| Reference | Chains (ch) | Hectometers (hm) |
|---|---|---|
| A sheet of A4 paper (long side) | 0.0147638 ch | 0.00297 hm |
| Average adult human height | 0.0845065 ch | 0.017 hm |
| A football pitch (length) | 5.21952 ch | 1.05 hm |
| A marathon | 2097.5 ch | 421.95 hm |
| Height of Mount Everest | 439.881 ch | 88.49 hm |
The chain is a unit of length equal to 66 feet, 22 yards, or exactly 20.1168 metres. It occupies an unusual position in the imperial system as the unit that connected everyday measurement to land surveying.
It is named after a physical object. Edmund Gunter, an English mathematician, introduced a surveying chain of 100 iron links in 1620. The design solved a real problem: land area was recorded in acres, a unit inherited from medieval agriculture, while distances were measured in feet and yards. Gunter chose a length that reconciled the two systems. Ten square chains make exactly one acre, and 80 chains make one mile, so a surveyor could measure in chains and compute both area in acres and distance in miles without awkward conversion factors.
The chain shaped the landscape of several countries. The United States Public Land Survey System, which divided much of the American west into townships and sections from 1785, was laid out in chains. A section is one square mile, or 80 chains on a side, and the resulting grid remains visible in road patterns and field boundaries across the Midwest. Similar surveys in Canada, Australia and New Zealand used the same unit, and older property deeds in those countries still cite chain measurements.
Cricket preserves it in sporting form. The pitch measures 22 yards between the wickets, which is exactly one chain, a length fixed long before the game's laws were formally written.
British railways continue to use the chain for locating features along a line. Positions are recorded as a mileage and a chain figure, so 12 miles 40 chains identifies a point precisely. The convention survives in signalling diagrams and engineering records despite general metrication.
The chain subdivides into 100 links of 7.92 inches each, giving surveyors a decimal subdivision within an otherwise non-decimal system.
One chain equals 22 yards, 66 feet, or 20.1168 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.