Maritime unit converter
Knots, nautical miles, fathoms, hectopascals, long tons and barrels: the sea keeps units the rest of the world dropped a century ago, and half of a watch handover is spent turning one into another. Pick a category, type a value, read every unit at once.
A point or a comma, either works, and the result appears as you type. Click a tile and the value goes to your clipboard. The address bar carries your settings, so a particular conversion can be sent to a colleague as a link.
- The factors are exact by definition (a nautical mile is 1852 m, a foot is 0.3048 m), not rounded approximations.
- Everything is computed in your browser: no account, no ads, nothing sent anywhere.
- Speed comes with the Beaufort force read from the WMO thresholds.
Quick try:
The Beaufort force is read from the WMO thresholds of the unit you typed in. A knot is one nautical mile per hour, which is exactly 1.852 km/h.
A closer look at single units
The conversions people ask about most have their own pages, with tables of common values and answers to the questions that come up on every ship:
Frequent questions about maritime units
How many metres is a cable?
The international cable is 185.2 m, exactly one tenth of a nautical mile, and that is the definition this converter uses. Older sources give the US Navy cable (120 fathoms, 219.5 m) and the Royal Navy cable (608 feet, 185.3 m).
How long is a shackle of anchor chain?
Fifteen fathoms, that is 27.43 m. So "three shackles in the water" is about 82 m of chain. American usage calls the same length a shot; the Royal Navy used 12.5 fathoms (22.86 m) until 1949, which is why older books disagree.
How many litres are in a barrel of oil?
About 159 litres (exactly 158.987 l), because a barrel is defined as 42 US gallons. Converting barrels into tonnes needs the density of the cargo: for a typical crude it works out at roughly 7.3 barrels per tonne.
Is a US gallon the same as an imperial gallon?
No. A US gallon is 3.785 l, an imperial gallon 4.546 l. The imperial one is about a fifth larger, so stores, lubricants and fuel ordered in the wrong gallon arrive in the wrong quantity.
Why is gross tonnage (GT, NT) not in the converter?
Because gross and net tonnage are not units of mass or volume. They are dimensionless results of a formula from the Tonnage Convention, so there is no honest way to turn them into tonnes, and a calculator that does it is misleading you.
Can I type a comma instead of a point?
Yes. The converter reads 12.5 and 12,5 the same way, and it treats 1,852 as one thousand eight hundred and fifty two. Results are always shown with a decimal point.
Keep going
- Units quiz: ten questions on the figures a seafarer is expected to know by heart.
- Coordinates: the same position in DD, DM and DMS.
- Maritime glossary: 138 terms in plain English.