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.
1. What it gives you. Convert nautical miles, knots and fathoms in one place, and enter a wind speed to see its Beaufort force alongside. The conversion factors come straight from the NIST Guide to the SI, Appendix B — nautical mile 1,852 m (exact), knot 0.5144444 m/s.
2. What people get wrong. A nautical mile is 15.08% longer than a statute mile — eyeballing “45 m/s” against “90 knots” will not line up. And temperature converts differently as an interval than as a scale.
3. How to use it. Treat this as reference — actual navigation, fishing and diving decisions belong to official marine forecasts and charts.
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 |
Where these numbers come from
The conversion factors are taken from the US National Institute of Standards and Technology's Guide to the SI, Appendix B — a document that marks which factors are exact and which are rounded, so you can also see where a value stops being precise.
| Unit | NIST source | This converter | Match |
|---|---|---|---|
| Nautical mile | 1.852 E+03 m (exact) | 1,852 m | |
| Knot | 5.144 444 E−01 m/s | 0.5144444 m/s | |
| Mile | 1.609 344 E+03 m (exact) | 1,609.344 m | |
| Foot | 3.048 E−01 m (exact) | 0.3048 m | |
| Standard atmosphere | 1.013 25 E+05 Pa (exact) | 1,013.25 hPa | |
| Fathom | 1.828 804 E+00 m (based on the U.S. survey foot) | 1.8288 m | different basis |
Five matched exactly. The nautical mile at 1,852 m and the mile at 1,609.344 m are printed by NIST as exact, so there is no rounding to argue about.
Only the fathom diverged. “One fathom equals six feet” holds — the question is which foot.
· International foot 0.3048 m × 6 = 1.8288 m ← what this converter uses
· U.S. survey foot 1200/3937 m × 6 = 1.828804 m ← what NIST prints
The gap is 3.66 micrometres per fathom, or 0.37 mm over 100 fathoms (about 183 m, our calculation).
Irrelevant in practice — but “not the same six feet” is worth recording.
Nautical mile against mile — 15.08%
Dividing the two exact values: 1,852 ÷ 1,609.344 = 1.15078 (our calculation).
- A nautical mile is 15.08% longer than a land mile. The usual “about 15%” is 15.078% precisely.
- The definition of a knot checks out too — 1,852 ÷ 3,600 = 0.5144444… m/s, matching NIST to seven decimal places.
- A cable is one tenth of a nautical mile, so it lands on a round 185.2 m.
Why forecasts swapped millibars for hectopascals
1 millibar = 1.0 E+02 Pa = 100 Pa
1 hectopascal = 100 Pa
They are exactly the same unit. Not one digit changes.
Which is why “1013 mb” becoming “1013 hPa” on weather charts caused no confusion at all. The name moved into the SI system and the value stayed put. NIST prints the bar as exactly 1.0 E+05 Pa, so the millibar inherits that exactness.
| Pressure unit | NIST source (Pa) | In hPa |
|---|---|---|
| Millibar | 1.0 E+02 (exact) | 1 hPa |
| Standard atmosphere | 1.013 25 E+05 (exact) | 1,013.25 hPa |
| Inch of mercury | 3.386 389 E+03 | 33.86 hPa |
Converting aviation's 29.92 inHg gives 1,013.21 hPa (our calculation) — 0.04 hPa off the standard 1,013.25, showing that the inch figure is simply truncated at two decimals. Pressure and how it feels connects to the temperature work in our air conditioning electricity guide.
Temperature — an interval is not a scale
NIST lists Fahrenheit as “degree Fahrenheit (temperature interval)” at 5.555 556 E−01 — that is 5/9 = 0.5555… (our calculation).
That converts an interval, not a reading. Turning a sea temperature of 20°C into Fahrenheit still needs the separate step of adding 32. This converter's temperature tab handles that. Reading the same number on a different axis is the theme of our net pay table too.
Questions that remain
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?
Our general unit converter is currently offline. For length and mass, use the NIST table above; for body metrics see the BMI and calorie calculator.
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.
Sources
- US National Institute of Standards and Technology — agency source — Guide to the SI, Appendix B.8, conversion factors listed alphabetically (checked 30 July 2026). Source for the nautical mile at 1.852 E+03 m, the knot at 5.144444 E−01 m/s, the fathom (U.S. survey foot basis) at 1.828804 E+00 m, the bar at 1.0 E+05 Pa, the millibar at 1.0 E+02 Pa, the standard atmosphere at 1.01325 E+05 Pa, the inch of mercury at 3.386389 E+03 Pa and the Fahrenheit interval at 5.555556 E−01.
- US National Institute of Standards and Technology — agency source — Guide to the SI, Appendix B.9, conversion factors by kind of quantity (same date). Source for the mile at 1.609344 E+03 m, the foot at 3.048 E−01 m, the yard at 9.144 E−01 m and the inch at 2.54 E−02 m, and for which values are marked exact.
- Our own calculation — nautical mile ÷ mile = 1.15078, 1,852 ÷ 3,600 = 0.5144444, the U.S. survey foot fathom at 1.828803658 m and its 3.66 µm gap from the international-foot value (0.37 mm over 100 fathoms), 29.92 inHg = 1,013.21 hPa, and 5/9 = 0.5555…
Where to check further
- Which fathom definition your chart uses. The NIST table qualifies the fathom as “based on the U.S. survey foot”, while this converter uses the international foot (1.8288 m) — which applies to charts was not established (Korea's hydrographic office is blocked by robots.txt). The chart's own legend states its definition.
- The Beaufort bands against an agency source. The force bands shown here were not matched to an official original. Reading how your national marine forecast describes the same wind speed calibrates it faster.
- Safe limits for swell and depth. These vary with vessel size and wave period, so no figures appear here. Sailing decisions belong to marine warnings and coastguard or port authority guidance.
As of July 2026. Land distances and fuel appear in the cost of running a car in the US. The conversion factors are NIST's own; the ratios, gaps and checks are our arithmetic. Base actual navigation, fishing and diving decisions on official marine forecasts and charts, and follow official guidance and professional judgement on matters of safety.
