Knots vs MPH
Knots and miles per hour — why aviation and shipping use a different speed unit
Understanding the differences between Knot (kn) and Miles per Hour (mph) is essential for choosing the right system for your needs. This comparison covers the key differences, advantages of each approach, and practical guidance on when to use one over the other. Whether you are a student, engineer, scientist, or just curious about measurement systems, this guide will help you understand the trade-offs and make informed decisions.
Knot (kn)
One nautical mile per hour, exactly 1.852 km/h or about 1.15078 mph. The standard speed unit in aviation, maritime navigation and meteorology.
Miles per Hour (mph)
Statute miles per hour — the road-speed unit in the US and UK, and the one most people have intuition for.
Key Differences
- 1A knot is faster than a mile per hour: 1 kn = 1.15078 mph. A 100-knot approach speed is about 115 mph.
- 2The reason knots survive is navigational, not traditional. One nautical mile is one minute of latitude, so a speed in knots converts directly into degrees of latitude per hour on a chart. That property is lost with statute miles.
- 3Wind speed in aviation and marine forecasts is in knots worldwide, including in fully metric countries. Public weather forecasts in the same countries may use km/h or m/s for the same wind.
- 4Saying knots per hour is an error — a knot already contains the per-hour. It is a speed, not a distance.
- 5The nautical mile is now defined as exactly 1852 metres by international agreement, having originally been derived from the Earth's circumference, which varies slightly with latitude.
Background
The name comes from the chip log — a weighted board thrown overboard on a line knotted at fixed intervals. Sailors counted knots paid out against a sand glass, and the count gave speed directly. The unit outlasted the technique because of the latitude relationship: a navigator working on a chart in degrees and minutes gets distance and speed in the same family of units, which mattered enormously before satellite positioning and still simplifies dead reckoning today.
When to Use Knot (kn) vs Miles per Hour (mph)
Use Knot (kn) when…
Use knots for anything involving navigation over the Earth's surface — aircraft airspeed and groundspeed, ship speed, wind in aviation and marine forecasts, and ocean current.
Use Miles per Hour (mph) when…
Use mph for road vehicles and general public communication in the US and UK, where it is the unit people have a feel for.