A forecast says "maximum winds of 45 m/s" while the international bulletin reads "90 knots." Your fishing app shows 1.5 m swell; the chart marks depth in fathoms. The sea runs on its own units.
Convert nautical miles, knots and fathoms in one place — and enter a wind speed to see its Beaufort force alongside.
Type into either side and the other updates. 1 nautical mile = 1,852 m, 1 knot = 1 NM per hour and 1 fathom = 6 feet are fixed international definitions. The speed tab also shows the Beaufort force.
Why the sea uses different units
There's a reason. The nautical mile is built from the Earth itself.
One nautical mile equals one minute of latitude (1/60 of a degree). On a chart, the latitude scale is the distance scale — you can read distances straight off the margin. Land miles and kilometres can't do that. It's now fixed at exactly 1,852 m.
The knot comes from the same world. Sailors trailed a rope knotted at fixed intervals and counted how many ran out in a set time. Hence 1 knot = 1 nautical mile per hour.
Distance — nautical miles and cables
| Unit | Conversion · use |
| 1 nautical mile (NM) | 1,852 m = 1.852 km — standard for sea and air |
| 1 cable | 185.2 m (one tenth of a NM) — close-quarters distances |
| 1 statute mile | 1,609 m — land measure, not the same thing |
| 10 NM | About 18.5 km |
| 100 NM | About 185 km |
Confusing the two introduces a 15% error — potentially serious in passage planning. Charts and GPS use nautical miles; road navigation uses kilometres or statute miles.
Speed — getting a feel for knots
| Knots | km/h · m/s · context |
| 1 kn | 1.85 km/h · 0.51 m/s |
| 5 kn | 9.3 km/h · 2.6 m/s — small boat cruising |
| 10 kn | 18.5 km/h · 5.1 m/s — yacht at pace |
| 20 kn | 37 km/h · 10.3 m/s — ferry cruising |
| 30 kn | 55.6 km/h · 15.4 m/s — fast craft |
| 50 kn | 92.6 km/h · 25.7 m/s — storm-force wind |
Mental shortcut — knots × 2, minus 10% approximates km/h. Twenty knots: 40 − 4 = about 36 (actual 37). And m/s is roughly half the knots: 10 kn ≈ 5 m/s.
⭐ The Beaufort scale
Devised by the Royal Navy in 1805 to judge wind strength from the state of the sea, running from 0 to 12.
| Force · name | Wind (m/s) · sea state |
| 0 Calm | Under 0.3 · sea like a mirror |
| 1 Light air | 0.3–1.5 · ripples |
| 2 Light breeze | 1.6–3.3 · small wavelets |
| 3 Gentle breeze | 3.4–5.4 · large wavelets |
| 4 Moderate breeze | 5.5–7.9 · frequent whitecaps |
| 5 Fresh breeze | 8.0–10.7 · moderate waves — small craft caution |
| 6 Strong breeze | 10.8–13.8 · large waves, spray |
| 7 Near gale | 13.9–17.1 · foam blown in streaks |
| 8 Gale | 17.2–20.7 · high waves, dense spray |
| 9 Strong gale | 20.8–24.4 · very high waves |
| 10 Storm | 24.5–28.4 · sea surface white |
| 11 Violent storm | 28.5–32.6 · exceptionally high waves |
| 12 Hurricane force | 32.7 and above · air filled with spray |
Enter a wind speed in the speed tab above and the force appears automatically.
Depth — fathoms and swell
| 1 fathom | 6 feet = 1.8288 m — from the span of outstretched arms |
| 10 fathoms | About 18.3 m |
| 20 fathoms | About 36.6 m |
| 100 fathoms | About 182.9 m |
The fathom is literally an arm span — the length hauled hand over hand while sounding with a weighted line. Korean charts use metres, but overseas charts and sounder settings often retain fathoms and feet.
Swell is usually metric locally and imperial in English sources. Remember 1 m ≈ 3.3 ft. A 3 m swell is about 10 ft — serious water for a small boat.
Pressure — hPa and inHg
| 1 hPa | = 1 mb — identical values |
| Standard pressure | 1,013.25 hPa = 29.92 inHg = 760 mmHg |
| Typhoon central pressure | Lower means stronger (below about 950 hPa is severe) |
US sources typically use inches of mercury. Remembering that 29.92 is standard gives you the reference point.
Reading marine forecasts
- Strong wind radius — the radius from a typhoon centre where winds reach 15 m/s
- Storm radius — where winds reach 25 m/s
- To convert international bulletins, multiply knots by about 0.51: 30 kn ≈ 15.4 m/s, 50 kn ≈ 25.7 m/s
Note — the Korea Meteorological Administration changed its typhoon intensity classification in 2026, replacing the previous category names with a new system. Check the KMA's current guidance when reading typhoon bulletins.
Where this helps
| 🎣 Sea fishing | Force 5 (8 m/s) and above warrants rethinking a small-boat trip |
| ⛵ Sailing | Passage planning — 10 NM at 5 knots takes two hours |
| 🌊 Diving | Converting overseas dive-site depths in feet or fathoms |
| 🌀 Typhoon tracking | Comparing knot-based bulletins with local m/s figures |
| ✈️ Aviation | Aircraft use nautical miles and knots too |
FAQ
Why do nautical and statute miles differ?
The statute mile is a land custom (1,609 m); the nautical mile derives from the Earth's latitude (1,852 m) — about 15% apart.
Is "knots per hour" correct?
No. A knot already means per hour (one NM per hour), so "20 knots per hour" is redundant.
Is a 1 m swell safe?
It depends on vessel size and wave period. Forecast heights are averages, and individual waves run considerably higher. Small craft should allow margin.
Why do some sources use feet?
Metric is the international standard, but English-language charts and equipment often retain feet and fathoms. Check whether your device lets you switch units.
What about everyday units?
Length, weight, volume and temperature are covered in the general unit converter.
These units aren't awkward by accident — they exist to find your way across a globe. Knowing that one nautical mile is one minute of latitude changes how a chart reads.
This converter is for general information. Base actual navigation, fishing and diving decisions on official marine forecasts and charts, and follow official guidance and professional judgement on matters of safety.
