Comparison of Area units

Unit	SI
1 ca	1 m2
1 a	100 m2
1 ha	10,000 m2
100 ha	1 km2
non-SI comparisons
non-SI	metric
0.3861 sq mi	1 km2
2.471 acre	1 ha
107,639 sq ft	1 ha
1 sq mi	259.0 ha
1 acre	0.4047 ha

The hectare is a unit of area, defined as 10,000 square metres, and primarily used in the measurement of land. In 1795, when the metric system was introduced, the are was defined as being 100 square metres and the hectare ('hecto-' + 'are') was thus 100 ares or 1/100 km². When the metric system was rationalised in 1960 with the introduction of the International System of Units (SI), international recognition of the are was withdrawn, though the hectare continued to be recognised as a "unit of measure that may be used with SI".¹

Contents 1 History 2 The units 2.1 Centiare 2.2 Are 2.3 Decare 2.4 Hectare 3 Conversions 4 Visualising a hectare 4.1 Rugby Union 4.2 Cricket Fields 5 See also 6 References 7 External links

History

The metric system of measure was first given 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:²

• The metre for length
• The are (100 m²) for area [of land]
• The stere (1 m³) for volume of firewood
• The litre (1 dm³) for volumes of liquid
• The gram for mass

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 General Conference on Weights and Measures (GCPM). At the first meeting of the GCPM in 1889 when a new standard metre, manufactured by Johnson Matthey & Co of London³ 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 did not receive international recognition. The International Committee for Weights and Measures (CIPM) makes no mention of the are in the current (2006) definition of the SI, but classifies the hectare as a "Non-SI unit accepted for use with the International System of Units"⁴

In 1972, the European Economic Community (EEC) passed directive 71/354/EEC ⁵, which catalogued the units of measure that might be used within the community. The units that were catalogued replicated the recommendations of the GCPM, supplemented by a few other units including the are (and implicitly the hectare) whose use was limited to the measurement of land.

The units

The names centiare, decare and hectare are derived by adding the standard metric prefixes to the original base unit of area, the are.

Centiare

The centiare (symbol ca) is equal to one square metre. It is sometimes used in combination with the are, e.g. "10 ares 12 centiares", instead of the standard metric format of 10.12 are or 1012 centiare.^{citation needed}

Are

The are (symbol a, pronounced /ˈɛər/ or sometimes /ˈɑr/) is a unit of area, equal to 100 square metres (10 m × 10 m), used for measuring land area. It was defined by older forms of the metric system, but is now outside of the modern International System (SI).⁶

It is commonly used in many countries (in particular, in Indonesia and in French-, Portuguese-, Slovakian-, Czech-, Polish-, Dutch-, and German-speaking countries, to measure real estate.

The following units are identical to the are, but have different names:

• In Russia and other former Soviet Union states, the "sotka" (сотка 'hundredth') is used to describe the size of suburban dacha or allotment garden plots or small city parks where the hectare would be too large.

Decare

The decare (symbol daa) is derived from deka, the prefix for 10 and are, and is equal to 10 ares or 1000 square metres. It is not widely used, except in Norway, Bulgaria and Botswana.^{citation needed} The following legacy units of land measure have been defined as being equal to one decare:^{citation needed}

• Stremma in Greece
• Dunam, dunum, donum, or dönüm in Israel, Jordan, Lebanon, and Turkey

Hectare

The hectare (symbol ha, pronounced /ˈhɛktɛər/) is widely used throughout the world⁸ and is the legal unit of measure in domains concerned with land ownership, planning, and management, including law (land deeds), agriculture, forestry, and town planning throughout the European Union⁹. The United States, Burma, and to some extent Canada instead use the Imperial measurement unit of area, the acre (0.404686 ha)^{citation needed}.

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.^{citation needed}

In many counties the advent of metrication has meant that national units of measure were either redefined or clarified in terms of metric units. The following legacy units of area has been redefined as being equal to one hectare:¹⁰

• Jerib in Iran
• Djerib in Turkey
• Gong qǐng in mainland China
• Manzana in Argentina
• Bunder in The Netherlands

Conversions

Metric and Imperial Comparisons
Units	Symbol	Metric Equivalents			Imperial Equivalents
centiare	ca		1 m2	0.01 a	1.19599 sq yd
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 sq mi

The most commonly used units are in bold.

One hectare is also equivalent to:

• 2.4710439 U.S. survey acres
• 15 mǔ (Chinese)
• 0.15 qǐng
• 10 dunam or dönüm (Middle East)
• 10 stremmata (Greece)
• 6.25 rai (Thai)
• ≈ 1.008 chō (Japanese)
• 7.47 bigha (Bangladesh, India, Nepal)

Visualising a hectare

Visitors to London can visualize a hectare by visiting Trafalgar square. However a number of sports fields also have an area of about a hectare.

Rugby Union

Rugby Union is played in many Commonwealth countries and is also popular in, amongst others, France, Ireland, Italy and Argentina. The posts on an international rugby union field are placed on the goal line up to 100 metres apart. Behind the goal line is the dead-ball area (which is also a playing area). This area extends between 10 and 22 metres behind the goal line, giving a maximum length of 144 metres for the playing area. The maximum width of the pitch is 70 metres, giving a maximum playing area of 10,080 square metres or 1.008 hectares.

Cricket Fields

The laws of cricket do not stipulate the size of a cricket ground. The groundsman will usually prepare a different pitch for each match by rotating the playing batting area. However, since the boundary remains the same from match to match, the field’s area remains the same.¹² Although the field in this picture is a 1.55 ha, the fields used in village cricket are often smaller than those used in first-class or international matches and as a result a number are often just under a hectare.

See also

• 1 E+4 m² for further comparisons
• conversion of units
• hecto-
• Hectometre
• Orders of magnitude

References

External links

• Official SI website: Table 6. Non-SI units accepted for use with the International System of Units
• Agricultural Measurements and Conversions

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