Conversion from Square kilometers to Centiares

Information about the Square kilometer (km²)

The square kilometre is a unit of area equal to one million square metres, or one hundred hectares. It is written km². It is the area of a square one kilometre on a side, and it is the unit in which countries, cities, lakes, forests and administrative districts are measured almost everywhere in the world.

Its main work is geography. Monaco covers about 2 square kilometres, Manhattan about 59, Greater London around 1572, and France some 551,500. Because the unit spans that whole range without changing, it makes very different places directly comparable, which is why atlases, censuses and statistical yearbooks settle on it rather than on hectares above and square metres below.

Population density is its most quoted derived figure. Dividing inhabitants by square kilometres produces a number that is meaningful at every scale: a few per square kilometre across Mongolia or the Australian interior, a few hundred across most of western Europe, and eight thousand or more in Singapore and central Paris. The same arithmetic done in square miles gives figures 2.6 times larger, which is a common source of confusion in translated sources.

It is worth being careful about the phrasing. A square kilometre is one kilometre by one kilometre. An area described as two kilometres square is two kilometres on each side and therefore four square kilometres, not two. The distinction sounds pedantic until a fire, a flood or a proposed development is being described, at which point the difference is a factor of the side length itself.

Environmental and infrastructure figures live here. Rainfall converts neatly: one millimetre falling on one square kilometre is a thousand cubic metres, or a million litres of water. A utility-scale solar farm producing a gigawatt at peak needs roughly twenty square kilometres of ground. Wildfire extent, glacier retreat, deforestation and sea-ice cover are all reported in square kilometres because that is the resolution at which satellites usefully see them.

Against imperial units it faces the square mile, which is 2.59 square kilometres. Because a square mile also equals 640 acres, the three units chain together awkwardly, and any figure that has passed through more than one of them is worth checking. Below the square kilometre, land measurement passes to the hectare, and one square kilometre is exactly one hundred of them.

One square kilometre equals 1,000,000 square metres, 100 hectares, about 0.386 square miles, or about 247.1 acres.

Information about the Centiare (ca)

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.