The SI system is based on seven base units. These are defined as the units for length (meter), mass (kilogram), time (second), current (ampere), temperature (kelvin), amount of substance (mole) and luminous intensity (candela). All other SI units are combinations of these seven units and are thus called derived units.

We see that speed is not one of the base units. So we must breakdown speed until we find a relationship that describes speed using the base quantities.

Speed is a measure of how fast something is moving, or rather, how much distance is the object moving per unit time.

`Speed = (distance)/(time)`

Distance is a length. Since length and time are base quantities, we can then deduce that the SI unit for speed is the SI unit of length (meters) over the SI unit of time (seconds), or `(meters)/(seconds)` which is abbreviated m/s.

The International System of units or simply SI units consist of seven base units. They are -

meter(m) for length, kilo gram(kg) for mass, second(s) for time, kelvin(k) for temperature, ampere(A) for electric current, candela(cd) for luminous intensity and mole(mol) for amount of substance.

As speed is not a base unit, we need to derive the units for speed from base units. Speed is the ratio of two basic physical quantities, which is,

Speed = distance/time

We know the unit for distance or length is meter(m) and also time is second(s)

So we can write the unit for the speed as meter/second

So, **the SI unit of speed is meter/second or m/s**