Knots to Miles per Hour

The name is literal. Sailors from the seventeenth century measured speed with a chip log: a weighted wooden board tossed over the stern on a line with knots tied at regular intervals. A small sandglass, often about 28 or 30 seconds, was turned, and the number of knots that ran out through the sailor's hands in that time was the ship's speed. The spacing was chosen so that one knot on the line equaled one nautical mile per hour.

The nautical mile itself was based on one minute of latitude, which varies slightly over the Earth. In 1929 an international hydrographic conference in Monaco fixed it at exactly 1,852 m, and the US adopted that value in 1954. Since then the knot has been exactly 1,852 m per hour, or 1.150779 mph.

Ships, aircraft and weather services. Airspeed indicators, air traffic control instructions and aviation wind reports are in knots almost worldwide, and marine forecasts give wind and current the same way. A sailboat cruising at 6 kn is doing 6.9 mph, a container ship at 20 kn about 23 mph, and an airliner approaching to land at 140 kn roughly 161 mph.

Hurricanes are a good example of both units in use. The US National Hurricane Center works in knots, while public advisories show mph. Category 1 on the Saffir-Simpson scale begins at 64 kn, which is 74 mph, and Category 5 at 137 kn, 157 mph.

Saying knots per hour is redundant, since a knot already means per hour; knots per hour would describe acceleration. The other trap is treating knots like miles per hour, which understates speed by about 15%. A 25-knot wind sounds like a 25 mph breeze but is really 28.8 mph.

For metric speeds, see knots to km/h. Distances in nautical miles are covered on nautical miles to miles, and road speeds on mph to km/h.