A fathom is a unit of length in the imperial and the U.S. customary systems equal to 6 feet (1.8288 m), used especially for measuring the depth of water.[1] The fathom is neither an International Standard (SI) unit, nor an internationally-accepted non-SI unit. Historically, however, it is the most frequently employed maritime measure of depth in the English-speaking world.

1 fathom in ...... is equal to ...
   imperial/US units   6 ft
   SI unit equivalent   1.8288 m
Standard units in Regensburg: the metal rods are (from left to right) a fathom (Klafter), foot (Schuch) and ell (Öln).

There are two yards (6 feet) in an imperial fathom.[1] 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+12 feet (1.5–1.7 m).


The name (pronounced /ˈfæðəm/) derives from the Old English word fæðm, cognate to the Danish (via the Vikings) word "favn" meaning embracing arms or a pair of outstretched arms. Cognate maybe also via the Old High German word "fadum" of the same meaning.[2][3][4][5] In Middle English it was fathme.


Ancient fathoms

The Ancient Greek measure known as the orguia (Greek: ὀργυιά, orgyiá, lit. "outstretched") is usually translated as "fathom".[6] By the Byzantine period, this unit came in two forms: a "simple orguia" (ἁπλὴ ὀργυιά, haplē orguiá) roughly equivalent to the old Greek fathom (6 Byzantine feet, c.1.87 m) and an "imperial" (βασιλικὴ, basilikē) or "geometric orguia" (γεωμετρικὴ ὀργυιά, geōmetrikē orguiá) that was one-eighth longer (6 feet and a span, c.2.10 m).[7][8]

International fathom

One fathom is equal to:

  • 1.8288 metres exactly (Official international definition of the fathom)[9]
  • 1.828804 m (Obsolete measurement of the fathom based on the US Survey Foot, only for use of historical and legacy applications)[10]
  • 2 yards (1 yard is exactly 12 fathom)
  • 6 feet (1 foot is exactly 16 fathom)
  • 18 hands
  • 72 inches
  • 1 metre is about 0.5468 fathoms

In the international yard and pound agreement of 1959 the United States, Australia, Canada, New Zealand, South Africa, and the United Kingdom defined the length of the international yard to be exactly 0.9144 metre. In 1959 United States kept the US survey foot as definition for the fathom.

In October 2019, U.S. National Geodetic Survey and National Institute of Standards and Technology announced their joint intent to retire the U.S. survey foot, with effect from the end of 2022. The fathom in U.S. Customary units is thereafter defined based on the International 1959 foot, giving the length of the fathom as exact 1.8288 meters in the United States as well.[11][12]

British fathom

The British Admiralty defined a fathom to be a thousandth of an imperial nautical mile (which was 6080 ft) or 6.08 feet (1.85 m). In practice the "warship fathom" of exactly 6 feet (1.8 m) was used in Britain and the United States.[13] No conflict between the definitions existed in practice, since depths on Imperial nautical charts were indicated in feet if less than 30 feet (9.1 m) and in fathoms for depths greater than that. Until the 19th century in England, the length of the fathom was more variable: from 5+12 feet on merchant vessels to either 5 or 7 feet (1.5 or 2.1 m) on fishing vessels (from 1.7 to 1.5 or 2.1 m).[13]

Derived units

At one time, a quarter meant one-quarter 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.

Use of the fathom

Water depth

Excerpt of a 1920 map centred at 16°N 114.5°E / 16; 114.5, a notable focal bank of the South China Sea, with depths in whole fathoms only. The Hydrographic Office highlights hazardous depth shallows (shoals) with dashed lines.[15] Click for broader map and to enable varied magnification.

Most modern nautical charts indicate depth in metres. However, the U.S. Hydrographic Office uses feet and fathoms.[16] 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 estimated by the user.[17] Water near the coast and not too deep to be fathomed by a hand sounding line was referred to as in soundings or on soundings.[18] The area offshore beyond the 100 fathom line, too deep to be fathomed by a hand sounding line, was referred to as out of soundings or off soundings.[19] A deep-sea lead, the heaviest of sounding leads, was used in water exceeding 100 fathoms in depth.[20]

This technique has been superseded by sonic depth finders for measuring mechanically the depth of water beneath a ship, one version of which is the Fathometer (trademark).[21] The record made by such a device is a fathogram.[22] A fathom line or fathom curve, a usually sinuous line on a nautical chart, joins all points having the same depth of water, thereby indicating the contour of the ocean floor.[23]

Some extensive flat areas of the sea bottom with constant depth are known by their fathom number, like the Broad Fourteens or the Long Forties, both in the North Sea.

Line length

The components of a commercial fisherman's setline were measured in fathoms. The rope called a groundline, used to form the main line of a setline, was usually provided in bundles of 300 fathoms. A single 50-fathom (300 ft; 91 m) skein of this rope was referred to as a line. Especially in Pacific coast fisheries the setline was composed of units called skates, each consisting of several hundred fathoms of groundline, with gangions and hooks attached. A tuck seine or tuck net about 70 fathoms (420 ft; 130 m) long, and very deep in the middle, was used to take fish from a larger seine.

A line attached to a whaling harpoon was about 150 fathoms (900 ft; 270 m). A forerunner — a piece of cloth tied on a ship's log line some fathoms from the outboard end — marked the limit of drift line.[24] A kite was a drag, towed under water at any depth up to about 40 fathoms (240 ft; 73 m), which upon striking bottom, was upset and rose to the surface.

