Speed is distance over time
Every speed unit has the form distance per time, so converting the distance and the time separately gives the factor. One metre per second means covering 1 m each second. An hour has 3,600 seconds, so in an hour you cover 3,600 m, or 3.6 km. That makes 1 m/s exactly 3.6 km/h, and to go from km/h to m/s you divide by 3.6. When the arithmetic gets confusing, rather than memorising a formula, write the units out: so many metres each second, so how far in an hour? That alone prevents most slips.
Values worth knowing
Keeping a few conversions in your head makes news reports and road signs instantly meaningful. Decimals are rounded.
- 10 m/s = 36 km/h
- 100 km/h ≈ 27.8 m/s
- 60 km/h ≈ 16.7 m/s
- 1 km/h ≈ 0.278 m/s
- Speed of light = 299,792,458 m/s (defined value)
Why forecasts give wind in m/s
Korean weather forecasts report wind speed in metres per second. A gale of 20 m/s is therefore 72 km/h. To someone used to car speeds that may sound modest, but the force wind exerts on people and structures grows quickly: double the wind speed and the force roughly quadruples. Because the same number differs by a factor of 3.6 depending on whether it is m/s or km/h, check the unit before comparing with foreign forecasts or other sources. For gale and typhoon warning criteria and what to do, follow the official guidance of the national weather service.
Car speed and stopping distance
Road signs and speedometers use km/h, but converting to m/s helps the danger sink in. At 100 km/h a car covers about 27.8 m every second. If it takes one second to notice a hazard and press the brake, the car has already travelled nearly 30 m. Braking distance comes on top of that, and it grows with the square of speed, so doubling speed roughly quadruples it. Wet roads or worn tyres stretch it further. This is where the advice to leave plenty of space to the car ahead comes from.
Knots at sea and in the air
Ships and aircraft use knots. One knot is one nautical mile per hour, and the nautical mile is defined internationally as exactly 1,852 m. One knot is therefore exactly 1.852 km/h, or about 0.514 m/s. The nautical mile was originally based on the distance of one minute of latitude along a meridian, which makes calculations easy in navigation, where positions are given in latitude and longitude. Since charts and aviation measure distance in nautical miles, it is natural to give speed in knots as well. You will also see knots in marine forecasts and vessel information.
Miles per hour and running pace
Roads in the United States and the United Kingdom use miles per hour. A mile is exactly 1,609.344 m, so 1 mph is exactly 1.609344 km/h. Runners often use pace instead of speed: the minutes taken per kilometre. Pace and speed are reciprocals, so dividing 60 by the minutes per kilometre gives km/h.
- 60 mph ≈ 96.6 km/h
- 100 km/h ≈ 62.1 mph
- 5 minutes per km = 12 km/h
- 6 minutes per km = 10 km/h
Mach is a ratio, not a speed
The Mach number is an object's speed divided by the local speed of sound. Mach 1 means as fast as sound; Mach 2 means twice that. The catch is that the speed of sound is not fixed. In air it depends mainly on temperature: in dry air at 15 °C it is about 340 m/s, or roughly 1,225 km/h, but it is slower at high altitude where the air is much colder. So the statement that Mach 1 equals 1,225 km/h is only an approximation for particular conditions, and the same Mach number means a different actual speed at different altitudes. When an article converts Mach to km/h, ask what conditions it assumed.
Common conversion mistakes
Most speed conversion errors come from dropping the time unit or mixing units with similar names. Simply writing the unit next to every number you copy prevents most of them.
- Mixing m/s and km/h and missing the factor of 3.6
- Treating knots as if they were land miles per hour
- Thinking of Mach as a fixed speed
- Reading pace (minutes per km) as if a bigger number meant faster
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