The acre began as a day's ploughing and has no fixed shape
Medieval farmers thought of an acre as the land one man could plough in a day behind a team of eight oxen. The classic shape was a long strip, one furlong by one chain, 660 by 66 feet. Today any patch of 43,560 square feet counts, whether square, round or ragged.
A furlong literally meant a furrow long, and the original acre was forty perches long and four wide. Modern acres all contain 4,840 square yards, or one six-hundred-fortieth of a square mile, about 4,047 square metres or roughly 40 percent of a hectare. Squared off, an acre measures about 63.6 metres, or 208 feet 9 inches, on each side.
The exact size depends on which yard you use. In 1959 the United States and five Commonwealth countries fixed the international yard at exactly 0.9144 metre, but American officials kept the older survey foot for land surveying, based on the 1893 Mendenhall Order's rule that a metre equals exactly 39.37 inches. The result was two American acres. The survey version is about 4,046.872 square metres, while the international acre works out to 4,046.8564224, although no treaty actually defines it.
The gap is just 160 square centimetres, about a quarter of a sheet of printer paper, and land is almost never measured finely enough to notice. Even so, in October 2019 two federal agencies, NIST and the geodetic survey office, declared that the supposedly temporary survey units would be retired once 2022 ended.
The acre lives on mainly by habit. The UK dropped it as a statutory unit in 2010, and Australia, New Zealand and South Africa did so decades ago, but it may still appear alongside hectares. Ireland legally uses hectares, yet farmers there still talk in acres, the British statute kind rather than the larger old Irish acre. Indian farmland may be measured in bighas, acres or hectares, and in Sri Lanka an acre is often split into 4 roods or 160 perches. In the continental United States, Puerto Rico's cuerda, 0.97 acres, is sometimes nicknamed the Spanish acre.
Source: Acre
