About the Speed Converter
This speed converter handles the six units that matter across driving, sport, navigation and physics: kilometres per hour, miles per hour, metres per second, feet per second, knots and Mach. Everything is routed through metres per second, the SI unit, using exact definitions where they exist — a mile per hour is precisely 0.44704 m/s and a knot is exactly 1.852 km/h. The result comes with the single-unit formula and a table giving your speed in all six units, which is handy when a weather forecast quotes knots and your instruments read kilometres per hour.
How to use the Speed Converter
- 1
Enter the speed figure in the Value box.
- 2
Select the unit it is measured in from the From dropdown.
- 3
Select the unit you want it converted to from the To dropdown.
- 4
Read the result, then check the table to see the same speed expressed in every supported unit.
What people use it for
Driving abroad
A 100 km/h motorway limit is 62 mph and a 30 mph urban limit is 48 km/h. Converting the common limits once before a trip is easier than doing it at speed.
Sailing, flying and weather
Wind and vessel speeds are given in knots. A 20 knot wind is 37 km/h or 23 mph, which is around force 5 on the Beaufort scale.
Physics and sports analysis
Equations of motion need metres per second. A sprinter covering 100 m in 9.8 seconds averages 10.2 m/s, which is 36.7 km/h.
The factors, and how to convert them in your head
Metres per second is the base here, and the rest follow from the length and time definitions. Kilometres per hour is 1,000 metres over 3,600 seconds, so exactly 1/3.6 m/s — multiply m/s by 3.6 to get km/h, and divide to reverse it. A sprinter at 10 m/s is doing 36 km/h. Miles per hour is 1,609.344 m over 3,600 s, giving exactly 0.44704 m/s, and km/h to mph is a multiply by 0.621371. Feet per second is simply 0.3048 m/s, and there is a neat coincidence worth knowing: 60 mph is exactly 88 ft/s, because 60 × 5,280 ÷ 3,600 works out cleanly. A knot is one nautical mile per hour, exactly 1,852 metres over 3,600 seconds, or 0.514444 m/s, which is 1.852 km/h and 1.15078 mph. For mental work, take 60 percent of a km/h figure to approximate mph, and add about 15 percent to a knot figure to get mph.
Mach is not a fixed speed
Mach number is a ratio rather than a unit: it is the object's speed divided by the local speed of sound. That divisor is not constant. Sound travels through air at a speed determined almost entirely by temperature, following roughly 331.3 + 0.606 × temperature in Celsius metres per second, with pressure and humidity having only a minor effect. At the International Standard Atmosphere sea-level temperature of 15 °C, that gives about 340.3 m/s, which is the value this converter uses, equivalent to 1,225 km/h or 761 mph. At the minus 56.5 °C of the stratosphere, where airliners cruise, the speed of sound falls to about 295 m/s, so Mach 1 there is only around 1,062 km/h. This is why an aircraft can approach Mach 1 simply by climbing at a constant true airspeed, and why fighter performance figures always come with an altitude attached. Chuck Yeager's 1947 supersonic flight reached about 1,127 km/h at around 13,700 m — well under the sea-level figure yet comfortably past Mach 1 in the thin cold air up there. Treat any Mach conversion here as a sea-level standard-day approximation.
Tips
- Multiply km/h by 0.6 for a quick mph estimate — it is under by about 3.5 percent, which is close enough for road signs.
- Divide km/h by 3.6 for metres per second, the unit almost every physics formula expects.
- Wind speeds in weather bulletins may be sustained or gust figures; the two can differ by a third or more.