Speed units and conversions

Two common units

In physics, two units of speed are commonly used:

• the kilometer per hour (km/h), used for vehicles
• the meter per second (m/s), the official SI unit

Converting km/h to m/s

To go from km/h to m/s, divide by 3.6:

v(m/s) = v(km/h) / 3.6

Example: 90 km/h -> 90 / 3.6 = 25 m/s

Converting m/s to km/h

To go from m/s to km/h, multiply by 3.6:

v(km/h) = v(m/s) x 3.6

Example: 10 m/s -> 10 x 3.6 = 36 km/h

Why 3.6?

1 km = 1000 m and 1 h = 3600 s, so 1 km/h = 1000/3600 m/s = 1/3.6 m/s. This factor comes up in almost every exercise, so it's worth knowing by heart.

Summary table

km/h	m/s
36	10
72	20
90	25
108	30

Classic pitfall

Watch out for mixed units in a calculation: if d is in meters and t is in hours, the result v = d/t will be in m/h, a rarely used unit and a common source of errors. Always check that the distance and time units match the desired speed unit before calculating.