Nautical miles per minute
Converting groundspeed from knots into nautical miles per minute is one of the quickest pieces of mental math a pilot can use. It makes ETA estimates, holding distance, top-of-descent checks and position updates much easier.
The basic conversion
Knots are nautical miles per hour, so divide groundspeed by 60 to get nautical miles per minute. At 120 knots, the airplane covers 2 NM each minute. At 90 knots, it covers 1.5 NM per minute.
Seconds per nautical mile
Another useful form is seconds per NM. Divide 3,600 by groundspeed in knots. At 120 knots, one NM takes about 30 seconds. At 180 knots, one NM takes about 20 seconds.
Quick ETA example
If 24 NM remain and groundspeed is 120 knots, the airplane is covering 2 NM per minute, so the time remaining is about 12 minutes.
Why groundspeed matters
Use groundspeed for distance-over-ground timing. True airspeed does not include the wind effect and can produce the wrong ETA when a meaningful headwind or tailwind exists.
Holding and procedure math
If groundspeed is 90 knots, a one-minute leg covers roughly 1.5 NM in still conditions. Actual holding tracks are affected by wind correction, turn geometry and procedure requirements, so the simple calculation is only a planning estimate.
Descent planning
NM per minute helps connect altitude to distance. If a descent must lose 3,000 feet over 10 NM and groundspeed is known, you can quickly estimate both the time available and the vertical speed required.
Related PilotDesk tools
Top of descent · Holding leg distance · 3° descent rate