Fathom Fathom A fathom is a unit of length in the imperial and the U.S. customary systems equal to , 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 . The name derives from the Old English word "fæðm", cognate
Fathom more variable: from feet on merchant vessels to either on fishing vessels (from ). At one time, a quarter meant one-fourth of a fathom. A cable length, based on the length of a ship's cable, has been variously reckoned as equal to 100 or 120 fathoms. Most modern nautical charts indicate depth in metres. However, the U.S. Hydrographic Office uses feet and fathoms. A nautical chart will always explicitly indicate the units of depth used. To measure the depth of shallow waters, boatmen used a sounding line containing fathom points, some marked and others in between, called "deeps", unmarked but