Acre

The acre is a unit of land area used in the imperial and US customary systems, traditionally defined as the area of one chain by one furlong (66 by 660 feet), which is exactly equal to 10 square chains, 1⁄640 of a square mile, 43,560 square feet, approximately 4,047 square meters, or about 40% of a hectare.

Definition and Equivalents

The most common variant of the acre in use today is the international acre. In 1959, the United States and five countries of the Commonwealth of Nations defined the international yard as exactly 0.9144 meters. Consequently, the international acre is exactly 4,046.8564224 square meters.

In the United States, a slightly different variant known as the US survey acre was historically used, primarily in land surveying. The US survey acre is based on the US survey foot, which is defined as exactly 1200/3937 meters. This makes the US survey acre approximately 4,046.872609874252 square meters. The difference between the international acre and the US survey acre is roughly two parts per million, or about four-thousandths of a square meter, which is negligible for most practical purposes. However, the US National Geodetic Survey and the National Institute of Standards and Technology officially deprecated the US survey foot at the end of 2022, making the international acre the standard across the United States.

An acre can also be visualized as a rectangular plot of land measuring one furlong (660 feet) long and one chain (66 feet) wide. In terms of other imperial units, one acre is equal to 4,840 square yards or 160 square rods.

History and Etymology

The word "acre" derives from the Old English word "æcer," which originally meant "open field" and is cognate with the Latin "ager" and the Greek "agros," both meaning "field." In medieval times, the acre was not a strictly defined unit of measurement but rather a practical concept: it represented the approximate amount of land that could be plowed by a yoke of oxen in a single day.

Because of this agricultural origin, the shape of an acre was typically long and narrow. The length of a furrow that a team of oxen could plow without resting was a furlong (from "furrow long"), which became standardized at 660 feet. The width of the plowed strip was a chain, or 66 feet.

The acre was formally standardized in England by various statutes. King Edward I, Edward III, and Henry VIII all passed laws defining the acre to ensure uniformity in taxation and land transactions. The Weights and Measures Act of 1878 officially defined the acre in terms of square yards, cementing its place in the imperial system.

Usage

The acre remains a standard unit of land measurement in several countries, most notably the United States, where it is widely used in agriculture, real estate, and land development. In the United Kingdom, while the metric system is the official standard for most trade and commerce, the acre is still commonly used in informal contexts, particularly in agriculture and when describing the size of rural estates.

Other countries that historically used the British imperial system, such as Canada, Australia, and New Zealand, have largely transitioned to the metric system and use the hectare for official land measurement. However, the acre is still understood and occasionally used in these regions, especially by older generations or in specific agricultural contexts. In India, the acre is used alongside local units of measurement like the bigha and the cent, particularly in states that were under British influence.

In real estate, particularly in the United States, residential lot sizes are often expressed in fractions of an acre, such as a "quarter-acre lot." Larger tracts of land, such as farms, ranches, and national parks, are typically measured in acres or square miles.

Comparison with Other Units

The acre is frequently compared to the hectare, the standard unit of land area in the metric system. One hectare is defined as exactly 10,000 square meters. Therefore, one international acre is approximately 0.4047 hectares, and one hectare is approximately 2.471 acres.

When compared to larger imperial units, 640 acres make up one square mile. A standard American football field, including the end zones, covers an area of about 57,600 square feet, which is approximately 1.32 acres. Conversely, an acre is roughly equivalent to 75% of a standard American football field. In urban planning, city blocks vary in size, but a standard city block in many US grid systems is often around 2.5 to 3 acres.