A shot, one of the forged lengths of chain joined by shackles to form an anchor cable, was usually 15 fathoms (90 ft; 27 m).[25]

A shackle, a length of cable or chain equal to 12+12 fathoms (75 ft; 22.9 m).[26] In 1949, the British navy redefined the shackle to be 15 fathoms (90 ft; 27 m).[27]

The Finnish fathom (syli) is occasionally used: 11000 nautical mile or 1100 cable length.


A burial at sea (where the body is weighted to force it to the bottom) requires a minimum of six fathoms of water. This is the origin of the phrase "to deep six" as meaning to discard, or dispose of.[28]

The phrase is echoed in Shakespeare's The Tempest, where Ariel tells Ferdinand, "Full fathom five thy father lies".

On land

Until early in the 20th century, it was the unit used to measure the depth of mines (mineral extraction) in the United Kingdom.[29] Miners also use it as a unit of area equal to 6 feet square (3.34 m2) in the plane of a vein.[2] In Britain, it can mean the quantity of wood in a pile of any length measuring 6 feet (1.8 m) square in cross section.[2] In Central Europe, the klafter was the corresponding unit of comparable length, as was the toise in France. In Hungary the square fathom ("négyszögöl") is still in use as an unofficial measure of land area, primarily for small lots suitable for construction.

See also



  1. Chisholm, Hugh, ed. (1911). "Fathom" . Encyclopædia Britannica. Vol. 10 (11th ed.). Cambridge University Press. p. 201.
  2. Oxford English Dictionary, second edition, 1989;
  3. Bosworth, Joseph (1898). Thomas Toller (ed.). An Anglo-Saxon Dictionary. Oxford, England: Clarendon Press. Archived from the original on 2007-03-14.
  4. "Definition of FATHOM". www.merriam-webster.com. Retrieved 21 March 2018.
  5. "Zoekresultaten".
  6. "3712. orguia", Bible Hub, 2016.
  7. Loizos, Demetris I. (2010), "Byzantine Measures" (PDF), Digital Humanities: Diophant Ancient Measures Converter, p. 1–2, retrieved 6 April 2015.
  8. Schilbach, E.; et al. (1991), "Orgyia", The Oxford Dictionary of Byzantium, Oxford University Press, pp. 1532–1533, ISBN 978-0-19-504652-6.
  9. "Sea measures". Admiralty Manual of Seamanship. HMSO. 1995. p. 1·19. ISBN 0-11-772696-6.
  10. NIST Guide to the SI, Appendix B: Conversion Factors
  11. "NGS and NIST to Retire U.S. Survey Foot after 2022". National Geodetic Survey. 31 October 2019. Retrieved 4 March 2020.
  12. "U.S. Survey Foot: Revised Unit Conversion Factors". NIST. 16 October 2019. Retrieved 4 March 2020.
  13. Fenna (2000: 88-89)
  14. Nautical chart of the Macclesfield Bank by the Hydrographic Office 1920
  15. Nautical chart of the Macclesfield Bank by the Hydrographic Office 1920
  16. "NOAA Chart". Retrieved 2008-05-22.
  17. Sounding lead. By James Mathews. Navy & Marine Living History Association.
  18. "Burney: "Vocabulary of Sea Terms", 1876". www.bruzelius.info. Retrieved 21 March 2018.
  19. MarineWaypoints.com - Nautical Glossary. SandyBay.net - Marine Directory (MarineWaypoints.com) and Reference Directory (StarDots.com).
  20. The new way and the old; how the sounding machine has superseded the deep sea lead. The New York Times, June 6, 1892, page 5.
  21. Field Procedures Manual, National Oceanic and Atmospheric Administration (NOAA), Office of Coast Survey. Archived 2017-07-03 at the Wayback Machine May 2008. In chapter 7, Glossary, page 252.
  22. Hydrographic Manual. Archived 2008-07-04 at the Wayback Machine By Captain Karl B. Jeffers. Publication 20-2, Coast and Geodetic Survey, U. S. Department Of Commerce. Posted by the Hydrographic Society of America.
  23. Glossary of Marine Navigation. Archived 2008-12-18 at the Wayback Machine Page 763. I'd Rather Be Sailing.
  24. Scofield, William Launce (1947). Drift and set line fishing gear in California. Sacramento, California: California State Printing Office. Retrieved 18 May 2017 via Calisphere. As opposed to drifting, a piece of fishing gear is considered set when it is anchored or attached to the bottom or shore so that it is not free to move about with water or wind currents. By contrast, a drift line or net has no such attachment to the bottom or shore and is therefore free to drift or move with any currents.
  25. Dept. of the Army Technical Bulletin TB 43-0144: Painting of Watercraft. U.S. Government Printing Office. 1990. pp. D-2.
  26. "Shackle n.1, 9.". Oxford English Dictionary. Retrieved 23 February 2015.
  27. Jerrard, H. G.; McNeill, D. B. (1986). A Dictionary of Scientific Units. Springer Science & Business Media. ISBN 9789400941113. Retrieved 3 March 2015.
  28. Hirsch Jr, E.D.; Kett, Joseph F.; Trefi, James (2002). The New Dictionary of Cultural Literacy. Boston, MA: Houghton Mifflin. ISBN 0-618-22647-8.
  29. "Mining Encyclopaedia". U.K. Mine and Quarry Information and Exploration. Retrieved 2007-05-28.


  • Fenna, Donald (2002). "fathom". A Dictionary of Weights, Measures, and Units. Oxford: University Press. ISBN 0-19-860522-6..
This article is issued from Wikipedia. The text is licensed under Creative Commons - Attribution - Sharealike. Additional terms may apply for the media files.