| Reference | Acres (ac) | Square furlongs (sq fur) |
|---|---|---|
| A sheet of A4 paper | 0.000015412 ac | 0.0000015412 sq fur |
| A tennis court | 0.0644451 ac | 0.00644451 sq fur |
| A football pitch | 1.76433 ac | 0.176433 sq fur |
| One hectare | 2.47105 ac | 0.247105 sq fur |
| Central Park, New York | 842.629 ac | 84.2629 sq fur |
The acre is a unit of area equal to 4046.86 square metres, or 43,560 square feet. Its symbol is ac. It is the principal land measure of the English-speaking world, and unlike most imperial units it survives not as a nostalgic remnant but as a legally operative measure in the United States, Britain, Ireland, India and much of the former British empire.
Its shape explains its odd size. The acre was defined as one chain by one furlong: a strip 22 yards wide and 220 yards long, which is the area a team of oxen was reckoned to plough in a day. Ploughing with oxen favoured long narrow strips because turning the team was slow, so the unit encodes a working method rather than a geometric idea, and that is why it is not a square number of anything.
The chain that fixes its width had a second life in surveying. Gunter's chain of 22 yards divides a mile into eighty parts and an acre into ten square chains, which made land arithmetic tractable before decimal calculation. American public land was surveyed on this basis, so a section is one square mile and contains exactly 640 acres, a quarter section 160, and those numbers still shape property boundaries across the western United States.
In practice the acre is used wherever land changes hands. Farms, building plots, forestry blocks and conservation holdings in the United States and Britain are advertised in acres, agricultural yields are quoted in bushels or tonnes per acre, and pesticide and fertiliser application rates are printed per acre on the label. In India and Pakistan the acre coexists with local units, and a Punjabi killa is an acre by another name.
A useful mental picture is that an acre is a little smaller than an association football pitch, which runs to about 1.76 acres. A city block in a gridded American town is often between two and five acres. A hectare, the metric competitor, is about two and a half acres, so converting between the two never gives a round number in either direction.
There is no single legal acre worldwide. The international acre, based on the international yard of exactly 0.9144 metres, is used almost everywhere; the United States survey acre, retired at the end of 2022, was larger by about two parts in a million, which matters only when re-tracing old survey lines across long distances.
One acre equals 4046.86 square metres, 43,560 square feet, 4840 square yards, or about 0.4047 hectares.
The square furlong is a unit of area equal to ten acres, or 40,468.6 square metres. Its symbol is sq fur. A furlong is 220 yards, an eighth of a mile, so a square furlong is a hundred square chains and sixty-four of them make a square mile. It is one of the tidiest relationships in the imperial system and one of the least used.
The furlong is older than the acre and gave it its shape. The name means furrow long, and it was the distance an ox team was expected to plough before resting and turning. In the open fields of medieval England a furlong was a block of cultivated strips as well as a length, so the word described a place and a measurement at the same time, and the acre was defined as one furlong by one chain because that was what a day's ploughing produced.
That history explains why the square furlong is arithmetically neat but practically absent. Land was allotted in strips and acres, not in blocks a furlong on a side, so the square form never had a use even when the linear one was in daily service. A surveyor who needed ten acres wrote ten acres, and the square furlong was left as an entry in tables.
The length itself has survived by a peculiar route. Horse racing kept it, and races in Britain, Ireland, the United States, Australia and much of Asia are still described in furlongs: a six-furlong sprint, a mile and a half meaning twelve furlongs. Nothing else in modern life uses the unit, but racing uses it constantly, which keeps the word familiar to people who would struggle to say how long it is.
As an area it is a useful size to picture even so. Ten acres is a small farm, a large public park, about fourteen football pitches, or a city block of generous proportions. New York's Central Park covers roughly eighty-four square furlongs. The unit is large enough that anything measured in it is being described as a place rather than a property.
Converting is cleanest through the acre. Multiply by ten for acres, then by 0.4047 for hectares, or use the direct factor of 4.047 hectares to the square furlong. Going the other way, a hectare is about a quarter of a square furlong, which is close enough for rough comparison of old and new land records.
One square furlong equals 10 acres, 100 square chains, about 40,469 square metres, or about 4.047 hectares.