How Deep Is A Fathom?

A fathom equals six feet,  it was originally used as a broad phrase to describe the distance between a man’s outstretched arms.

It might indicate five feet, five and a half feet, or six feet.

Fortunately, someone noticed a few hundred years ago that six feet were nearly precisely one-thousandth of a nautical mile (6,080 feet).

Origin Of Fathom

Fathom is derived from the Old English word faethm, which means “outstretched arms.”

The term fathom, which today frequently refers to a measurement (particularly of depth) of six feet, was originally used to describe the distance generated by reaching one’s arms straight out from the sides of the body, fingertip to fingertip.

The verb fathom was used as a synonym for our contemporary embrace in one of its oldest forms: to fathom someone meant to hold the person in your arms.

Fathom had taken to the waters by the 1600s, when the word was employed to signify “to measure with a sounding line.”

Simultaneously, the verb evolved meanings synonymous with probe or explore, and it is today widely used to allude to the process of symbolically getting to the bottom of something.

When Fathoms Are Used?

Water Depth

The depth on most current nautical charts is shown in meters.

The United States Hydrographic Office, on the other hand, employs feet and fathoms.

The units of depth used on a nautical chart are always fully stated.

Boatmen utilized a sounding line with fathom points, some marked and others in between termed deeps, unmarked but assessed by the user, to determine the depth of shallow waters.

In soundings or on soundings were used to describe water close to the coast that was not too deep to be fathomed by a hand sounding line.

Line Length

Fathoms were used to measure the parts of a fisherman’s setline.

The rope known as a groundline, which was used to construct the primary line of a setline, was often supplied in bundles of 300 fathoms.

The setline, particularly on the Pacific coast, was made up of skates, which were made up of a few hundred fathoms of groundline with hooks connected.

To catch fish from a bigger seine, a tuck seine or tuck net approximately 70 fathoms long and very deep in the center was utilized.

On Land

This unit was used to figure out the depth of mines in the United Kingdom until the early twentieth century.

Miners also use it as a measure of area in the plane, equal to six feet square (see also ‘How Long Is 50 Feet?‘).

In the United Kingdom, it can refer to the amount of wood in a pile of any length that is six feet square in cross-section.

The Klafter was the analogous unit of comparable length in Central Europe, as was the Toise in France.

The square fathom is still used as an unofficial land area measurement in Hungary, mainly for small parcels suitable for building.

What Is A Cable Length?

A unit of nautical measurement equivalent to one-tenth of a nautical mile or around 100 fathoms is what is known as a cable length.

It can be between 169 and 220 meters, varying on the standard used, due to differing measurement procedures and anachronisms.

The unit is called after the length of an anchor chain on a ship during the Age of Sail.

How Deep Is a Fathom (1)

Measuring Ocean Depth

Before the discovery of employing sound and radar to detect the depth of the ocean, a variety of methods would be used.

Sailors would use a lead line, which was simply a lead weight linked to a rope that was marked every six feet with a strip of leather or rag, a length known as a fathom.

A crew member would then toss the line into the sea, and after the lead weight hit the bottom, the sailor would use the rope’s strips to measure and record the distance to the ocean floor.

Since the fifth century BCE, the lead line has been the most valuable technique for determining depth for navigation.

The instrument informed sailors about the depth of the sea and if the ship would hit the bottom.

The lead weight was filled with grease, and it was used to collect bits from the ocean floor to assist sailors in determining if the ocean bed was sand, mud, or gravel.


Sound is the most frequent and fastest method of gauging ocean depth.

Sonar technology, which stands for sound navigation and range, allows ships to survey the topography of the ocean floor.

The technology delivers sound waves to the ocean below and records the time it takes for an echo to return.

It works by sending out a sound wave “echo” that bounces off the seafloor and returns to the sonar equipment.


Radar is another option, however, it is slower than sonar. Radar, like sonar, involves putting out a form of a wave that pings off an object and reflects back.

The distinction is that radar employs radio waves, which are a type of electromagnetic wave.

However, because electromagnetic waves move slower in water than in air and get weakened as they pass through it, they are more suited for atmospheric observations.


However, there is another method for measuring ocean depth that combines radar and satellite data.

A radar altimeter is a device that estimates the distance between land and air by measuring the time it takes radio waves to reflect from the surface back to a satellite.

The ocean surface, which bulges outward and inward in ways that human eyes cannot identify, matches the morphology of the ocean bottom, and hence the radar altimeter may be used to measure ocean depth against the ocean surface.

Read More: How Far Is 100 Meters? 


Fathom is equal to six feet and has an interesting history and was used for a long, long time to measure the depth of the ocean in particular.

However, today we use sonar, radar, and satellite technology to measure these distances and use miles instead of fathoms. 

Nick Spieth

Leave a Comment