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What is a fathom?

Traditional marine depth unit — 6 feet exactly

Definition

The fathom equals exactly 6 feet or 1.8288 meters. It is the traditional marine unit for water depth, used on older nautical charts and in recreational sailing. Modern charts mark depths in metres, but fathoms still appear in maritime literature and older US charts.

Common uses

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

🌍 Real-world scale

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

Convert from fathoms

14 conversions
Fathom to Meter1 ftm = 1.8288 mFathom to Kilometer1 ftm = 0.001829 kmFathom to Centimeter1 ftm = 182.88 cmFathom to Millimeter1 ftm = 1,828.8 mmFathom to Mile1 ftm = 0.001136 miFathom to Yard1 ftm = 2 ydFathom to Foot1 ftm = 6 ftFathom to Inch1 ftm = 72 inFathom to Nautical Mile1 ftm = 0.0009875 nmiFathom to Micrometer1 ftm = 1,828,800 µmFathom to Nanometer1 ftm = 1,828,800,000 nmFathom to Light Year1 ftm = 1.9330e-16 lyFathom to Furlong1 ftm = 0.009091 furFathom to Angstrom1 ftm = 18,288,000,000 Å

Convert to fathoms

14 conversions
Meter to Fathom1 m = 0.5468 ftmKilometer to Fathom1 km = 546.807 ftmCentimeter to Fathom1 cm = 0.005468 ftmMillimeter to Fathom1 mm = 0.0005468 ftmMile to Fathom1 mi = 880 ftmYard to Fathom1 yd = 0.5 ftmFoot to Fathom1 ft = 0.1667 ftmInch to Fathom1 in = 0.01389 ftmNautical Mile to Fathom1 nmi = 1,012.69 ftmMicrometer to Fathom1 µm = 5.4681e-7 ftmNanometer to Fathom1 nm = 5.4681e-10 ftmLight Year to Fathom1 ly = 5.1732e15 ftmFurlong to Fathom1 fur = 110 ftmAngstrom to Fathom1 Å = 5.4681e-11 ftm

Tips

  • 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.

FAQ about the fathom

What is a fathom?
The fathom equals exactly 6 feet or 1.8288 meters. It is the traditional marine unit for water depth, used on older nautical charts and in recreational sailing. Modern charts mark depths in metres, but fathoms still appear in maritime literature and older US charts.
Where is the fathom used?
The fathom is commonly used for: Water depth on older US nautical charts; Maritime literature and historical navigation; Sport diving and scuba depth references in the US.
What is an example of a fathom in real life?
A "mark twain" reading — made famous by the Mississippi riverboats — meant 2 fathoms (3.66 m) of water, the minimum safe depth.
What category does the fathom belong to?
The fathom is a unit of length, part of the 15-unit length category on UnitConv.
How accurate are fathom conversions?
All conversion factors follow NIST and BIPM international standards to 8 significant figures — the highest practical accuracy.
Explore all 15 length units and their conversions.📏 Length hub →