Convert kilometers per hour to knots

The formula

knots = kilometers per hour × 0.5399568

kilometers per hour to knots — quick reference

Halve it and add eight per cent

Halving and adding eight per cent is unusually accurate here: 100 km/h gives 50 plus 4, or 54 knots, against a true 53.996. The factor 0.54 sits within a thousandth of correct, so the mental answer and the calculated one agree to the decimal place for any speed a boat, a car or a coastal gust will reach.

The anchors are easy to hold: 10 km/h is 5.4 knots, 50 km/h is 27, 100 is 54, 130 is 70. A brisk sailing breeze of 30 km/h is 16 knots, and a 90 km/h gust in a coastal forecast is 49 - one knot short of the 50 that usually comes with a warning attached.

Aviation and marine reports expect the number in knots and will not take km/h

Airspeed, groundspeed and reported surface wind in aviation are all in knots; METAR and TAF reports abbreviate them KT, with metres per second used instead in a handful of countries including Russia and China. Marine forecasts, shipping bulletins and harbour reports use knots throughout. Nothing on land uses them at all.

The common reason to convert this way is a general weather app reporting wind in km/h when the table being read against is defined in knots. The Beaufort scale, small craft advisories and gale warnings all carry knot thresholds, so the metric figure has to be converted before it can be classified as anything.

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