Nautical miles vs statute miles: what's the difference?

In aviation, a nautical mile (NM) is exactly 1,852 metres, while a statute mile (mi or SM) is exactly 1,609.344 metres. One nautical mile therefore equals about 1.151 statute miles. Pilots use nautical miles for navigation and knots for speed; statute miles mainly appear in some weather and visibility reporting, especially in the United States.

Why does aviation use nautical miles?

A nautical mile suits geographic navigation because it was historically based on one minute of latitude. A degree contains 60 minutes, so latitude and longitude charts made nautical distances practical to measure and calculate.

Because Earth is not a perfect sphere, the physical length of an arc minute varies slightly. The modern nautical mile avoids that ambiguity by fixing the distance at exactly 1,852 metres.

Flight plans, airways, airspace radii, GPS distances and DME indications normally use nautical miles. Our explanation of how aircraft navigation systems calculate and display distance covers the equipment behind those readings.

How do you convert nautical miles to statute miles?

Multiply nautical miles by 1.15078 to obtain statute miles; multiply statute miles by 0.868976 to obtain nautical miles.

• statute miles = nautical miles × 1.15078
• nautical miles = statute miles × 0.868976
• 10 NM equals approximately 11.51 statute miles.
• 50 statute miles equals approximately 43.45 NM.

For metric flight planning, see our separate guide to converting nautical miles and kilometres in aviation.

Which type of mile do pilots use?

Pilots use nautical miles for most operational distances, including route legs, distance to a waypoint, radar separation and distance from an airport. If ATC reports an aircraft or fix as five miles away in a normal aviation context, that generally means five nautical miles unless a local procedure states otherwise.

Not every measurement around an aircraft uses NM. Runway lengths are usually published in feet or metres, altitude is commonly given in feet, and horizontal visibility may be reported in statute miles, metres or kilometres according to the national weather format.

Why do METAR reports sometimes use statute miles?

United States METAR visibility is reported in statute miles, which is why groups such as 3SM, 1/2SM and P6SM appear. The last example means visibility greater than six statute miles.

Many ICAO-style reports instead encode prevailing visibility in metres. For example, 9999 means 10 kilometres or more, not 9,999 miles. Runway visual range may be reported in feet or metres depending on the local format, so read the unit rather than assuming every aviation distance is nautical.

Are nautical miles and knots the same thing?

A nautical mile measures distance, while a knot measures speed: one knot is one nautical mile per hour. The statute-mile equivalent is miles per hour, so 100 knots is approximately 115.1 mph.

KIAS means indicated airspeed expressed in knots; it is not automatically true airspeed or groundspeed. We explain that distinction in our guide to KIAS, speed conversions and true airspeed.

What mistakes occur in flight simulators?

The most common mistake is treating every displayed “mile” as a statute mile. Cockpit avionics and flight plans usually retain aviation units even when a simulator's general interface uses imperial or metric measurements.

• Flight-plan distance: Treat FMS, GPS and DME distances labelled NM as nautical miles. DME also measures slant range to the station, so it can exceed horizontal distance when the aircraft is close and high.
• Weather settings: Check whether a visibility field expects statute miles, nautical miles, kilometres or metres. Entering 10 without checking the unit can produce very different conditions.
• Approach planning: The familiar descent estimate of roughly 300 feet per mile means 300 feet per nautical mile. Our visual-approach planning method shows how that rule is applied.
• Abbreviations: Use NM for nautical miles and SM or mi for statute miles. Lower-case nm is the SI symbol for nanometres, not nautical miles.