How Fast Is a Knot? Knots to mph and km/h for Boats

How fast is a knot?

One knot is 1.852 km/h, or 1.151 mph: one nautical mile per hour. If you are used to road speeds, a yacht cruising at 7 knots is doing about 13 km/h, which is walking pace on land and quite brisk on water.

Knots to mph and km/h

Knots	mph	km/h
5	5.75	9.26
10	11.51	18.52
15	17.26	27.78
18	20.71	33.34
20	23.02	37.04
25	28.77	46.30
30	34.52	55.56
40	46.03	74.08

Multiply knots by 1.852 for km/h and by 1.151 for mph. For anything not in the table, the speed converter does it instantly.

Why not just use km/h?

Because a nautical mile is not an arbitrary distance. It is defined as one minute of arc of latitude, one sixtieth of a degree. That makes navigation arithmetic trivial: travel 60 nautical miles due north and you have moved exactly one degree of latitude, which you can read straight off the side of a chart with dividers. No conversion factor, no calculator.

The nautical mile was fixed at exactly 1,852 metres by the International Hydrographic Conference in 1929, and that is the figure every conversion above uses.

How knots were measured with a rope

The name comes from the chip log. A flat wooden board, the chip, was weighted on one edge so it stood upright in the water and stayed put while the ship sailed away from it. It was tied to a line wound on a free-running reel, and the line had knots tied in it at equal spacing. A sailor threw the chip over the stern, another turned a sandglass, and a third counted the knots that slipped through his fingers until the sand ran out. That count was the ship’s speed in knots.

The arithmetic is what makes the method work. The knots were spaced 47 feet 3 inches apart and the glass ran for 28 seconds.

• A nautical mile is 1,852 m, or about 6,076 feet.
• An hour is 3,600 seconds. 28 seconds is 1/128.6 of an hour.
• A ship doing one nautical mile per hour covers 6,076 / 128.6 = 47.25 feet in 28 seconds. That is 47 feet 3 inches.

So each knot that passed in 28 seconds meant exactly one nautical mile per hour. The spacing and the glass were chosen together so that no conversion was needed on deck. The unit outlived the method by about two centuries.

Speed over ground vs speed through water

This is the distinction that catches people out, and it is where a GPS speedometer differs fundamentally from a boat’s own log.

• Speed through water (STW) is what a paddlewheel or impeller log measures: your speed relative to the water around you.
• Speed over ground (SOG) is what GPS measures: your speed relative to the seabed.

In a 2-knot tidal stream, a boat making 6 knots through the water is doing 8 knots over the ground with the tide and 4 against it. Both numbers are correct; they answer different questions. GPS gives you SOG, which is what you want for estimating arrival time. STW is what you want for trimming sails.

Comparing the two is the standard way to measure the set and drift of a current, a genuinely useful trick that costs nothing when you have both readings in front of you.

Typical speeds in knots

Numbers on the water are smaller than most people expect. These are typical ranges, not limits.

Craft	Typical range	In km/h
Sailing yacht, cruising	5 to 8 kn	9 to 15
Ferry	15 to 25 kn	28 to 46
Cruise ship	18 to 22 kn	33 to 41
Speedboat or RIB	30 to 50 kn	56 to 93
Container ship	18 to 24 kn	33 to 44

A sailing yacht’s top speed is set by its waterline length, not its engine: a displacement hull cannot climb over its own bow wave, so a 10 m boat tops out near 7.5 knots however hard it is pushed. Anything faster has to plane, which is why speedboats look the way they do.

Using a phone as a speed log

Conditions on the water are close to ideal for GPS: an unobstructed horizon in every direction and no buildings to bounce signals off. Accuracy is usually better on a boat than anywhere on land. The phone reports speed over ground through the Geolocation API’s coords.speed value, in metres per second, and the app converts it to knots.

Practical notes:

• Switch the boat speedometer to kn. The dial rescales to a sensible 0 to 140 range and the distance readout switches to nautical miles automatically.
• GPS needs no data connection, so speed and trip statistics keep working well offshore.
• Save the passage at the end and export it as GPX. Every chartplotter and route-planning tool reads GPX.
• Spray and salt are the real enemy. Use a waterproof case, or keep the phone below.
• Continuous GPS on a long passage will flatten a phone. Bring a power bank.

Sources

• NIST Special Publication 811, Appendix B, lists the nautical mile as exactly 1,852 m, the value adopted by the 1929 International Hydrographic Conference.
• NOAA Ocean Service: What is the difference between a nautical mile and a knot? describes the chip log and the 47 foot 3 inch spacing.
• W3C Geolocation API defines the coords.speed attribute a phone reports.