hectare
Areaexact by definition
The hectare is exactly 10,000 square metres, the area of a square 100 m on a side. It is not an SI unit but is accepted for use with SI, and it is the standard unit of land area in most of the world. The factor is exact by definition.
Watch out: A hectare is about 2.471 acres, so the two are not casually interchangeable: a "10 hectare" parcel is nearly 25 acres.
| 1 ha | 10,000 m² |
The name is the prefix "hecto" attached to the are, a unit of 100 m² created in the French metric legislation of the 1790s. The are itself has all but disappeared, leaving the hectare as an orphan hundred of a unit nobody uses.
About the hectare
A hectare is easy to picture because it is a square 100 m on a side. That is roughly a large city block, about two and a half acres, or a little under one and a half association football pitches. Its convenience is that it links straight to the rest of the metric system: 100 ha is one square kilometre, and one hectare of rain at 10 mm depth is exactly 100 m³ of water. Agricultural yields, forestry inventories, fertiliser rates and conservation areas are all quoted per hectare for that reason, since converting to a volume or a mass needs no lookup table.
The unit it was built from has almost vanished. The are, 100 m², was intended as the metric replacement for small land parcels, and the hectare was simply a hundred of them. In practice the are was too small for farmland and too large for gardens, so it faded while its multiple thrived. A few countries kept relatives of it in property records, and the centiare, one square metre, still appears in some French land documents.