| Reference | Ares (a) | Centiares (ca) |
|---|---|---|
| A sheet of A4 paper | 0.0006237 a | 0.06237 ca |
| A tennis court | 2.608 a | 260.8 ca |
| A football pitch | 71.4 a | 7140 ca |
| One hectare | 100 a | 10000 ca |
| Central Park, New York | 34100 a | 3410000 ca |
The are is a unit of area equal to one hundred square metres, written a. It is a square ten metres on a side. It was created by the French metric legislation of 1795 as the basic decimal unit of land, taking its name from the Latin word for a level open space, and it is the parent of the hectare that eventually overshadowed it.
The original scheme was tidy. The are would measure a garden or a plot, the hectare a field, and the centiare a single square metre, giving three decimal steps that covered everything from a doorway to a farm. In practice the middle rung carried almost all the weight: fields were described in hectares and buildings in square metres, and the are was left with the narrow band between.
It nevertheless survives in land registration, and there it is entirely alive. French, Belgian and Luxembourgish deeds record parcels as hectares, ares and centiares in three columns, so a plot might be written as 1 ha 25 a 30 ca, meaning 12,530 square metres. In Poland the ar is the ordinary way of describing a building plot, and in Indonesia and parts of central Europe it fills the same role.
For scale, one are is a square ten metres on a side: a generous garden, a small yard, or the footprint of a modest house with a little room to spare. A badminton court is a little over eight tenths of an are, a single-car garage about one fifth, and a tennis court about two and a half. It is the largest unit that still describes something a person can pace out in a few seconds.
In strict SI terms the are is exactly the square decametre, since a decametre is ten metres. The two are the same quantity written two ways, and the SI form is essentially never used, which makes the are one of the few cases where the older unit is the readable one.
Its main practical trap is the symbol. A single lower-case a is easily lost in running text or mistaken for an abbreviation, which is why deeds usually write it in a fixed column and why anything ambiguous is safer converted to square metres before it is passed on.
One are equals 100 square metres, 0.01 hectares, or about 1076.4 square feet.
The centiare is a unit of area equal to one hundredth of an are, which makes it exactly one square metre. Its symbol is ca. It is the only unit in common use that is an exact duplicate of another under a different name, and the reason it exists at all is a matter of bookkeeping rather than of measurement.
When the French metric law of 1795 established the are as the decimal unit of land, it created a full ladder around it: the hectare above and the centiare below. Land registries then adopted a three-column format, recording every parcel as so many hectares, so many ares and so many centiares. The last column needed a name, and the square metre, though identical, did not fit the pattern.
That format is still in force. French, Belgian, Luxembourgish and Dutch land registers write parcel areas as three numbers in sequence, so 1 ha 25 a 30 ca means 12,530 square metres. The middle and last figures never exceed 99, which makes the notation self-checking: a value like 1 ha 130 a is malformed and signals a transcription error before anyone has to do arithmetic.
Reading it is a matter of place value. Each hectare is 10,000 square metres, each are is 100, and each centiare is one, so the three columns simply spell out a number in base one hundred. A notary's figure of 0 ha 04 a 62 ca is 462 square metres, and a surveyor's plan showing the same plot would print 462 m² without further comment.
Outside the land registry the centiare has no life. Nobody specifies a floor, a wall or a plot of ground in centiares when the square metre says the same thing with a symbol everyone recognises, and no branch of science or engineering has ever adopted it. It is a unit that exists to fill a column.
The one hazard is the symbol. In many documents ca is an abbreviation for the Latin circa, meaning approximately, so a figure that reads ca 400 could mean 400 centiares or about 400 of something else entirely. In land records the position of the number resolves it; in running text, converting to square metres removes the ambiguity for good.
One centiare equals exactly 1 square metre, 0.01 ares, or about 10.764 square feet.