How to calculate average speed

We work out average speed, for example of a car, by finding out how many kilometres we travelled by car and how long it took us, and dividing one by the other. For example, if we travelled 200 km and it took us 2 hours, we drove at an average speed of 100 kilometres per hour. The basic formula is therefore that the average speed v is equal to

$$v=\frac{\text{distance}}{\text{time}}$$

So

$$\frac{200\text{km}}{2\text{h}}=100\text{km}/\text{h}$$

Other units

Another example: we travelled 180 km and it took us 90 minutes. If we substitute into the formula, we calculate

$$\frac{180\text{km}}{90\text{min}}=2\text{km}/\text{min}$$

Notice that this time we did not get the speed in kilometres per hour, but in kilometres per minute! If we want to convert the speed to the more usual km/h, we have two options. The first is that instead of working with minutes in the formula, we convert them to hours. So 90 minutes is an hour and a half, that is, 90 min = 1.5 h. Now we can substitute the values into the formula again:

$$\frac{180\text{km}}{1.5\text{h}}=120\text{km}/\text{h}$$

So we travelled at an average speed of 120 kilometres per hour. The second option is to say that if we travel two kilometres per minute on average, then in an hour, which has 60 minutes, we must travel sixty times as many kilometres. So we can multiply

$$2\text{km}/\text{min}\cdot 60\text{min}/\text{h}=120\text{km}/\text{h}$$

See also

• How to calculate an average grade