How this instrument works
One knot equals one nautical mile per hour. Since delegates at Monaco's 1929 hydrographic conference fixed an international nautical mile at exactly 1852 metres, a knot became exactly 1852 metres per hour, which is 1.852 kilometres per hour. No measurement uncertainty rides along with that number; it is a definition, agreed by treaty, and every digit past 1.852 is zero.
That awkward-looking 1852 has tidy ancestry. Navigators wanted their distance unit equal to one minute of arc along the meridian, so that one degree of latitude scaled off the chart would read as sixty units of travel. The metre was originally pegged to one ten-millionth of the quarter meridian, putting that arc minute at 1851.85 metres — Monaco simply rounded up. Earth being an oblate spheroid rather than a ball, the real arc minute runs from roughly 1843 metres near our equator out to 1862 at each pole, and 1852 sits comfortably between.
Even older is where this unit got its name. The chip log — a weighted triangular board — was heaved astern on a line tied with knots about 47 feet 3 inches apart, then counted while the 28-second sand glass emptied. Knots slipping through the mate's fingers gave speed directly, and sailors have counted them ever since. Aviation and shipping keep knots because charts are ruled in degrees and minutes; most road signs on Earth are ruled in kilometres per hour. Anyone reading marine forecasts in one and driving home under another needs this arithmetic.
- Type your speed into Knots (kn). It opens at 20 kn, a solid Force 5 on Beaufort's scale.
- Read your answer on Kilometres per hour (km/h); it recomputes on every keystroke.
- Going backwards from a road speed? Divide that km/h figure by 1.852 and type the result into Knots (kn) to confirm it.
- Wind, ship, and aircraft speeds all convert identically — a definitional factor does not care what happens to be moving.
Worked example — a Force 5 forecast
Saturday's inshore forecast gives 20 knots from south-west: middle of Force 5, fresh breeze, whitecaps across open water, one reef tucked into your mainsail. Leave Knots (kn) at its default 20 and this sheet returns 37.04 kilometres per hour.
That is a figure you can feel. Around 37 km/h is what a club cyclist holds on flat tarmac, or a scooter through town. Arithmetic short enough for a napkin — 20 × 1.852 = 37.04 — and it terminates, because 1.852 is 463⁄250, and denominators built only from twos and fives can never yield recurring decimals.
Questions
Is 1.852 exact, or has it been rounded?
Exact. An international nautical mile is defined as precisely 1852 metres, so one knot is precisely 1852 metres per hour, or 1.852 km/h. Nothing needs to propagate: any speed in knots written with finitely many decimals converts to kilometres per hour with finitely many decimals, since 1.852 reduces to 463⁄250. Only your screen rounds anything.
Should I ever write knots per hour?
No. A knot already contains its own per-hour, so knots per hour would mean nautical miles per hour per hour — an acceleration, and not what anybody intends. Say 20 knots, or spell out 20 nautical miles per hour. Same trap produces kn/h in writing; correct symbols are kn on its own, or kt in aviation practice, with kts sometimes seen informally for plurals.
How do knots compare with miles per hour?
A knot is larger. One knot is 1.15077944 mph, because a nautical mile of 1852 metres beats a statute mile of 1609.344 metres. So 20 kn is 23.02 mph but 37.04 km/h. Confusing those two miles is far and away this conversion's commonest error: a boat doing 20 at sea moves noticeably faster than a car doing 20 past a British road sign.
Which trades and countries actually use knots?
At sea and in flight, essentially all of them. ICAO and IMO practice, marine VHF forecasts, METAR and TAF weather bulletins, plus both Beaufort and Saffir-Simpson scales, are keyed to knots whatever local road signs say. Kilometres per hour is a civil road unit almost everywhere beyond America and Britain, which is exactly why forecasts get converted so often at a chart table.
What do familiar speeds look like in both units?
Round numbers help: 1 kn is 1.852 km/h, 10 kn is 18.52, 20 kn is 37.04, and 100 kn is 185.2. A cruising yacht makes 5 to 7 kn (9 to 13 km/h); a container ship running to schedule holds 20 to 24 kn (37 to 44 km/h); an airliner cruises near 470 kn true airspeed, about 870 km/h; and 64 kn, which promotes a tropical storm into a hurricane, is 118.5 km/h.
Was a nautical mile always 1852 metres?
No. Britain kept an Admiralty nautical mile of 6080 feet (1853.184 m) until 1970, and America used 6080.20 feet until adopting international practice in 1954. Both were attempts at that same arc minute, measured against different reference ellipsoids. Anything published before those dates may quote a knot around 0.06 percent larger than today's, which matters over an ocean passage but vanishes inside normal forecast rounding.