What is a Knot? Understanding Speed at Sea and Air
Why do sailors and pilots use Knots instead of MPH? Learn the history of the log line, the definition of a Nautical Mile, and how to convert to land speed.
What is a Knot? Speed at Sea and Air
If you board a plane or a boat, speed is measured in knots. Why not use "miles per hour" (mph) or "kilometers per hour" (km/h)? Is it just tradition, or is there a scientific reason?
Quick Answer (TL;DR)
- 1 Knot (kt) = 1 Nautical Mile per hour.
- Value: 1.15 mph or 1.85 km/h.
- Why use it?: One nautical mile matches one minute of latitude on the Earth's surface. It makes navigating maps incredibly easy.
The Nautical Mile Explained
To understand Knots, you must understand the Nautical Mile (NM).
- Statute Mile (Land): 5,280 feet (arbitrary Roman history).
- Nautical Mile (Sea/Air): Based on the Earth's geometry.
- The Earth is a circle (360 degrees).
- Each degree has 60 minutes.
- 1 Nautical Mile = 1 Minute of Latitude.
This means if you travel 60 nautical miles straight North, you have traveled exactly 1 degree of latitude on the chart. No calculator needed.
Why is it called a "Knot"?
The name comes from the old method of measuring speed: the Chip Log.
- Sailors threw a wooden panel (the chip) tied to a rope into the water.
- The rope had actual knots tied at specific intervals (47 feet, 3 inches).
- They let the line pay out for 28 seconds (measured by a sandglass).
- They counted how many knots went overboard.
- If 10 knots went out, the speed was 10 knots.
The math works out: The spacing of the physical knots vs the 28-second timer ratio is exactly proportional to Nautical Miles per Hour.
Conversion Guide
- Knots to MPH: Multiply by 1.15.
- Example: A cruise ship doing 20 knots is doing ~23 mph.
- Knots to km/h: Multiply by 1.85.
- Example: A plane doing 500 knots is doing ~925 km/h.
Common Speeds in Knots
| Vehicle | Speed (Knots) | Speed (MPH) |
|---|---|---|
| Cargo Ship | 12 - 22 kts | 14 - 25 mph |
| Cruise Ship | 20 - 24 kts | 23 - 28 mph |
| Nuclear Sub | 30+ kts | 35+ mph |
| Speedboat | 30 - 60 kts | 35 - 70 mph |
| Small Cessna | 110 kts | 126 mph |
| Boeing 737 | 450 kts | 517 mph |
| F-18 Fighter | 1,000+ kts | 1,150+ mph |
Aviation Use
Planes use Knots because early navigation was done exactly like ships—using charts and latitude.
- Airspeed: Measured in Knots (Indicated Airspeed - IAS).
- Wind: Measured in Knots.
- Distance: Measured in Nautical Miles.
Trivia: Road distance to the airport is in statute miles, but as soon as the wheels leave the ground, the pilot switches to nautical miles.
Conclusion
The Knot isn't just an old sailing term; it is the only unit of speed tied directly to the geography of planet Earth.
- 1 Knot = 1 Minute of Latitude per Hour.
- It is roughly 15% faster than mph.
Last Updated: February 7, 2026
Reading Time: 5 minutes