Depth at sea was once measured by the length of a man’s outstretched arms. A fathom is a unit of length equal to six feet (1.8288 meters), traditionally used for water depth and the length of anchor cable.

Lead lines were marked in fathoms, and older charts and sailing directions still refer to depths in fathoms in some regions. Modern charts predominantly use meters, yet the term persists in speech and in some equipment. Anchor chain is still often described in shots of 15 fathoms.

Navigators converting between fathoms and meters must be precise; a mistake of units has caused groundings. Understanding fathoms remains useful when reading older documents or working with traditional sounding gear.

• Traditional depth unit equal to six feet
• Historically used with lead lines and charts
• Still appears in cable length and older references
• Requires careful conversion to metric units