Conversion from Square inches to Centiares

Information about the Square inch (sq in)

The square inch is a unit of area equal to 6.4516 square centimetres exactly. Its symbol is sq in, sometimes written in². It is the area of a square one inch on a side, and one hundred and forty-four of them make a square foot. Its most important use is not as an area at all but as the denominator of the pound per square inch, the pressure unit that governs tyres, hydraulics and gas supply across the English-speaking world.

That pressure unit does most of the work. A car tyre is inflated to around 32 pounds per square inch, a bicycle tyre to 80 or more, a domestic water supply arrives at 40 to 60, and hydraulic machinery runs at several thousand. Because a hydraulic cylinder produces a force equal to pressure times piston area, the square inch is what turns a pressure gauge reading into a load, which is why the unit is printed on machinery even in workshops that measure everything else in millimetres.

Materials strength uses a multiple of it. Steel and aluminium are rated in thousands of pounds per square inch, written ksi, and a common structural steel yields near 50 ksi. Concrete in the United States is specified in pounds per square inch, with 3000 to 5000 typical for building work. The corresponding metric figures in megapascals are smaller by a factor of about 6.9, and mixing the two produces some of the most dangerous errors in engineering.

As a plain area it describes small flat things. A postage stamp is about one square inch, a large coin rather less, a credit card about 3.3, and the die inside a large processor around one. Gaskets, contact patches, wound dressings, printed labels and machined faces are all quoted in square inches in American workshops.

The conversion to metric is exact rather than approximate. Because the inch is defined as exactly 2.54 centimetres, one square inch is exactly 6.4516 square centimetres, or 645.16 square millimetres. The exactness is worth knowing, because it means a converted figure can be checked rather than merely trusted.

Its awkwardness lies in the ladder above it. A square foot is 144 square inches and a square yard is 1296, so imperial area arithmetic never reduces to shifting a decimal point. That is the practical reason engineering drawing in Britain moved to millimetres while American practice stayed with the inch and simply accepted the multiplication.

One square inch equals 6.4516 square centimetres, 645.16 square millimetres, or about 0.006944 square feet.

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.