acre
Areaexact by definition
The acre is exactly 43,560 square feet, or 4,046.8564224 square metres. It is one chain by one furlong, that is 66 ft by 660 ft, and 640 acres make a square mile. The factor is exact by definition, following from the exact international foot.
Watch out: The US survey acre, based on the survey foot, is about 4 parts per million larger. It is negligible for a farm and not for a legal boundary description, and the United States retired the survey foot at the end of 2022.
| 1 ac | 4,046.8564 m² |
The acre is commonly explained as the area a man with a yoke of oxen could plough in one day, which is why it is a long thin rectangle rather than a square: turning the team was slow, so the furrow ran a full furlong before the plough turned. Its width, the chain of 66 ft, comes from Gunter's surveying chain of 1620, a 100-link chain designed so that ten square chains make exactly one acre.
About the acre
The acre is the clearest surviving fossil of medieval agriculture in modern law. Its shape is the giveaway: 66 ft by 660 ft, a ratio of ten to one, because a plough team wasted time turning and so ploughed the longest furrow the field allowed. That furrow length, the furlong, and that width, the chain, are still the acre's factors today. Nothing about the number 43,560 makes sense until you know it is 66 times 660.
Edmund Gunter's chain of 1620 is what carried it into surveying. His chain was 66 ft long and divided into 100 links, a deliberate hybrid: it measured in decimal for arithmetic but its length was chosen so that 10 square chains equal exactly 1 acre and 80 chains equal exactly 1 mile. American land records still use chains and links, and section corners across the Midwest were set with them. The result is a grid where a section is 640 acres, a quarter section is 160 acres, and the standard homestead grant of the Homestead Act was that quarter section, sized to what a family could work.