Fathom

A unit of length equal to six feet, or 1.8288 metres, traditionally used for water depth and cable length.

Older charts and sounding records may show depths in fathoms, sometimes with additional feet. Modern charts commonly use metres, and the chart’s unit and vertical datum must always be checked before using a depth.

One fathom is exact under the international foot definition. Converting the number alone is insufficient for under-keel clearance because tide height, squat, vessel draught, waves and charted-depth uncertainty also matter.