Conversion from Deciares to Hectares

Information about the Deciare (da)

The deciare is a unit of area equal to one tenth of an are, or ten square metres. Its symbol is da. It completes the decimal ladder that the French metric law of 1795 built around the are, alongside the centiare below it and the hectare above, but unlike either of those it never found a job and is today the least used member of the family.

Ten square metres is an easily pictured quantity. It is a small bathroom, a single bedroom in a modest flat, or four fifths of a car parking space. It is the floor of a garden shed, the area a market trader is allotted at a fair, or the footprint of a large dining table with chairs pushed back. Nothing about the size is obscure; only the name is.

The reason it failed is structural. Land registration adopted a three-column system running hectare, are and centiare, each step a factor of one hundred, which mirrors how areas actually behave: squaring a tenfold change in length produces a hundredfold change in area. A tenth of an are sits between two columns and belongs to neither, so no register ever needed a place for it.

Its symbol makes matters worse. In the International System, da is the prefix symbol for deca, so a reader meeting da in a technical document is far more likely to take it as the start of a compound like dam or daN than as a unit in its own right. The decare, ten times larger, is written daa, which does nothing to help.

Where the quantity does need naming, ordinary metric notation handles it without effort. Ten square metres is written 10 m², which is shorter than one deciare, needs no explanation, and sorts correctly beside every other figure on a plan or in a contract.

The unit therefore survives almost entirely in conversion tables and in teaching material that runs through the complete prefix ladder. Encountering it in a real document usually means the document is old, translated from an older source, or reproducing a table rather than making a measurement, and converting it to square metres at once is the safe response.

One deciare equals 10 square metres, 0.1 ares, 0.001 hectares, or about 107.6 square feet.

Information about the Hectare (ha)

The hectare is a unit of area equal to 10,000 square metres, written ha. It is a square one hundred metres on a side. It is not strictly an SI unit, but the International Bureau of Weights and Measures accepts it for use with the SI in the one field where it is indispensable: the measurement of land.

It descends from the are, a unit of one hundred square metres introduced in revolutionary France in 1795. The are proved too small for fields and the square kilometre too large, so the hundredfold multiple settled into the gap and outlived its parent entirely. Today almost nobody writes ares, while hectares appear on title deeds, farm subsidies and environmental reports across most of the world.

Agriculture is built on it. Crop yields are quoted in tonnes per hectare, and the figures carry a great deal of information in very little space: wheat in north-western Europe runs to about eight tonnes per hectare, against a world average nearer three. Wine appellations cap production in hectolitres per hectare, and European farm support has been paid per hectare rather than per tonne since the 1990s, which changed what farmers grew as much as any agronomic advance.

Environmental accounting uses the same unit. Deforestation, wetland loss, protected area and reforestation targets are all reported in hectares, because the quantity is large enough to describe a forest and small enough to describe a field. Urban planning uses it inverted, as dwellings per hectare, which is the standard way of comparing the density of one development with another.

For a sense of scale, an association football pitch of the usual size covers about 0.7 hectares, so a hectare is roughly a pitch and a half. A hectare of productive farmland feeds a handful of people on a mixed diet, and a hectare of dense city holds several hundred residents. Traditional Japanese and Korean land measures happen to land close by: one chō and one jeongbo are both about 0.99 hectares.

The imperial competitor is the acre, and the two do not convert cleanly: one hectare is 2.471 acres, and one acre is 0.4047 hectares. Where both systems are in use, as in Ireland, parts of Canada and much of South Asia, land is often quoted in both, and the conversion factor is one of the few worth committing to memory.

One hectare equals 10,000 square metres, about 2.471 acres, 0.01 square kilometres, or about 107,639 square feet.