How Fast Is a Knot?
One knot is 1.15 mph — a strange-looking number that exists because the nautical mile was built from the shape of the Earth rather than from a king’s foot.
Published September 21, 2026
One knot is 1.1508 mph, or 1.852 km/h. A ship doing 20 knots is doing 23 mph — a figure that sounds unimpressive until you remember it is doing it continuously, for weeks.
The awkward conversion factor is not an accident of history. It is the one unit in common use that was derived from the planet itself.
Why the nautical mile exists
A nautical mile is one minute of latitude — one sixtieth of a degree, measured north to south.
That definition is enormously practical at sea. A navigator with a chart and a sextant measures angles; if distance and angle share a unit, a position fix becomes arithmetic instead of a lookup table. Sixty nautical miles is one degree of latitude, anywhere on Earth, forever.
The statute mile has no such property. It descends from the Roman mille passus, a thousand paces, and its modern length was fixed by an English statute in 1593 for reasons that had nothing to do with navigation.
The nautical mile is now defined as exactly 1,852 metres, which is a rounding of the true average minute of latitude — the Earth is slightly flattened, so a minute of latitude is a little longer at the poles than at the equator.
A knot, then, is simply one nautical mile per hour. Not "knots per hour", which is a common slip and would be an acceleration.
Where the name came from
From an actual knot in an actual rope.
Sailors measured speed by throwing a weighted board — the chip log — over the stern and letting the attached line run out freely. The line had knots tied at fixed intervals, and the crew counted how many passed through their hands while a sandglass emptied.
Get the ratio right and the count is the speed. With knots spaced 47 feet 3 inches apart and a 28-second glass, one knot passing per glass equals one nautical mile per hour. The unit is named after the measuring instrument, and it stuck long after anyone stopped throwing wood off the back of ships.
What things actually do
| Speed | In mph | What moves at it |
|---|---|---|
| 6 knots | 7 mph | Cruising sailing yacht |
| 12 knots | 14 mph | Small ferry, fast trawler |
| 20 knots | 23 mph | Container ship at sea |
| 30 knots | 35 mph | Fast ferry, naval frigate |
| 135 knots | 155 mph | Airliner on final approach |
| 470 knots | 541 mph | Airliner at cruising altitude |
Convert any of these with the knots to mph converter.
Aviation kept knots for the same reason shipping did: charts, flight levels and navigation all work in nautical miles, and mixing units into that is how mistakes happen. It is also why aircraft altitudes are still given in feet worldwide, even in countries entirely metric on the ground.
Knots in wind forecasts
Marine and aviation forecasts give wind in knots, which is where most people meet the unit without owning a boat:
- 10 knots (12 mph) — pleasant sailing breeze
- 20 knots (23 mph) — whitecaps everywhere, small boats uncomfortable
- 34 knots (39 mph) — gale force, the threshold for a gale warning
- 64 knots (74 mph) — hurricane force
The Beaufort scale, which those thresholds come from, was written in 1805 to describe the effect of wind on a full-rigged ship rather than its speed, because measuring wind speed at sea was impractical. The numbers were attached later.
If you are converting a forecast rather than a boat speed, mph to km/h may be the second conversion you need.