| Reference | Acres (ac) | Square kilometers (km²) |
|---|---|---|
| A sheet of A4 paper | 0.000015412 ac | 0.00000006237 km² |
| A tennis court | 0.0644451 ac | 0.0002608 km² |
| A football pitch | 1.76433 ac | 0.00714 km² |
| One hectare | 2.47105 ac | 0.01 km² |
| Central Park, New York | 842.629 ac | 3.41 km² |
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 kilometre is a unit of area equal to one million square metres, or one hundred hectares. It is written km². It is the area of a square one kilometre on a side, and it is the unit in which countries, cities, lakes, forests and administrative districts are measured almost everywhere in the world.
Its main work is geography. Monaco covers about 2 square kilometres, Manhattan about 59, Greater London around 1572, and France some 551,500. Because the unit spans that whole range without changing, it makes very different places directly comparable, which is why atlases, censuses and statistical yearbooks settle on it rather than on hectares above and square metres below.
Population density is its most quoted derived figure. Dividing inhabitants by square kilometres produces a number that is meaningful at every scale: a few per square kilometre across Mongolia or the Australian interior, a few hundred across most of western Europe, and eight thousand or more in Singapore and central Paris. The same arithmetic done in square miles gives figures 2.6 times larger, which is a common source of confusion in translated sources.
It is worth being careful about the phrasing. A square kilometre is one kilometre by one kilometre. An area described as two kilometres square is two kilometres on each side and therefore four square kilometres, not two. The distinction sounds pedantic until a fire, a flood or a proposed development is being described, at which point the difference is a factor of the side length itself.
Environmental and infrastructure figures live here. Rainfall converts neatly: one millimetre falling on one square kilometre is a thousand cubic metres, or a million litres of water. A utility-scale solar farm producing a gigawatt at peak needs roughly twenty square kilometres of ground. Wildfire extent, glacier retreat, deforestation and sea-ice cover are all reported in square kilometres because that is the resolution at which satellites usefully see them.
Against imperial units it faces the square mile, which is 2.59 square kilometres. Because a square mile also equals 640 acres, the three units chain together awkwardly, and any figure that has passed through more than one of them is worth checking. Below the square kilometre, land measurement passes to the hectare, and one square kilometre is exactly one hundred of them.
One square kilometre equals 1,000,000 square metres, 100 hectares, about 0.386 square miles, or about 247.1 acres.