The figures
A knot is one nautical mile per hour, and a nautical mile is defined as exactly 1,852 metres. The "per hour" is already built in, so "knots per hour" is a common and wrong construction — it would describe acceleration.
For mental work, knots to mph is add about 15%, and knots to km/h is add about 85%. A 20-knot wind is roughly 23 mph or 37 km/h.
The reason knots survive
A nautical mile is one minute of latitude along a meridian. Sixty nautical miles is one degree of latitude, everywhere on Earth.
That makes chart navigation direct in a way no arbitrary unit allows: distance can be read straight off the latitude scale printed on the edge of any chart, with no conversion and no separate distance scale. Before satellite positioning that was decisive, and it remains convenient enough that neither shipping nor aviation has any real interest in changing. The definition also means a nautical mile is tied to the size of the Earth rather than to a metal bar in a vault.
Where the name comes from
Ships once measured speed with a chip log: a weighted board on a line knotted at fixed intervals, thrown overboard and allowed to run out for a timed period, usually measured with a sandglass. Counting the knots that passed through the sailor's hands gave the speed directly — hence the unit.
The knot spacing was calibrated so the count corresponded to nautical miles per hour, which is a rather elegant piece of analogue computation. The method persisted into the twentieth century before mechanical and then electronic logs replaced it.
Reading wind and speed figures
Marine and aviation forecasts give wind in knots. A useful scale: 10 knots is a gentle breeze, 20 knots raises whitecaps and is a brisk sailing wind, 34 knots is the threshold for a gale, and 64 knots marks hurricane force. Tropical cyclone categories are defined in knots and converted for public forecasts, which is why the mph thresholds look like odd numbers.
Aircraft speeds are quoted in knots almost universally — a jet cruising at 480 knots is doing about 552 mph or 889 km/h. Ground speed differs from airspeed by the wind, which is why the same flight takes different times in each direction. For land-based speeds, the mph to km/h converter is the tool you want.
Frequently asked questions
How fast is 20 knots in mph?
About 23 mph, or 37 km/h. A knot is 1.15078 mph, so adding roughly 15% converts knots to mph.
Why do ships and aircraft use knots?
A nautical mile is one minute of latitude, so sixty nautical miles is one degree everywhere on Earth. Distance can be read directly off a chart’s latitude scale without any conversion.
Is it wrong to say knots per hour?
Yes. A knot already means one nautical mile per hour, so knots per hour would describe acceleration rather than speed.