Knots to km/h

Multiply by 1.852. That figure is exact, not rounded.

The formula

km/h = knots × 1.852 mph = knots × 1.150779

One knot is one nautical mile per hour, and one nautical mile is defined as exactly 1,852 metres.

Why 1,852 metres?

Because a nautical mile is one minute of latitude. The Earth is roughly 40,000 km around, divided into 360 degrees of 60 minutes each — 21,600 minutes in total, giving about 1,852 metres each.

This is the reason the unit refuses to die. On a nautical chart, one minute of latitude on the side scale is one nautical mile, so a navigator can measure distance with dividers against the latitude scale and needs no conversion at all. No other unit has that property.

Where the name comes from

From an actual knot in an actual rope. Sailors trailed a weighted wooden board — the chip log — behind the ship on a line knotted at regular intervals. A sandglass was turned, the line paid out, and the knots that ran through the sailor's hands in the allotted time were counted.

The spacing was calibrated so that the count equalled the ship's speed in nautical miles per hour. Hence "making eight knots", and hence the ship's log.

Typical speeds

Speed over ground versus through water

GPS gives you speed over ground. A boat's paddlewheel log gives speed through water. The difference between them is the current, which is why sailors watch both — see boat speedometer.

Self Speed reads out in knots and reports distance in nautical miles when you select those units.

Knots in aviation

Aircraft use knots for the same reason ships do: navigation charts are laid out in degrees and minutes of latitude, and one minute is one nautical mile. An aviation chart and a marine chart share that geometry.

Pilots deal with several different speeds that are easy to confuse:

Air is thinner at altitude, so the same indicated airspeed corresponds to a much higher true airspeed high up. A jet indicating 280 knots at 35,000 ft is actually moving through the air at around 480 knots.

Ground speed is what GPS measures — the aviation equivalent of speed over ground at sea, and what determines your arrival time. A 100 knot jet stream tailwind is the difference between a seven-hour and a nine-hour Atlantic crossing.

Wind speed and the Beaufort scale

Wind is also reported in knots at sea. The Beaufort scale, devised by Francis Beaufort in 1805, described wind by its observable effect on a ship rather than by measurement — useful when no instrument existed.

Force 6 is the point at which most small-boat sailors stay in harbour, and 34 knots — the bottom of force 8 — is the threshold at which a gale warning is issued.

Frequently asked

Is a knot the same as a nautical mile?

No. A nautical mile is a distance, 1,852 m. A knot is a speed, one nautical mile per hour. Saying "knots per hour" is a common error — it would mean acceleration.

Why not just use km/h at sea?

Because a nautical mile is one minute of latitude, so distance can be measured straight off a chart's latitude scale with dividers, with no conversion and no risk of arithmetic error. That property has kept the unit alive through two centuries of metrication.

What speed does a sailing yacht make?

A typical 10-metre cruising yacht sails at 5 to 7 knots, which is 9 to 13 km/h — slower than a competent cyclist. Hull speed, the practical limit for a displacement hull, scales with the square root of waterline length, so longer boats are faster almost by definition.