Conversion from 2 Square hands to Centiares

Information about the Square hand (sq hh)

The square hand is a unit of area equal to sixteen square inches, or 103.2256 square centimetres. Its symbol is sq hh. A hand is four inches, so a square hand is a square four inches on a side, roughly the area of an adult palm. It is a complete and consistent unit that almost nobody has ever needed, because the hand it derives from measures one thing only.

That one thing is horses. The height of a horse or pony is measured at the withers, the ridge between the shoulder blades, and it is given in hands almost everywhere in the world, including in countries that are otherwise entirely metric. A hand was fixed at four inches by an English statute of 1541 under Henry VIII, and the equestrian world has held to that figure ever since.

The notation contains a trap worth knowing. A horse described as 15.2 hands is not fifteen and two tenths but fifteen hands and two inches, which is 62 inches or 157.5 centimetres. The digit after the point runs from 0 to 3 and then rolls over, so 15.3 is followed by 16.0. Anyone converting a list of horse heights with ordinary decimal arithmetic will produce numbers that are wrong by up to three inches.

Where the boundary between pony and horse falls is decided by this measurement. Most classifications put it at 14.2 hands, which is 58 inches or 147.3 centimetres, and competition classes at every level are drawn on the same scale. Because a placing can depend on a fraction of an inch, official measurement is a regulated procedure carried out on a level surface by an approved measurer.

The area form has never followed the length into use. Body-derived units were invented for laying along things, the height of an animal, the length of a cloth, the depth of water, and not for enclosing ground, which is why the acre and the chain came from ploughing rather than from anatomy. A square hand is simply what appears when the rule for forming areas is applied to a unit that had no such purpose.

As a quantity it is easy enough to place. A hundred square centimetres is a large sticky note, a beer mat and a half, or the palm of a hand, which is the same comparison used to estimate burn area in first aid. Sixteen square hands make a square foot, and 2304 of them make a square yard.

One square hand equals 16 square inches, about 103.23 square centimetres, or about 0.0103 square metres.

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.