| Reference | Chains (ch) | Millimeters (mm) |
|---|---|---|
| A sheet of A4 paper (long side) | 0.0147638 ch | 297 mm |
| Average adult human height | 0.0845065 ch | 1700 mm |
| A football pitch (length) | 5.21952 ch | 105000 mm |
| A marathon | 2097.5 ch | 42195000 mm |
| Height of Mount Everest | 439.881 ch | 8849000 mm |
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 millimetre is one thousandth of a metre, written mm. It is the smallest division marked on most rulers and tape measures, which makes it the practical floor of everyday measurement in countries using the metric system.
Technical drawing has settled on the millimetre almost universally. Mechanical engineering drawings state dimensions in millimetres by default, usually without writing the unit at all, because the convention is understood. This avoids the decimal points that centimetres would introduce and the large numbers that micrometres would require. A component 45.5 mm long is easier to read and harder to misinterpret than one 4.55 cm or 45500 µm long.
Meteorology uses the millimetre for rainfall. A reading of 25 mm means that rain would stand 25 millimetres deep on a flat surface that did not drain, which corresponds to 25 litres per square metre. Snowfall is usually reported as depth in centimetres and separately as liquid water equivalent in millimetres.
Medicine relies on the unit for tumour dimensions, wound measurement and the size of anatomical structures on imaging. Ophthalmic prescriptions specify pupillary distance in millimetres. Blood pressure retains the older unit of millimetres of mercury, a pressure measurement whose name preserves the height of a mercury column.
Paper, sheet metal and glass are specified by thickness in millimetres. A standard sheet of office paper is about 0.1 mm thick. Domestic window glass is commonly 4 mm. Firearm and ammunition calibres are frequently given in millimetres, as in 9 mm.
Fastener sizes follow the unit closely. Metric bolts and screws are designated by their nominal diameter in millimetres, so an M8 bolt has an eight-millimetre thread. Spanner and socket sizes are marked the same way, measuring across the flats of the fastener head. This is why a metric toolkit contains an unbroken run of sizes rather than the fractional inch steps of an imperial set.
Medicine measures pressure with it as well as distance. Blood pressure is reported in millimetres of mercury, the height of a mercury column the pressure would support, so a healthy reading of 120 over 80 refers to two lengths rather than to any force directly. The convention dates from the mercury manometer and has survived every instrument that replaced it, because the numbers are familiar to every clinician alive. The same unit measures intraocular pressure in the eye, central venous pressure and the partial pressures of gases in blood, and it appears in vacuum work as the torr, which is one millimetre of mercury under another name.
One millimetre equals 1000 micrometres, 0.1 centimetres, or 0.001 metres. It is roughly 0.03937 inches, so 25.4 millimetres make exactly one inch.