Quick Answer: Divide by 100.

To do any conversion, you find the relationship between the two units. In this case, 100 cm = 1 m.

This gives you two conversion factors:

##(100 cm)/(1 mm)## and ##(1 m)/(100 cm)##

Example

Convert 789 cm into metres.

First method

You multiply by a conversion factor: ##(100 cm)/(1 mm)## or ##(1 m)/(100 cm)##

Choose the one that gives you the correct units for the answer.

789 cm × ##(1 m)/(100 cm)## = 7.89 m

Note that the “cm” cancels out and leaves you with the answer in “m”. If you had chosen the other conversion factor, you would have gotten

789 cm × ##(100 cm)/(1 m)## = 78 900 cm²/m.

The strange units for the answer tell you that you have made a mistake.

Second Method

If you can’t remember that 100 cm = 1 m, you can always use the definition of centi.

centi = 10⁻², so 1 cm = 10⁻² m

Then you can convert “cm” to “m”.

789 cm × ##(10⁻² m)/(1 cm)## = 7.89 m

Third Method

Here is an easy acronym for you to memorize.

khan’s hot dogs use dead cow meat → k h d u d c m

The letters stand for kilo hecto deka (unit) deci centi milli.

‘u’ always stands for whatever base unit you are in, in this case, “metres”.

You want to go from “c” to the base unit “u” (metres). “u” is two places to the left of “c”, so you move the decimal point two places to the left.

789 cm= 7.89 m