Speed Converter

Speeds convert through metres per second. Each factor is a length divided by a time, so a kilometre per hour is 1,000 metres over 3,600 seconds — exactly 1/3.6 m/s, which is why the 3.6 appears in every metric speed calculation.

The knot is the interesting one. It is one nautical mile per hour, and since a nautical mile is one minute of latitude, a vessel doing one knot covers one minute of latitude in an hour. That relationship is why aviation and shipping still use it: with a chart marked in degrees, speed in knots converts to position without any arithmetic at all.

Metres per second is the physicist's unit and the one to reach for in any calculation involving acceleration or energy, because it is the SI unit and needs no conversion factors in the formulas.

One useful mental anchor: 100 km/h is 62 mph, and 60 mph is 96.6 km/h. The two motorway speeds people quote at each other are closer than they sound.

You are renting a car abroad. The limit signs read 120 and the dashboard reads mph.

Type 120, choose km/h: 74.6 mph. So the motorway limit is about 75 — meaningfully faster than a UK 70 and slower than an American 80 limit, which is worth knowing before you set the cruise control by habit.

A weather example. A storm warning gives winds of 45 knots. Type 45, choose knot: 83 km/h, or 52 mph. That is a gale — enough to make driving a high-sided vehicle unpleasant. Knots are quoted in every marine and aviation forecast and are roughly 15% larger than mph, which is easy to underestimate.

A running example. A 4-minute kilometre pace is 15 km/h. Type 15: 4.17 m/s, or 9.3 mph. Sprinters peak above 12 m/s, which puts a comfortable running pace at about a third of the fastest a person has ever moved.

For distance rather than speed, the length converter handles kilometres and miles.