Converting and Calculating Units of Length

The metric units of length, from smallest to largest, are the millimetre, the centimetre, the metre, and the kilometre. A millimetre is one tenth of a centimetre, and a metre is the standard unit. A kilometre is the largest, used for long distances such as those between cities.

First recall how the two units relate, such as 1 metre equalling 100 centimetres. If you are converting to a smaller unit, you multiply by 10, 100, or 1000. If you are converting to a larger unit, you divide by that same factor instead.

One metre equals 100 centimetres. Because the centimetre is smaller than the metre, you multiply by 100 to convert metres into centimetres. For example, 47 metres becomes 47 × 100, which gives 4,700 centimetres. To go back from centimetres to metres, you divide by 100 instead.

One kilometre equals 1000 metres, so to convert metres into kilometres you divide by 1000. For example, 5020 metres divided by 1000 gives 5.020 kilometres, which you can also write as 5.02 kilometres. You divide because the kilometre is the larger unit.

Standardise both measurements to one unit first, then add. For example, take 1.75 metres and 40 centimetres. Convert the 40 centimetres to 0.4 metres, then add to reach 2.15 metres. You can align to either the larger or the smaller unit, whichever you find easier.

In the metric system, "kilo" indicates multiplication by 1000, so one kilometre equals 1000 metres. This is why the kilometre is the largest unit of length covered here. We use it to describe longer distances, such as the distance between two cities.