|1 ca||1 m2||1 m2|
|1 a||1 dam2||102 m2|
|1 ha||1 hm2||104 m2|
|100 ha||1 km2||106 m2|
|0.00386102 sq mi||1 ha||104 m2|
|2.471 acre||1 ha||104 m2|
|107,639 sq ft||1 ha||104 m2|
A hectare (symbol ha, pronounced /ˈhɛktɛər/) is a unit of area equal to 10,000 square metres (107,639 sq ft), or one square hectometre (100 metres, squared), and is commonly used for measuring land area.
Its base unit, the are, was defined by older forms of the metric system, but is no longer part of the modern metric system known as the International System of Units (SI). The International Committee for Weights and Measures (CIPM) classifies the hectare as a unit that is accepted for use with SI.
The metric system of measure was first put onto a legal basis in 1795 by the French Revolutionary government. The law of 18 Germinal, Year III (7 April 1795) defined five units of measure:
Although the law defined the length of the metre, there was no practical way of accurately measuring the metre (and hence the are) until 1799 when the first standard metre was manufactured and adopted.
The standard metre remained in the custody of successive French governments until 1875 when, under the Convention of the Metre, its supervision passed into international control under the auspices of the GCPM. At the first meeting of the GCPM in 1889 when a new standard metre, manufactured by Johnson Matthey & Co of London was adopted, the are and hectare were automatically redefined.
In 1960, when the metric system was updated as the International System of Units (SI), the are had not received international recognition. The CGPM made no mention of the are in its publication, but gave the hectare a qualified recognition. In 2006 the hectare was recognised as a "Non-SI units accepted for use with the International System of Units"
In 1972, the European Economic Community (EEC) passed directive 71/354/EEC  which catalogued the units of measure that might be used within the community. The units that were catalogued replicated the recommendations of the CGPM, supplemented by a few other units including the are (and implicity the hectare) whose use was limited to the measurement of land.
The hectare is widely used throughout the world and is the legal unit of measure in domains concerned with land ownership, land planning, and management, including law (land deeds), agriculture, forestry, and town planning throughout the European Union, though the United States, Myanmar, and to some extent Canada instead use the Imperial measurement unit of area - the acre (0.404686 ha). Some of the former Ottoman countries and Norway and Botswana use the decare, one tenth of a hectare..
Even in countries that have undergone a general conversion from traditional English measurements to metric measurements (e.g. Canada), legal descriptions relating to land, which frequently use the acre, have not been converted, as doing so would require a resurvey of the land.
In many counties the advent of metrication has meant that national units of measure were either redefined or clarified in terms of metric unit. The following legacy units of area has been redefined as being equal to one hectare:
|Metric and Imperial Comparisons|
|Units||Symbol||Metric Equivalents||Imperial Equivalents|
|centiare||ca||1 m2||0.01 a||1.19599 yd2|
|are||a||100 ca||100 m2||0.01 ha||3.95369 perches|
|decare||daa||10 a||1,000 m2||0.1 ha||0.98842 roods|
|hectare||ha||100 a||10,000 m2||0.01 km2||2.47105 acres|
|square kilometre||km2||100 ha||1,000,000 m2||0.38610 miles2|
The most commonly used units are in bold.
One hectare is also equivalent to: