GreatCalc
العربية

Speed Converter

Convert between km/h, mph, m/s, knots, Mach and the speed of light.

Enter the values to see the result here.

Formula

All speeds are stored in metres per second, then converted:

xto=xfrom×ffromftox_{\text{to}} = x_{\text{from}} \times \frac{f_{\text{from}}}{f_{\text{to}}}

with ff in m/s: km/h =1/3.6= 1/3.6, mph =0.44704= 0.44704, knot =1852/3600=0.514444= 1852/3600 = 0.514444, ft/s =0.3048= 0.3048, Mach =340.29= 340.29, and c=299,792,458c = 299{,}792{,}458.

Worked example

Convert 100 km/h to mph and to m/s.

  • To m/s: 100/3.6=27.78100 / 3.6 = 27.78 m/s.
  • To mph: 27.78/0.44704=62.1427.78 / 0.44704 = 62.14 mph.
  • In knots: 27.78/0.514444=54.027.78 / 0.514444 = 54.0 kn.

A 120 km/h motorway limit is therefore 74.6 mph.

Common mistakes

  • Dividing by 1.6 and by 0.62 interchangeably. mph to km/h multiplies by 1.609; km/h to mph multiplies by 0.621.
  • Treating the knot as a distance. It is a speed.
  • Using a fixed Mach number for aircraft at altitude.
  • Forgetting that stopping distance grows with the square of speed, so a 20% speed rise raises it 44%.

Frequently asked questions

What is the quick way to go from km/h to mph?

Multiply by 0.62, or take 60% and add a tenth of that. 100 km/h is about 62 mph. The exact factor is 0.621371.

Why do ships and aircraft use knots?

A knot is one nautical mile per hour, and a nautical mile is one minute of latitude. That makes distance on a chart easy to read off against speed. 1 knot is 1.852 km/h.

Is Mach 1 always the same speed?

No. The speed of sound depends on air temperature. This tool uses 340.29 m/s (15 °C at sea level), about 1,225 km/h. At cruising altitude, where it is colder, Mach 1 is nearer 1,060 km/h.

How do I convert m/s to km/h?

Multiply by 3.6. A sprinter at 10 m/s runs 36 km/h; a car at 25 m/s travels 90 km/h.

Related calculators