| Reference | Hands (hh) | Hectometers (hm) |
|---|---|---|
| A sheet of A4 paper (long side) | 2.92323 hh | 0.00297 hm |
| Average adult human height | 16.7323 hh | 0.017 hm |
| A football pitch (length) | 1033.46 hh | 1.05 hm |
| A marathon | 415305 hh | 421.95 hm |
| Height of Mount Everest | 87096.5 hh | 88.49 hm |
The hand is a unit of length equal to exactly four inches, or 101.6 millimetres. It survives in one specialised application: measuring the height of horses and ponies. Almost nowhere else is it still in use.
The measurement is taken from the ground to the withers, the ridge between the shoulder blades where the neck meets the back. The withers are chosen because they are a fixed skeletal point that does not move as the horse raises or lowers its head, so the figure is reproducible in a way that measuring to the top of the head would not be.
Notation is unusual and frequently misread. Heights are written with a decimal point that does not indicate a decimal fraction. A horse of 15.2 hands stands fifteen hands and two inches, which is 62 inches or 157.5 centimetres. Because a hand is four inches, the digit after the point runs only from 0 to 3, and 15.4 is not a valid height. The abbreviation hh, for hands high, is common.
The boundary between a pony and a horse is conventionally set at 14.2 hands, though the exact threshold varies between breed registries and competitive disciplines. Draught breeds such as the Shire commonly exceed 17 hands. The tallest horses on record have stood over 20 hands.
The unit derives from the breadth of a human palm, an origin shared with several ancient measures. Egyptian and Roman systems both used a palm of comparable size. Its standardisation at four inches in England is usually credited to a statute of Henry VIII. The persistence of the hand in equestrian use is largely conservatism, and metric heights in centimetres are now given alongside it in international competition rules.
Measuring accurately is harder than it appears. The horse must stand square on level ground with its head in a natural position, and a purpose-made measuring stick with a spirit level and sliding crossbar is used rather than a tape. Shoeing adds height, so official measurements specify whether the horse is shod, and competition rules that impose height limits require certification by an appointed measurer.
One hand equals 4 inches, 101.6 millimetres, or 0.1016 metres.
The hectometre is one hundred metres, written hm. It uses the SI prefix hecto, from the Greek hekaton meaning hundred. Like the decametre and the decimetre it is a legitimate unit that ordinary usage has largely bypassed in favour of metres and kilometres.
Its clearest application is in land measurement, where it appears indirectly. A square hectometre is 10,000 square metres, which is exactly one hectare. The hectare is the standard unit of land area in agriculture, forestry and property registration across most of the world, so the hectometre is embedded in daily practice even though the length itself is seldom named. A square field measuring one hectometre on each side covers one hectare.
Rail and road infrastructure provides the other common use. Several European railway networks mark distances with hectometre posts, placing a marker every hundred metres along the track. These give maintenance crews and signallers a precise location reference, and incident reports commonly cite a kilometre and hectometre figure. Dutch and Belgian motorways use hectometre markers in the same way, and drivers are directed to quote them when reporting a breakdown.
Athletics offers a familiar length without using the name. The straight of a standard outdoor track is one hectometre, and the 100 metres is the shortest standard sprint distance. Nobody calls it the one-hectometre sprint.
Meteorology uses the unit for cloud base height in some aviation reporting formats, where reporting in hundreds of metres or hundreds of feet keeps the figures compact.
Nautical charts and aviation both work in comparable steps without adopting the name. Runway visual range is reported in hundreds of metres in many countries, and visibility in aviation weather reports is given in metres up to five thousand, effectively in hectometre increments. Where the unit is used explicitly, as on Dutch motorways, its advantage is precision: a hectometre marker locates an incident to within fifty metres.
Radio engineering names a whole band after it. The international classification calls waves between 100 and 1000 metres hectometric, corresponding to frequencies from 300 kilohertz to 3 megahertz, and that band is the medium wave used for broadcasting since the 1920s. A transmitter working at 1000 kilohertz radiates a wave three hectometres long, which is why medium-wave aerials are tall masts rather than the short whips that serve higher frequencies. The naming continues in both directions: the band below is kilometric and the one above decametric, so the ladder of metric prefixes maps directly onto the divisions of the radio spectrum.
One hectometre equals 100 metres, 10 decametres, or 0.1 kilometres. It is approximately 328.084 feet, or 109.361 yards.