2 min read · Updated 22 June 2026
A knot is a unit of speed equal to one nautical mile per hour — about 1.85 km/h or 1.15 mph. It is how boats and aircraft measure how fast they are going. A boat doing 10 knots is travelling at roughly 18.5 km/h. It is one of the first pieces of nautical language newcomers meet, and it comes with a genuinely interesting history.
Knots as speed
The key thing to get right: a knot is a speed, not a distance. You say a boat is "doing ten knots" — never "ten knots per hour", which would be like saying "miles per hour per hour". One knot is one nautical mile per hour, with the "per hour" already built in.
Converting knots
The conversions are worth memorising:
- 1 knot ≈ 1.852 km/h (multiply knots by about 1.85 for km/h)
- 1 knot ≈ 1.151 mph (multiply knots by about 1.15 for mph)
So a cruising yacht ambling along at 6 knots is doing about 11 km/h; a fast racing yacht surfing downwind at 20 knots is doing about 37 km/h; and the outright sailing speed records, set by specialist craft, exceed 60 knots.
Why nautical miles?
Boats use knots because they are built on the nautical mile, which is tied directly to the size of the earth: one nautical mile equals one minute of latitude. That makes navigation elegant — distances and speeds relate directly to the latitude lines on a nautical chart. Aircraft use knots for exactly the same reason, and the system has been standard at sea for centuries.
Where the name comes from
The word "knot" is a lovely piece of maritime history. Before instruments, sailors measured speed with a chip log: a weighted wooden panel on a long rope, with knots tied in the rope at regular intervals. The panel was thrown off the stern, and as the rope paid out over a set time (measured with a small sandglass), a sailor counted how many knots ran through their hands. More knots meant more speed — and the unit took its name from the knots on the rope.
Measuring speed today
Modern boats have moved on from the chip log. Speed through the water is measured by a small paddlewheel or sonic sensor in the hull, while speed over the ground comes from GPS — and the two can differ because of current. Racing crews watch both, along with derived figures like velocity made good, on their instruments. For the rest of the language of the sea, see the sailing terms glossary.
Frequently asked questions
- What is a knot in sailing?
- A knot is a unit of speed equal to one nautical mile per hour. It is how boats and aircraft measure speed. One knot is about 1.85 kilometres per hour, or about 1.15 miles per hour, so a boat doing 10 knots is travelling at roughly 18.5 km/h. The word describes a speed, not a distance — you would say a boat is doing 'ten knots', never 'ten knots per hour'.
- How fast is one knot?
- One knot equals one nautical mile per hour, which is approximately 1.852 kilometres per hour or 1.151 miles per hour. So to convert knots to km/h, multiply by about 1.85; to convert to mph, multiply by about 1.15. Ten knots is roughly 18.5 km/h or 11.5 mph.
- Why do boats use knots instead of kilometres per hour?
- Because a nautical mile is tied directly to the size of the earth — one nautical mile equals one minute of latitude. That makes navigation by chart and latitude simple: distances and speeds relate directly to the lines on a nautical chart. Aircraft use knots for the same reason, and the convention has stuck at sea for centuries.
- Where does the word 'knot' come from?
- From how sailors once measured speed. A rope with knots tied at regular intervals, weighted at one end by a wooden panel called a chip log, was thrown off the stern. As the rope ran out over a fixed time measured by a sandglass, a sailor counted the knots that passed through their hands — more knots meant more speed. The unit took the name of the knots on the rope.
- How do modern boats measure speed?
- Modern boats measure speed through the water with a small paddlewheel or sonic sensor in the hull, and speed over the ground with GPS. The two can differ because of current. Racing boats display both on their instruments, since the difference matters for tactics and navigation.