A fathom is a unit of length in the US customary and imperial systems of measurement that is equal to 6 feet, 2 yards, 72 inches, or 1.828804 meters. It is not part of the International System of Units (SI), nor is it accepted internationally as a non-SI unit. However, historically, the fathom is the most frequently used unit of length for measuring water depth.

A fathom is about the height of a man. The unit originated from the British imperial system and is still used to some degree in the US to measure water depth (meters are also used), such as the depth of water below this boat:

The fathom is not used to measure length outside of nautical contexts. In most other parts of the world, meters (m) are used to measure water depth instead. The list below shows the relationship between fathoms and other commonly used measurements of length.

Up until the early 20th century, the fathom was also used in the United Kingdom to measure the depth of mines. Since most other countries around the world use other units of measurement, it can be helpful to know the conversion factors listed above. Converting to feet (and therefore yards and inches) is relatively simple, though it is worth noting that the US fathom has measurements that slightly differ from the definition used outside the US: 1.828804 m (US) vs. 1.8288 (outside US).


Convert the following measurements between fathoms, meters, and feet.

1. 12 fathoms:

12 × 1.828804 = 21.946

12 fathoms × 6 = 72 feet

2. 43 meters:

43 ÷ 1.828804 = 23.513 fathoms

43 × 3.28084 = 141.076 feet

3. 215 feet:

215 ÷ 6 = 35.833 fathoms

215 × 0.3048 = 65.532 meters

Did you know?

Samuel Langhorne Clemens was one of the most famous Americans of the 1800s. The author of several books, including The Adventures of Tom Sawyer, he is best remembered by his pen name, Mark Twain. This name may have come from his work as a steamboat pilot. To steer the steamboat, the water needed to be 2 fathoms deep, or "by the mark, twain" to be in safe water.