La '''brasse''' (anglais ''fathom'', symbole '''fm''') est une ancienne mesure de longueur correspondant à l'envergure des bras. Cette unité, bien qu'autrefois utilisée pour la mesure des terres, n'est encore usitée que dans la marine pour mesurer les cordages, les filins ainsi que la profondeur de l'eau. Dans ce dernier cas, c'est la traduction française de l’unité anglo-saxonne « fathom » qui vaut 1,8288 mètre. 

Le sens général de la brasse – en partant de la Angleterre (« fathom ») – était toujours la mesure de six pieds. À partir du , cette mesure de six pieds fut appelée toise. (Étymologiquement aussi:&nbsp; « l’étendue des bras »).

En France seulement, où la longueur du pied était supérieure à celle pratiquée dans les pays voisins, la brasse fut redéfinie en ''brasse nouvelle'', valant seulement 5 pieds, soit environ 1,624 mètre. Dans les systèmes basés sur un pied inférieur ou à peine supérieur à 30 cm, cinq pieds correspondent au double-pas ''(lat. passus)''.

{{unit of length|
|m= 1.8288000
|accuracy=5 <!--Number of significant figures-->
}}

A '''fathom''' is a unit of Imperial system (and the derived U.S. customary units). The name derives from the Old English word ''fæðm'' meaning 'a pair of outstretched arms'.<ref>''Oxford English Dictionary'', second edition, 1989;</ref><ref>{{cite book
  | last = Bosworth
  | first = Joseph
  | authorlink = Joseph Bosworth
  | coauthors = Thomas Toller (ed.)
  | title = An Anglo-Saxon Dictionary
  | publisher = Clarendon Press
  | date = 1898
  | location = Oxford, England
  | pages = 
  | url = http://beowulf.engl.uky.edu/cgi-bin/Bosworth-Toller/ebind2html3.cgi/bosworth?seq=285
  | doi = 
  | id = 
  | isbn = }}</ref>
In Middle English it was ''fathme''. Its size varied slightly depending on its base definition, either based on a thousandth of an (Admiralty) nautical mile  or as a multiple of the imperial feet) in a fathom.<ref name=britannica11>''Encyclopædia Britannica'' eleventh edition 1911.</ref>

== International Fathom ==
One  fathom is equal to:
* 1.8288 metres (1 metre is about 0.5468 fathoms)
* 2 yards (1 yard is exactly 0.5 fathoms)
* 4 cubits (1 cubit is exactly 0.25 fathoms)
* 6 feet (1 foot is about 0.1667 fathoms)
* 18 hands
* 72 inches

In 1954 the United States and countries of the Commonwealth of Nations defined the length of the international yard to be exactly 0.9144 metres. With the adoption of the metric SI system the use of fathoms declined. 

==British Fathom==
The imperial nautical mile (which was 6080 ft) or 6.08 feet. In practice the fathom was always regarded as exactly 6 feet. No conflict in the real world existed as depths on Imperial nautical charts were indicated in feet if less than 30 feet and in fathoms for depths above that.

==Use of the fathom==
The fathom is now of mainly historical interest.  In most countries, it is an obsolete unit and has not been used in maritime affairs for decades. All nautical charts produced by Hydrographic Offices worldwide (except for those produced by the U.S. Hydrographic Office, which uses feet and fathoms.<ref>{{cite web
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  | title =NOAA Chart
  | work =
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  | url =http://www.oceangrafix.com/o.g/Charts/chartViewer.html?viewRegion=GreatLakes&viewChart=Lake-Huron
  | format =
  | doi =
  | accessdate = 2008-05-22 }}</ref>
) indicate depths in metres.  Until early in the twentieth century, it was the unit used to measure the depth of mines (mineral extraction) in the United Kingdom.<ref>{{cite web
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  | title =Mining Encyclopaedia
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  | url =http://www.mine-explorer.co.uk/open-htm-white-paper.asp?id=4
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  | accessdate = 2007-05-28 }}</ref>

== Burial==
It is customary, when burying the dead, to inter the corpse at a fathom's depth, or ''six feet under''. 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'', meaning to discard, or dispose of.<ref>{{cite book
  | last =Hirsch, Jr
  | first =E.D.
  | authorlink =
  | coauthors =Kett, Joseph F; Trefi, James
  | title =The New Dictionary of Cultural Literacy
  | publisher =Houghton Mifflin
  | date =2002
  | location =Boston, MA
  | pages =
  | url =
  | doi =
  | id =  ISBN 0618226478}}</ref>

==Other fathoms and similar units of length==
Units of length similar to the size of the fathom can be found in many cultures. Some are listed below.
{| class="wikitable"
! Culture !! Name !! Length in metres
|-
| Croatian || ''hvat'' || 1.896484
|-
| Czech || ''sáh'' || 1.7928
|-
| Danish || ''favn'' || 1.883124
|-
| Dutch || ''vadem, vaam'' || 1.883679
|-
| Estonian || ''süld'' || n/a
|-
| Finnish || ''syli'' || n/a
|-
| French || ''toise'' (circa 1150), ''brasse'' (1409) || ~1.949
|-
| German || ''Klafter'','' Faden = 6 Fuß'' || n/a resp. 1.7
|-
| Ancient Greek || ''orguia'' || 1.8542
|-
| Hungarian || ''öl'' || 1.8964838 (Viennese)
|-
| India (State of Manipur) || ''Sana lamjel'' || n/a
|-
| Italian || ''braccio'' || n/a
|-
| Japanese || ''hiro'' (尋) || ~1.818
|-
| Maltese || ''qasba'' || ~2.096
|-
| Norwegian || ''favn'' || 1.882<ref>{{cite web
  | last =
  | first =
  | authorlink =
  | coauthors =
  | title = Bokmålsordboka
  | work =
  | publisher =
  | date =
  | url = http://www.dokpro.uio.no/perl/ordboksoek/ordbok.cgi?OPP=favn&bokmaal=S%F8k+i+Bokm%E5lsordboka&ordbok=bokmaal&s=n&alfabet=n&renset=j
  | format =
  | doi =
  | accessdate = 2008-01-29 }}</ref>