Before measuring something, you need to know what unit to use. You can do this by estimating its approximate size. For measures such as speed you will use a formula.

Part of

Speed is worked out by dividing the distance travelled by the time taken:

The formula can also be written in this form:

- Question
If you travel in hours, what is your average speed?

The distance in km and the time in h - so the units for speed are km/h.

Another suitable metric unit for speed is metres per second. Using metres per second would be suitable when measuring short distances and short periods of time. For example, an athlete might run the race in seconds. What is the average speed for the race?

( decimal places)

If you know the speed, you can work out the time or the distance by rearranging the equation:

or

or

The triangle below is a good way to remember the equations:

To use the triangle, cover the value required and the formula that you need will be revealed by the location of the two values that you can see.

For example:

- For
**time**, you see distance above speed, so - For
**distance**, you see speed alongside time, so

- Question
If you walk for hours at an average speed of , how far will you have walked?