Primary sources · 4
- [1] First International Extraordinary Hydrographic Conference — Monaco 1929 conference that fixed the international nautical mile at exactly 1,852 metres · Conference proceedings via International Hydrographic Organization · 1929 https://iho.int/en/
- [2] BIPM SI Brochure (9th edition) — Lists the nautical mile as a non-SI unit accepted for use with SI · Bureau International des Poids et Mesures · 2019 https://www.bipm.org/en/publications/si-brochure
- [3] ICAO Annex 5 — Units of Measurement — Specifies the nautical mile as the primary unit for navigation distances in international civil aviation · ICAO Annex 5 to the Chicago Convention · Fifth edition, July 2010 https://www.icao.int/Pages/freepublications.aspx
- [4] U.S. Federal Aviation Regulations 14 CFR 1.1 — Defines distance units used in US flight rules; aligns with the international nautical mile · U.S. Federal Aviation Administration · Current https://www.ecfr.gov/current/title-14/chapter-I/subchapter-A/part-1/section-1.1
One nautical mile is exactly one minute of arc on a meridian — which is why pilots and ship navigators use it: a single number on a chart translates directly to a distance. The metric world has not been able to retire it because nothing else is as convenient for working on the globe.
Where 1,852 comes from
In 1929 the First International Extraordinary Hydrographic Conference in Monaco fixed the international nautical mile at exactly 1,852 metres. The value derives from a historical metric convention: if Earth's quarter-circumference along a meridian is 10,000,000 metres (the historical definition of the metre), then a minute of arc is 10,000,000 ÷ (90 × 60) = 1,851.85 m, rounded to 1,852.
The Mercator chart and the navigation use case
Gerardus Mercator's 1569 projection made the nautical mile the natural distance unit by accident. Mercator stretched the latitude axis non- linearly so that any constant-bearing course (a rhumb line) appears as a straight line on the chart. The latitude scale at the side of the chart is graduated in degrees, minutes, and seconds — and one minute of latitude is one nautical mile. A navigator wanting to measure a leg of a planned course simply walks a pair of dividers along the rhumb-line heading and counts ticks off the side scale, converting bearing into distance and back again with no unit conversion.
This is why the unit survives in modern jet aviation despite the calculator on the flight deck doing all the math. The chart is still graduated in minutes of latitude, the autopilot still steers magnetic courses, and the airway distances published by Eurocontrol and the FAA are still in nautical miles. The historical advantage has compounded into an installed-base advantage.
How fast in knots is a modern airliner?
The standard cruise speeds expressed in knots are deeply familiar to pilots and dispatchers in a way that km/h are not. The cluster between 440 and 490 knots is where almost every commercial passenger aircraft operates.
| Aircraft | Knots (KTAS) | km/h | Mach |
|---|---|---|---|
| Boeing 737-800 | ≈ 454 | ≈ 842 | 0.789 |
| Airbus A320neo | ≈ 449 | ≈ 833 | 0.78 |
| Embraer E195-E2 | ≈ 449 | ≈ 833 | 0.78 |
| Boeing 787-9 / A350-900 | ≈ 487 | ≈ 903 | 0.85 |
| Boeing 777-300ER | ≈ 488 | ≈ 905 | 0.84 |
| Airbus A380-800 | ≈ 486 | ≈ 900 | 0.85 |
Why navigators love it
A navigator looking at a chart can measure a distance directly with parallel rules off the latitude scale on the side: count the minutes of latitude between the two points and you have the nautical-mile distance, no calculator required. Statute miles or kilometres demand a separate distance scale and a unit conversion; nautical miles are baked into the chart.
| Domain | Primary unit | Notes |
|---|---|---|
| International civil aviation | Nautical miles | ICAO Annex 5 requires NM for navigation distance |
| U.S. domestic aviation | Nautical miles | FAA 14 CFR 1.1, aligned with ICAO |
| Maritime (international) | Nautical miles | International Maritime Organization conventions |
| Maritime (US inland) | Statute miles | Historically distinct from open-ocean NM |
| Surface road / rail | km or statute miles | Nautical miles are not used on land |
| Survey / geodesy | Metres | Nautical miles are an aviation/maritime artefact only |
Knots are nautical miles per hour
Aircraft and ships measure speed in knots — one knot is one nautical mile per hour. A Boeing 777 cruises at about Mach 0.84, which at FL370 is roughly 488 knots true airspeed. The unit aligns with the navigation- distance unit, so distance / speed gives time directly: 5,000 NM at 500 knots = 10 hours, no unit conversion.
Why a meter-based nautical mile would not help
Aviation could in principle adopt kilometres for navigation distance — some Russian-fleet specifications already use km internally — but the existing global chart infrastructure, the trained pilot population, and the convenient one-minute-of-arc property of the nautical mile mean the unit will likely outlive most other "imperial" survivors. The BIPM has listed it as accepted-for-use-with-SI since the 1960s, which is metric- system endorsement in all but the strictest interpretation.