A fathom is a unit of length in the imperial and the U.S. customary systems equal to 6 feet (1.8288 metres), used especially for measuring the depth of water.

There are two yards (6 feet) in an imperial fathom. Originally the span of a man's outstretched arms, the size of a fathom has varied slightly depending on whether it was defined as a thousandth of an (Admiralty) nautical mile or as a multiple of the imperial yard. Formerly, the term was used for any of several units of length varying around 5–5 ¹⁄₂ feet.

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A cable length or length of cable is a nautical unit of measure equal to one tenth of a nautical mile or approximately 100 fathoms. The unit is named after the length of a ship's anchor cable in the Age of Sail. The definition varies:

• International:  ¹⁄₁₀ nautical mile, or 185.2 m
• Imperial (Admiralty):  ¹⁄₁₀ Admiralty mile, or 608 ft (185.32 m), about 101 fathoms
  • The traditional British fathom varied from 5½ feet to 7 feet in the Merchant Navy.
• U.S. customary (US Navy): 120 fathoms (720 feet, 219.456 m)

conversion table