| Reference | Feet (ft) | Chains (ch) |
|---|---|---|
| A sheet of A4 paper (long side) | 0.974409 ft | 0.0147638 ch |
| Average adult human height | 5.57743 ft | 0.0845065 ch |
| A football pitch (length) | 344.488 ft | 5.21952 ch |
| A marathon | 138435 ft | 2097.5 ch |
| Height of Mount Everest | 29032.2 ft | 439.881 ch |
The foot is a unit of length equal to exactly 0.3048 metres, or twelve inches. Its plural is feet and its symbol is ft, though a single prime is used in technical drawing and a straight apostrophe informally.
Units based on the human foot appear across the ancient world, and their lengths varied considerably. The Roman pes measured about 296 millimetres and was divided into twelve unciae, an arrangement English inherited directly. Medieval Europe supported dozens of local feet, and a merchant crossing a few borders might encounter several. The English foot was standardised in stages, but its exact modern value dates only from the International Yard and Pound Agreement of 1959.
Aviation is the foot's most significant surviving international domain. Aircraft altitude is reported in feet almost everywhere, and flight levels are expressed in hundreds of feet, so FL350 means 35,000 feet. Russia and China historically used metres, and China's transition to feet for most flight levels in 2011 removed a genuine safety hazard at the boundaries of adjoining airspace. Vertical separation standards are defined in feet, and reduced vertical separation minima allow 1000 feet between aircraft above 29,000 feet.
Construction in the United States works in feet and inches throughout, and lumber, ceiling heights and room dimensions all follow. Broadcasting and film retain the foot for lens focus scales and for measuring film stock. Water depth in diving is given in feet in American practice and metres elsewhere, a divergence that dive computers accommodate by offering both.
Twelve as a divisor is the foot's practical strength. It divides evenly by two, three, four and six, which suits carpentry and layout work where thirds and quarters are common. A decimal unit divides cleanly only by two and five.
Water and timber keep two of its derivatives alive. The fathom, six feet, measured the depth of water for as long as a sounding line was thrown by hand, and old charts are still marked in fathoms even where new ones use metres. The board foot, a volume of one foot square by one inch thick, remains the trading unit of sawn timber in North America, so a lumber yard quotes prices per thousand board feet. Both illustrate a general habit of the imperial system: rather than adding prefixes, it names each new quantity after the job it does, which makes the units memorable to those who use them and opaque to everyone else.
One foot equals 12 inches, 0.3048 metres, or 30.48 centimetres exactly.
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.