values is exact by law, and everything running the other direction is a rounded approximation of an irrational-looking reciprocal.

The practical consequence is directional. Converting square feet into metric loses nothing. Converting metric back into square feet with a three-decimal factor introduces a small error that compounds on large areas, which is why surveyors carry more digits than any consumer table prints. The guides on square meters, square yards and square inches each work one of these through.

The United States retired one of its two feet on 31 December 2022

For most of the twentieth century the country ran two slightly different feet at once. The international foot, defined in 1959 as exactly 0.3048 meters, and the US survey foot, defined as exactly 1200/3937 meters, or about 0.3048006 meters. The gap is 2 parts per million.

NIST and the National Geodetic Survey published a joint final determination in the Federal Register on 5 October 2020. It states that use of the US survey foot would be deprecated on 31 December 2022, and that from 1 January 2023 the international foot definition supersedes it in all applications.

The difference on one acre is 0.000004 acre, which the Federal Register puts at about 0.17 square feet, or roughly 25 square inches. Over a mile of surveyed distance the two feet diverge by about 0.01 feet, an eighth of an inch. Nobody buying flooring will ever notice. It matters for boundary surveys, state plane coordinate systems and legal descriptions of land, which is exactly where the deprecation was aimed.

What actually counts as square footage in a house, and who decides

There is a written American National Standard for this, and since April 2022 most mortgage appraisals have been required to follow it. Fannie Mae's selling guide names it directly: appraisers must follow ANSI Z765-2021, Square Footage, Method for Calculating. Compliance became mandatory for applicable appraisals with an effective date on or after 1 April 2022.

The rules are more specific than most people expect, and several of them explain discrepancies that otherwise look like errors.

1. Ceiling height. Finished areas must have a ceiling at least 7 feet high. Under a sloping ceiling, at least 50 percent of the room's finished square footage must have 7 feet of headroom, and no part of the counted area may be under 5 feet. This is why a finished attic often adds far less than its floor area.
2. Stairs. A staircase is included in the square footage of the floor it descends from. It is counted once, on the upper floor, not on both.
3. Below grade. Any space partially or completely below grade must be reported as below-grade area. Fannie Mae treats a level as below grade if any portion of it is below grade, regardless of finish quality or window area. A finished walk-out basement is reported separately from the above-grade total and adjusted on its own line, which is the single biggest reason listing square footage and appraisal square footage disagree.
4. Precision. Measurements must be reported to the nearest tenth of a foot, taken from exterior dimensions.
5. Scope. The standard cannot be used for apartment or multifamily buildings. It applies to single-family dwellings including townhouses, rowhouses, detached houses and manufactured homes. If state law mandates a different standard, the report must say which one and how it was applied.

Three benchmarks people use to picture a square footage, checked against the source

The White House figure needs the same care. The 55,000 square feet from the National Park Service covers the Executive Residence and explicitly excludes the wings, and the East Wing on its own is roughly 60,000 square feet. Whitehouse.gov publishes room counts, 132 rooms, 35 bathrooms and 6 levels, but no floor area at all. How Many Square Feet Is the White House? keeps those apart.

On homes, the useful comparison is that the median new build in 2025 was 2,142 square feet on a 8,855 square foot lot. That means the house covers under a quarter of the ground it sits on, and the whole property is about a fifth of an acre. How Big Is 1000 Square Feet? is the guide for scaling that down.

All 24 square footage guides, grouped by the kind of question

The archive above runs newest first. These groups run from the unit itself out to the benchmarks people use to picture it. Anything that turns into arithmetic hands off to the square feet calculator, and material quantities are covered further by the home calculators.

Frequently asked questions

How many square feet are in an acre?

Exactly 43,560. That relationship is defined in feet, so it did not change when the US survey foot was deprecated. What changed is the acre expressed in square meters, from 4,046.872609874 under the old foot to 4,046.8564224 under the current one.

Did the definition of a foot change?

The United States retired one of its two feet. The US survey foot was deprecated on 31 December 2022, leaving the international foot at exactly 0.3048 meters. The two differed by 2 parts per million, about 0.01 feet over a mile.

Which square footage conversions are exact?

One square foot to 0.09290304 square meters, one square yard to nine square feet, 144 square inches to a square foot, 43,560 square feet to an acre and 640 acres to a square mile are all exact by definition. Square meters back to square feet is a rounded 10.764.

Why does my house have three different square footages?

Because listings, tax records and appraisals often count different space. Since 1 April 2022 most mortgage appraisals must follow ANSI Z765-2021, which reports any partially below-grade level separately and requires seven feet of ceiling height for finished area.

Does a finished basement count toward square footage?

Not toward the above-grade total. Under the standard Fannie Mae requires, any level with any portion below grade is reported as below-grade area on its own line. It still adds value, particularly when the finish quality is high.

Is a football field one acre?

No. An NFL field measures 360 by 160 feet including end zones, which is 57,600 square feet or 1.32 acres. The 100-yard field of play alone is 48,000 square feet, or 1.10 acres. Neither figure is one acre.

Every figure on this page, and where it came from

• The survey foot deprecation. Federal Register, 5 October 2020, final determination, for the 31 December 2022 date, the 2 parts per million figure and both acre and square mile values, and NIST on the US survey foot.
• Exact conversions. NIST Special Publication 811, Appendix B.9, where exact factors are printed in bold, and NIST Handbook 44, Appendix C, general tables of units of measurement.
• The measurement standard. Fannie Mae Selling Guide B4-1.3-05, naming ANSI Z765-2021, and the standardized property measuring guidelines fact sheet for the ceiling height, staircase, below-grade and precision rules. The announcement SEL-2021-11 sets the 1 April 2022 compliance date.
• Home sizes. US Census Bureau, Characteristics of New Housing, 2025 data published 1 July 2026.
• Field dimensions. Official Playing Rules of the National Football League, Rule 1, Section 1, Article 1: 360 feet by 160 feet.
• White House. National Park Service planning documentation for the 55,000 square foot Executive Residence figure, and whitehouse.gov for the 132 rooms, 35 bathrooms and 6 levels.

Where to go next

Run an area for any shape with the square feet calculator. For material quantities, the home calculators cover mulch, topsoil, paint, asphalt and cubic yards, and the pool collection handles the other big backyard measurement job. Everything else is on the Waldev homepage.