Imagine I told you the corridor outside our classroom is 30,000 millimetres long, and your pencil is one five-thousandth of a kilometre long. Both of those are true! But would you ever actually say them that way?

Which unit would you really choose for the corridor? Which for your pencil? And why does picking the wrong one make a number look silly?

The converter changes lengths between km, m, cm and mm, one example at a time. Look at each result: the decimal point stays put and the digits have stepped into new columns. Notice how many places they stepped, and which way.

We work through three conversions together on the converter. For each one, the whole class decides first: are we going up the ladder or down, and do we multiply or divide? Say your answer aloud so we can agree or correct it. One pupil comes up to drive the converter while everyone else watches the digits and checks the result.

Now we tackle fresh conversions, getting trickier as we go: 7.3 m to centimetres, 4200 mm to metres, 0.62 km to metres, and then the stretch — express 1250 cm in kilometres using the two-jump route we saw earlier.

Key Takeaways

• We choose the metric unit that gives a sensible-sized number for the real object.
• Converting between mm, cm, m and km means multiplying or dividing by powers of ten.
• There are 1000 mm in a metre, so the digits step three places between mm and m.