| Reference | Acres (ac) | Deciares (da) |
|---|---|---|
| A sheet of A4 paper | 0.000015412 ac | 0.006237 da |
| A tennis court | 0.0644451 ac | 26.08 da |
| A football pitch | 1.76433 ac | 714 da |
| One hectare | 2.47105 ac | 1000 da |
| Central Park, New York | 842.629 ac | 341000 da |
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 deciare is a unit of area equal to one tenth of an are, or ten square metres. Its symbol is da. It completes the decimal ladder that the French metric law of 1795 built around the are, alongside the centiare below it and the hectare above, but unlike either of those it never found a job and is today the least used member of the family.
Ten square metres is an easily pictured quantity. It is a small bathroom, a single bedroom in a modest flat, or four fifths of a car parking space. It is the floor of a garden shed, the area a market trader is allotted at a fair, or the footprint of a large dining table with chairs pushed back. Nothing about the size is obscure; only the name is.
The reason it failed is structural. Land registration adopted a three-column system running hectare, are and centiare, each step a factor of one hundred, which mirrors how areas actually behave: squaring a tenfold change in length produces a hundredfold change in area. A tenth of an are sits between two columns and belongs to neither, so no register ever needed a place for it.
Its symbol makes matters worse. In the International System, da is the prefix symbol for deca, so a reader meeting da in a technical document is far more likely to take it as the start of a compound like dam or daN than as a unit in its own right. The decare, ten times larger, is written daa, which does nothing to help.
Where the quantity does need naming, ordinary metric notation handles it without effort. Ten square metres is written 10 m², which is shorter than one deciare, needs no explanation, and sorts correctly beside every other figure on a plan or in a contract.
The unit therefore survives almost entirely in conversion tables and in teaching material that runs through the complete prefix ladder. Encountering it in a real document usually means the document is old, translated from an older source, or reproducing a table rather than making a measurement, and converting it to square metres at once is the safe response.
One deciare equals 10 square metres, 0.1 ares, 0.001 hectares, or about 107.6 square feet.