What is a Fathom?

The fathom is the traditional marine depth unit, equal to six feet, used in nautical charts and diving manuals.

Overview

The fathom equals exactly 1.8288 meters or 6 feet and is the traditional marine depth unit. The word derives from the Old English 'fæðm,' meaning the span of outstretched arms — historically the distance a sailor could measure rope by stretching it from fingertip to fingertip. Used since antiquity for sounding water depth (lowering a weighted line and counting fathom marks), the fathom remains common in nautical charts, diving manuals, and marine literature. Famous from Mark Twain's pen name (a riverboat lead's call meaning 'safe water,' 2 fathoms or 12 feet) and Shakespeare's 'full fathom five,' the unit retains cultural resonance. Modern oceanographic science prefers the meter, but the fathom persists in fishing, recreational diving, and historical maritime contexts. The fathom relates to the foot (1 fathom = 6 ft) and the yard (1 fathom = 2 yd).

Symbol

ftm

Convert Fathom to all units

Live result

ftm

Meter1.8288 m Kilometer0.001829 km Centimeter182.88 cm Millimeter1,828.8 mm Mile0.001136 mi Yard2 yd Foot6 ft Inch72 in Nautical Mile0.000987 nmi Micrometer1,828,800 µm Nanometer1,828,800,000 nm Light Year1.933 × 10^-16 ly Furlong0.009091 fur Angstrom18,288,000,000 Å

Relationship to Other Length Units

1 ftm equals

Visual reference for how the fathom relates to other length units. Each row links to the full converter for that pair.

1 ftm=1.8288 m 1 ftm=0.001829 km 1 ftm=182.88 cm 1 ftm=1,828.8 mm 1 ftm=0.001136 mi 1 ftm=2 yd 1 ftm=6 ft 1 ftm=72 in 1 ftm=0.000987 nmi 1 ftm=1,828,800 µm 1 ftm=1,828,800,000 nm 1 ftm=1.933 × 10^-16 ly

See all 15 length units in matrix view →

When Is the Fathom Used?

Water depth on older US nautical charts
Maritime literature and historical navigation
Sport diving and scuba depth references in the US

Real-world examples

A "mark twain" reading — made famous by the Mississippi riverboats — meant 2 fathoms (3.66 m) of water, the minimum safe depth.

Tips for Using the Fathom

1 fathom = 2 yards = 6 feet = 1.8288 m.
Modern charts use metres internationally; fathoms are mostly historical.
The word "fathom" comes from Old English for "outstretched arms" — originally the span of an adult's arms.

Common Mistakes

Reading a modern chart that uses metres as if it were fathoms — a factor-of-1.83 depth error.
Confusing fathoms with feet on mixed-unit charts.
Assuming all US charts still use fathoms — NOAA has largely migrated to metres.

Convert Fathom to Other Length Units

Fathom to Nautical Mile

1 ftm = 1,828,800,000 nm

Fathom to Light Year

1 ftm = 1.933 × 10^-16 ly

Fathom to Furlong

1 ftm = 0.009091 fur

Fathom to Angstrom

1 ftm = 18,288,000,000 Å

Convert Other Units to Fathom

Nautical Mile to Fathom

1 nmi = 1,012.6859 ftm

Micrometer to Fathom

1 µm = 5.468e-7 ftm

Nanometer to Fathom

1 nm = 5.468 × 10^-10 ftm

Light Year to Fathom

1 ly = 5,173,173,665,791,776 ftm

Furlong to Fathom

1 fur = 110 ftm

Angstrom to Fathom

1 Å = 5.468 × 10^-11 ftm

FAQ About the Fathom

3 questions

What does the Fathom (ftm) measure?

The fathom measures length. The fathom is the traditional marine depth unit, equal to six feet, used in nautical charts and diving manuals.

When is the Fathom used?

The fathom is used in: Water depth on older US nautical charts; Maritime literature and historical navigation; Sport diving and scuba depth references in the US. A "mark twain" reading — made famous by the Mississippi riverboats — meant 2 fathoms (3.66 m) of water, the minimum safe depth.

How accurate are conversions involving the Fathom?

All conversions on Units Converter use NIST SP 811 and BIPM reference values, accurate to 8 significant figures.