How fast is a knot, and how long is a nautical mile?
A nautical mile is exactly 1,852 metres—about 1.15 statute miles—and corresponds closely to one minute of latitude. A knot is a unit of speed: one nautical mile per hour.
Related terms
How it works
The nautical mile earns its place because it is built into the chart. One minute of latitude is very nearly one nautical mile, so the latitude scale on the side of any chart doubles as a distance scale—set dividers against it and step distances off directly. The international value was fixed at exactly 1,852 metres in 1929.
A knot is the matching rate: one nautical mile per hour, roughly 1.15 mph. Speed, time, and distance then work in one system—a passage of 30 nautical miles at 6 knots takes about five hours—and logs, GPS, AIS, and forecasts all report speeds in knots.
The log line and the minute of latitude
The word knot survives from the seventeenth-century common log: a wooden board streamed astern on a line knotted at regular intervals. Sailors counted the knots slipping through their fingers against a sandglass to estimate speed. Centuries later, the 1929 International Extraordinary Hydrographic Conference standardized the nautical mile the knot is built on at exactly 1,852 metres.
On the water
- A passage of 30 nautical miles at 6 knots takes about five hours.
- One minute on the chart’s latitude scale spans about one nautical mile—measure distance there, never on the longitude scale.
- A boat making 10 knots covers roughly 11.5 statute miles in an hour.
Common mistake
“Knots per hour” is wrong—a knot already contains the per hour. Ten knots per hour would describe acceleration, not speed.
Remember it
One minute of latitude ≈ one nautical mile; one nautical mile per hour = one knot.