Maritime / Nautical Converter
By KAMP Inc. / UnitOwl · Last reviewed:
Maritime measurement has its own vocabulary — knots, nautical miles, fathoms, and cables — developed over centuries of seafaring before modern standardization. These units remain in active use in navigation, weather forecasting, and admiralty law. Understanding how nautical units relate to metric and imperial equivalents is essential for anyone working in marine navigation, offshore energy, or international shipping.
Popular Maritime / Nautical Conversions
Nautical Miles, Knots, and Their Origins
The nautical mile is defined as exactly 1,852 meters (6,076.1 feet) — one arcminute of latitude along the Earth's meridian. This elegant definition made it a natural unit for celestial navigation: a position change of 1 arcminute in latitude equals exactly 1 nautical mile of distance. One knot (kn or kt) is one nautical mile per hour: 1.852 km/h or 1.151 mph. Ship and aircraft speeds are universally stated in knots. A modern container ship cruises at 18–22 knots (33–41 km/h); a sailboat races at 6–12 knots; a Navy destroyer can exceed 30 knots; a commercial airliner cruises at about 450–480 knots (834–890 km/h). Speed limits in harbors are typically 5–8 knots.
| Unit | Equivalent |
|---|---|
| 1 nautical mile (nm) | 1,852 m / 1.151 statute miles |
| 1 knot | 1.852 km/h / 1.151 mph |
| 1 fathom | 1.829 m / 6 feet |
| 1 cable (international) | 185.2 m / 0.1 nm |
| 1 league (nautical) | 3 nm / 5,556 m |
| 10 knots | 11.51 mph / 18.52 km/h |
Depth Measurement: Fathoms, Meters, and Sonar
Ocean depth was historically measured in fathoms (1 fathom = 6 feet = 1.829 m), using a lead weight on a marked line. The word 'fathom' derives from Old English for the span of outstretched arms. Modern charts increasingly use meters, though NOAA charts in US waters still show fathoms and feet on many editions. Average ocean depth is about 3,688 meters (2,018 fathoms); the Mariana Trench reaches 10,994 meters (6,010 fathoms). Sound travels in seawater at approximately 1,500 m/s — faster than in air — which is why sonar is effective for depth sounding. Echo sounders measure the round-trip time of a sound pulse: a ping that returns in 0.04 seconds indicates a depth of (0.04 × 1,500) / 2 = 30 meters.
Deadweight Tonnage and Ship Capacity
Maritime cargo capacity is measured in deadweight tonnage (DWT) — the maximum weight in metric tons a ship can carry including cargo, fuel, ballast, crew, and provisions. A Panamax container ship carries about 65,000–80,000 DWT; an ultra-large container vessel (ULCV) exceeds 200,000 DWT. Gross tonnage (GT) is a volumetric measure of a ship's enclosed spaces, not a weight: 1 GT = 100 cubic feet (2.83 m³) of enclosed volume. Displacement tonnage measures the actual weight of water displaced by the ship — equal to the total weight of the vessel. These tonnage measures are distinct and non-interchangeable, which causes significant confusion. Always check which tonnage is being referenced when comparing vessel capacities.
Sources & References
- NIST — Units and Conversion Factors — Official unit conversion factors from the National Institute of Standards and Technology.
- BIPM — The International System of Units (SI) — International SI unit definitions from the International Bureau of Weights and Measures.