| Centiares (ca) | Square decameters (dam²) |
|---|---|
| 1 Centiare | 0.01 dam² |
| 2 Centiares | 0.02 dam² |
| 3 Centiares | 0.03 dam² |
| 4 Centiares | 0.04 dam² |
| 5 Centiares | 0.05 dam² |
| 10 Centiares | 0.1 dam² |
| 20 Centiares | 0.2 dam² |
| 25 Centiares | 0.25 dam² |
| 50 Centiares | 0.5 dam² |
| 100 Centiares | 1 dam² |
| Reference | Centiares (ca) | Square decameters (dam²) |
|---|---|---|
| A sheet of A4 paper | 0.06237 ca | 0.0006237 dam² |
| A tennis court | 260.8 ca | 2.608 dam² |
| A football pitch | 7140 ca | 71.4 dam² |
| One hectare | 10000 ca | 100 dam² |
| Central Park, New York | 3410000 ca | 34100 dam² |
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.
The square decametre is a unit of area equal to one hundred square metres. It is written dam², and it is the area of a square ten metres on a side, since a decametre is ten metres. It is exactly the same quantity as the are, and the choice between the two names is a choice between strict metric grammar and two centuries of habit.
The International System is built from coherent units, meaning every derived unit is a product or quotient of base units with no extra factors. Squaring a prefixed length gives a perfectly coherent area, so the square decametre needs no special permission to exist. The are, by contrast, is listed as a non-SI unit accepted for use with the SI, tolerated for land measurement rather than derived from anything.
Despite that formal advantage, the square decametre lost decisively. The are was already in every French land register when the modern system was codified, and a register does not change its columns for a matter of notation. The result is that the correct SI form is almost never written, and its accepted non-SI equivalent is what appears on deeds across half of Europe.
The quantity itself is a useful one to be able to picture, whichever name is used. A hundred square metres is a square ten metres on a side: a generous garden, the ground floor of a small house, a market stall pitch, or a large classroom. It is the last area a person can pace out accurately without instruments, and it is the natural size for describing a plot rather than a field.
Where the symbol does appear, the exponent must be read carefully. The string dam already means decametre, a length, so dam² and dam differ by an entire dimension, and a dropped superscript turns an area into a distance. Software that strips formatting from tables is a common cause of exactly that error.
For any practical purpose, converting is the right response. Multiply by a hundred for square metres, divide by a hundred for hectares, and the resulting figure will match whatever else is on the page. One square decametre is one are, and both are one hundred square metres, so nothing is lost in either direction.
One square decametre equals 100 square metres, 1 are, 0.01 hectares, or about 1076.4 square feet.