| Square hands (sq hh) | Square inches (sq in) |
|---|---|
| 1 Square hand | 16 sq in |
| 2 Square hands | 32 sq in |
| 3 Square hands | 48 sq in |
| 4 Square hands | 64 sq in |
| 5 Square hands | 80 sq in |
| 10 Square hands | 160 sq in |
| 20 Square hands | 320 sq in |
| 25 Square hands | 400 sq in |
| 50 Square hands | 800 sq in |
| 100 Square hands | 1600 sq in |
| Reference | Square hands (sq hh) | Square inches (sq in) |
|---|---|---|
| A sheet of A4 paper | 6.04211 sq hh | 96.6737 sq in |
| A tennis court | 25265.1 sq hh | 404241 sq in |
| A football pitch | 691689 sq hh | 11067022 sq in |
| One hectare | 968752 sq hh | 15500031 sq in |
| Central Park, New York | 330344411 sq hh | 5.28551 × 109 sq in |
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.
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.