Hectare

• 1. Introduction

  ›

  The hectare sits quietly behind almost every land transaction on earth. When a farmer in New Zealand records the size of her paddock, when a city planner in Berlin zones a new district, when a forester in Canada files a timber lease, they all reach for the same unit: the hectare. One hectare is a square with 100-metre sides, covering exactly 10,000 square metres. That is a useful size for measuring land, not so small as to be tedious, not so large as to be abstract.

  Yet the hectare is also a quiet outlier. It is not an official unit of the International System of Units, the global framework that governs scientific measurement. It was never ratified into that system when the SI was established in 1960. It just stayed, widely used, legally recognised across the European Union, South Africa, New Zealand, Australia, Canada and the United Kingdom, and formally described as a unit whose use is expected to continue indefinitely.

  How did one non-official unit of measurement outlast every attempt to tidy it away? The answer reaches back to a revolutionary French decree from 1795, and to the peculiar gap between what measurement reformers want and what farmers and planners will actually use.

• 2. Born in Revolution

  ›

  The law of 18 Germinal, Year III, passed on the 7th of April 1795, was one of the French Revolutionary government's most ambitious acts. It gave the metric system its first legal foundation, defining five units of measure in a single stroke. Length got the metre. Volume of stacked firewood got the stere. Liquid volumes got the litre. Mass got the gram. And area of land got the are, defined as 100 square metres, one square decametre.

  The hectare appeared immediately alongside the are, not as a separate invention but as a compound unit built from the standard Greek prefix "hecto-" meaning one hundred, attached to the base unit "are". One hectare was 100 ares, which worked out to 10,000 square metres. The name itself was coined in French, from the Latin word area.

  This was a deliberate design. The metric system was built on powers of ten, so each unit in the area family was ten times the one before it: the centiare at one square metre, the deciare at ten, the are at one hundred, the decare at one thousand, the hectare at ten thousand. The names followed suit, drawn from the standard metric prefix ladder that also produced centimetres and kilometres.

  By 1960, when the metric system was rationalised into the International System of Units, the are was left out of official recognition. The International Committee for Weights and Measures made no mention of the are in the 2019 edition of the SI brochure. But the hectare was given a different status: a non-SI unit accepted for use with the International System, with its future described as expected to continue indefinitely.

• 3. The Unit That Stayed

  ›

  In 1972, the European Economic Community passed directive 71/354/EEC, cataloguing which units of measure member states could legally use. The EEC followed the recommendations of the international weights and measures body, but added a handful of further units, including the are and, implicitly, the hectare, specifically for measuring land. That carve-out for land measurement proved durable.

  The practical reason is scale. A square kilometre is too large for most agricultural and planning purposes. A square metre is too small. The hectare slots between them at exactly the right size for a farm field, a housing estate, or a city park. It contains 100 hectares per square kilometre, which makes mental arithmetic between the two scales straightforward.

  The United States and Myanmar, known also as Burma, chose a different path and continue to use the acre rather than the hectare. The acre is approximately 0.4047 hectares, or put the other way, one hectare is roughly 2.471 acres. In Japan, one hectare equals approximately 1.008 cho. In Egypt, one hectare corresponds to about 2.381 feddan. In Thailand, the equivalent is 6.25 rai. These local equivalences show how measurement systems overlap and translate, rather than one cleanly replacing another.

  The hectare is the only named unit of area accepted for use with SI units. Every other named area unit either belongs fully to the SI or sits outside it without that acceptance.

• 4. Legacy Units Redefined

  ›

  Across many countries, the spread of the metric system did not erase older land measures. Instead it redefined them, anchoring traditional units to exact metric values. Several units from different countries and legal traditions were set equal to exactly one hectare: the jerib in Iran, the djerib in Turkey, the gongqing in China, the manzana in Argentina, and the bunder in the Netherlands, though the bunder equivalence applied only until 1937.

  This pattern of redefinition allowed local legal and agricultural systems to preserve familiar names while connecting them to a universally understood measurement. A farmer in Iran who referred to land in jeribs was, after redefinition, speaking a unit exactly translatable to any metric context.

  The decare tells a parallel story. Equal to ten ares or 1,000 square metres, it is used in Norway and in the former Ottoman regions of the Middle East and Bulgaria. Several older land measures of similar size were redefined as exactly one decare: the stremma in Greece, the dunam or dönüm across the Balkans, Israel, Palestine, Jordan, Lebanon, Syria and Turkey, and the mal in Norway.

  In Mexico, a different convention took hold. Land area measurements are commonly expressed as combinations of hectares, ares, and centiares, written with hyphens separating each component. A parcel recorded as 1-21-00.26 ha would be read as one hectare, 21 ares, and 0.26 centiare, totalling 12,100.26 square metres. This notation reflects how granular land record-keeping can become, and how the full family of area units continues to function together.

• 5. The Are and Its Relatives

  ›

  The are family extends in both directions from its central unit. Below the are sits the centiare, equal to one square metre, used for the smallest cadastral divisions. The deciare, ten square metres, is rarely encountered in practice. Above the are, the decare bridges to the hectare, and the hectare bridges to the square kilometre.

  At the very bottom of the family sits the decimilliare, sometimes seen in cadastre area evaluations of real estate plots. It equals one ten-thousandth of an are, or one square decimetre, roughly a four-inch-by-four-inch square. Its name uses a double prefix, which is non-standard in the metric system.

  In Russian and several other languages of the former Soviet Union, the are carries a separate everyday name: sotka, meaning "a hundred", from the Russian word for that number. The sotka describes the size of suburban dacha plots and allotment gardens, and of small city parks where the hectare would be too large a unit to use. Many Russian dachas are six ares in size, a fact that speakers express as shest' sotok.

  The are also remains in active everyday speech in Indonesia, India, and various European countries, even though it no longer holds formal SI recognition. One hectare equals exactly 100 of these ares, a ratio that keeps the family coherent and the conversion trivial for anyone familiar with the system.

• 6. Encoding the Hectare

  ›

  The hectare has left a trace even in the Unicode character standard. A single character in the CJK Compatibility block was included specifically for compatibility with pre-existing East Asian character codes. It is not intended for use in alphabetic contexts; its purpose is purely to preserve compatibility with older encoding systems.

  The character is a combination of the Japanese katakana spelling of "hectare": hekutaru. Japan's own conversion from the hectare stands at approximately 1.008 cho per hectare, so the unit has been in active use in Japanese land administration long enough to earn its own compact representation in older character sets.

  The SI symbol for the hectare, ha, is two lowercase letters with no punctuation. That symbol is fixed by international convention and applies regardless of the language or script in use. The Unicode character is a separate legacy artefact, not a replacement for that symbol, and exists to allow older documents to be represented without data loss in modern systems.

  The fact that the hectare earned a place in the CJK Compatibility block at all reflects how deeply embedded it became in administrative record-keeping across East Asia before the Unicode era. Compatibility blocks are reserved for characters that already existed in widespread use; they are not granted to units that might someday become common.