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Nautical Unit Converter

Converter

Convert Nautical Units

Result

Enter a value and select units to see the conversion result.

About This Tool

Nautical Unit Converter – Distance, Speed, and Depth

The Nautical Unit Converter is a specialised tool for mariners, sailors, coastal navigators, maritime students, and aviation professionals who need to translate between the unique measurement system used at sea and everyday metric or imperial units. Whether you are planning an offshore passage, reading a tide table, interpreting a depth sounder, or studying for a maritime exam, this converter handles every common nautical measurement category in one place.

Distance and Length Conversions

The most fundamental nautical distance unit is the international nautical mile (nmi), defined as exactly 1,852 metresby international agreement. Its value is derived from one arc-minute of latitude on the Earth's surface, which is why pilots and navigators find it so convenient: one degree of latitude always equals exactly 60 nautical miles, making chart work and position calculation straightforward.

This converter supports nine distance units:

  • Nautical mile (nmi) – the standard for ocean navigation worldwide.
  • Statute mile (mi) – 1,609.344 m, used in US and UK inland contexts.
  • Kilometer (km) and meter (m) – the SI metric units.
  • Cable – International – exactly 185.2 m, one tenth of a nautical mile. Used to express short distances between vessels or between a vessel and a hazard.
  • Cable – UK Admiralty – 185.3184 m, historically one tenth of the Admiralty mile. Still referenced in older British publications and charts.
  • Fathom – exactly 6 feet (1.8288 m), traditionally used for sounding depths and still found on many charts.
  • Foot (ft) – 0.3048 m, used in US depth soundings and aviation altimetry.
  • Nautical league – 3 nautical miles (5,556 m), an older unit occasionally found in historical sailing texts.

Speed Conversions and the Beaufort Scale

At sea, speed is measured in knots — one knot equals exactly one nautical mile per hour, or 0.514444 m/s. The converter supports bidirectional conversion between knots, km/h, mph, m/s, and ft/s using exact factors so no precision is lost in the conversion chain.

When working in speed mode, the tool also displays the corresponding Beaufort wind force based on the WMO 2012 table. The Beaufort scale classifies wind speed from Force 0 (Calm, sea like a mirror) to Force 12 (Hurricane Force, visibility severely restricted by foam and spray). This helps mariners quickly gauge sailing or safety conditions alongside any speed conversion.

Depth Conversions

Water depth is measured in fathoms, metres, or feet depending on the chart edition and region. The converter handles all three with exact factors, making it easy to cross-reference a depth sounder reading in metres against an older chart that lists depths in fathoms.

How Conversions Are Calculated

All conversions use a single base unit per category (metres for distance and depth, m/s for speed) and apply exact multiplication factors:

result = inputValue × (fromUnitInBase / toUnitInBase)

For example, to convert 10 fathoms to metres: 10 × 1.8288 = 18.288 m. The formula and full calculation are shown below each result for transparency.

All Equivalent Units Table

The converter displays a table showing the input value expressed in every supported unit of the selected category at once. This lets you compare distances or speeds across all unit systems without repeating the conversion manually, which is particularly useful when writing passage plans or converting chart data.

Practical Examples

  • A vessel sailing at 12 knots covers approximately 22.22 km/h (13.81 mph or 6.17 m/s).
  • A depth of 10 fathoms equals 18.29 metres or 60 feet — a quick sanity check when interpreting a sounding.
  • A passage of 200 nautical miles equals 370.4 kilometres or about 230 statute miles.
  • A clearance listed as 3 cables (international) is 555.6 metres.

Tips for Accurate Navigation

When converting between nautical and statute measurements, always confirm which cable standard a publication uses (International vs. UK Admiralty) — the difference of about 0.1 m per cable is negligible for most purposes but can matter in precision harbour berthing data. For depth conversions, check whether a chart datum uses metres or fathoms before applying soundings to a passage plan.

Frequently Asked Questions

Is the Nautical Unit Converter free?

Yes, Nautical Unit Converter is totally free :)

Can I use the Nautical Unit Converter offline?

Yes, you can install the webapp as PWA.

Is it safe to use Nautical Unit Converter?

Yes, any data related to Nautical Unit Converter only stored in your browser (if storage required). You can simply clear browser cache to clear all the stored data. We do not store any data on server.

How does the Nautical Unit Converter work?

The converter uses exact conversion factors standardised by international bodies. For distance, it converts through meters as the base unit (1 nmi = 1,852 m exactly). For speed it uses meters per second as the base, and for depth it reuses the same distance factors. Simply select a category, enter a value, choose the source and target units, and the result appears instantly.

What is a nautical mile and why is it used at sea?

A nautical mile is defined as exactly 1,852 metres. It was historically tied to one arc-minute of latitude, making it a natural unit for celestial and chart navigation. One degree of latitude equals 60 nautical miles, so plotting positions and measuring distances on a chart is straightforward without conversion.

What is the difference between a knot and a nautical mile?

A knot is a unit of speed equal to one nautical mile per hour (1 kn = 1.852 km/h = 0.514444 m/s). A nautical mile is a unit of distance. Saying 'a vessel is doing 15 knots' means it covers 15 nautical miles every hour.

What is a fathom and where is it used?

A fathom is exactly 6 feet or 1.8288 metres. It is the traditional unit for measuring water depth and is still found on older nautical charts. Modern charts increasingly use metres, but fathom depths remain common in the US, UK, and Canada for historical and regulatory reasons.

How accurate are the conversion results?

All distance and depth conversions use exact or BIPM-standardised factors (e.g. 1 nmi = 1,852 m exactly; 1 fathom = 1.8288 m exactly). Speed conversions use the exact definition of the knot (1.852 km/h). Results are displayed to four decimal places by default; you can adjust precision from 0 to 10 decimal places.

What are the two different types of cable used in this converter?

An International Cable is exactly 185.2 m (one tenth of a nautical mile). A UK Admiralty Cable is slightly longer at 185.3184 m (historically one tenth of an Admiralty mile). The difference is small but matters in precise chart work. This converter distinguishes between the two so you can choose the correct standard.