length · International · Modern
SI base unit of length. Its definition fixes the speed of light in vacuum at exactly 299,792,458 m/s, with the second defined from the caesium-133 hyperfine frequency.
Converter
length · U.S./English customary; maritime · Modern + historical
fathom
exact
- Record
- M051
- Definition quality
- Historically documented
- Conversion mode
- multiplicative
- Calculator
- Fully calculable
Definition
1 fathom = 6 ft = 1.8288 m exactly under the international foot
Reference value
One fathom is exactly 1.8288 metres.
History & use
Traditionally associated with measuring water depth and line length; the modern numerical value follows the standardized foot.
Modern use
Modern use is context-dependent and varies by region or measurement system.
Sources & evidence
Source tier — · confidence high
- Office of Weights and Measures, National Institute of Standards and Technology — https://nvlpubs.nist.gov/nistpubs/hb/2026/NIST.HB.44-2026.pdf
- Office of Weights and Measures, National Institute of Standards and Technology — https://www.nist.gov/pml/owm/nist-handbook-133-current-edition
- National Bureau of Standards / Commonwealth national standards laboratories — https://www.nist.gov/pml/us-surveyfoot
- National Institute of Standards and Technology — https://www.nist.gov/pml/special-publication-811
Related measurements
length · International · Modern
1 km = 1000 m by the SI prefix kilo-.
length · International · Modern
1 cm = 0.01 m by the SI prefix centi-.
length · International · Modern
1 mm = 0.001 m by the SI prefix milli-.
inch
inlength · UK / US · Modern / historical
1 international inch = exactly 0.0254 m.
foot
ftlength · UK / US · Modern / historical
1 international foot = exactly 0.3048 m.
yard
ydlength · UK / US · Modern / historical
1 international yard = exactly 0.9144 m.
mile
milength · UK / US · Modern / historical
1 international mile = exactly 1609.344 m = 1760 yd.