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What is a Fathom?

Mary McMahon
Updated Jan 21, 2024
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A fathom is a unit of measurement which most often appears in the context of nautical depths. Like many seemingly odd units of measurement, the fathom was originally linked to a measurement on the human body, in this case the outstretched arms of an adult male. When measurements were standardized, the fathom as a unit of measurement was retained, and many sailors continued to use it to discuss the depth of water. As a result, many marine instruments give depths in fathoms as well as meters and feet.

Officially, a fathom measures six feet (1.8 meters) in length, although some countries quibble on fractional distances of the measurement. The fathom is an ancient unit of measurement, dating back at least to the times of Ancient Greece. The use of the old English term faethm for “outstretched arms” to discuss the measurement appears to be quite old, with the adoption of “fathom” for taking nautical soundings occurring in the 1600s. Typically, a knot would be made at each fathom length of rope, allowing sailors to count off the fathoms as they dropped the rope to the bottom.

Since one can speak of “fathoming” something in the sense of taking soundings or measurements, the term came to be used more generally to describe understanding or comprehending something. This usage of the word emerged in the early 1600s, shortly after the word began to be employed in the context of measuring depth. Something which cannot be measured or understood may be referred to as “fathomless.”

Fans of William Shakespeare may be familiar with the phrase “full fathom five thy father lies,” which appears in his play The Tempest. By convention, a burial at sea traditionally took place at a depth of at least five fathoms. It has been suggested that bodies buried at this depth are less likely to float back up, but more probably sailors simply wanted to ensure that their fellows were soundly submerged as a token of respect. On land, a traditional burial is usually performed at a depth of one fathom, leading to the slang term “six feet under.”

At one point, the fathom was also used for measuring distances on land. This trend has declined, as the fathom has been supplanted by yards, meters, feet, and other units of measurement which are smaller and therefore potentially more accurate. Old maps, however, may still indicate measurements in fathoms.

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Mary McMahon
By Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a AllTheScience researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

Discussion Comments

By anon67581 — On Feb 25, 2010

Here are some interesting things about fathoms as related to the distance of ones outstretched arms. Your reach, fingertip to fingertip is very nearly the same as your height.

Since the fathom is a very old measurement based on this reach one might conclude that either people were indeed six feet tall in ancient times as well as today or that people were just as big at exaggeration.

In use this is not a practical measurement unless you knot a rope as described because, as any sailor knows, to measure a rope with no knots you don't count by your fingertips since you are grasping and coiling but by something a bit less counting off your coils and multiplying.

My own reach is about 5' 9", larger than my 5' 7" height but my grasp is nominally only 5' which is actually more convenient when measuring off how much line to play out for anchor rodes and such.

By anon62749 — On Jan 28, 2010

this is very interesting to me. thank you for putting this up!

Mary McMahon

Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a...

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