Theory:

In our daily life, we used to calculate the physical quantity in units, like $$metre$$ and $$centimetre$$, but do you know how to convert one unit to another? Let us learn it.

One $$centimetre$$ ($$cm$$) has $$10$$ equal parts called $$millimetre$$ ($$mm$$).
$$1$$ $$cm = 10$$ $$mm$$

One $$metre$$ ($$m$$) is divided into $$100$$ equal parts called $$centimetre$$ ($$cm$$).
$$1$$ $$m = 100$$ $$cm$$

One $$kilometre$$ ($$km$$) is divided into $$1000$$ equal parts called $$metre$$ ($$m$$).
$$1$$ $$km = 1000$$ $$m$$

Important!
To convert $$millimetre$$ ($$mm$$) to the $$centimetre$$ ($$cm$$), divide the given number by $$10$$, and to convert from $$centimetre$$ ($$cm$$) to $$millimetre$$ ($$mm$$), multiply the number by $$10$$.
Example:

Convert $$1\ km$$ in terms of $$cm$$.

$$1$$ $$km = 1000$$ $$m$$

$$1$$ $$m = 100$$ $$cm$$

Hence, to convert $$kilometre$$ ($$km$$) to the $$centimetre$$ ($$cm$$), multiply the given number $$1$$ by $$100000$$, then

$$1$$ $$km = 100000$$ $$cm$$

Conversion of other units is given in the table below.

 Prefix Abbreviation Sub-multiple / Multiple In metre Milli $$m$$ $\frac{1}{1000}\phantom{\rule{0.147em}{0ex}}\mathit{or}\phantom{\rule{0.147em}{0ex}}{10}^{-3}$ $$1000\ millimetre$$ $$=$$ $$1\ metre$$ Centi $$c$$ $\frac{1}{100}\phantom{\rule{0.147em}{0ex}}\mathit{or}\phantom{\rule{0.147em}{0ex}}{10}^{-2}$ $$100\ centimetre$$ $$=$$ $$1\ metre$$ Deci $$d$$ $\frac{1}{10}\phantom{\rule{0.147em}{0ex}}\mathit{or}\phantom{\rule{0.147em}{0ex}}{10}^{-1}$ $$10\ decimetre$$ $$=$$ $$1\ metre$$ Nano $$n$$ $\frac{1}{1000000000}\phantom{\rule{0.147em}{0ex}}\mathit{or}\phantom{\rule{0.147em}{0ex}}{10}^{-9}$ $$1000000000\ nanometre$$ $$=$$ $$1\ metre$$ Kilo $$k$$ Multiple: $\mathit{1000}\phantom{\rule{0.147em}{0ex}}\mathit{or}\phantom{\rule{0.147em}{0ex}}{10}^{3}$ $$1\ kilometre$$ $$=$$ $$1000\ metres$$

Measuring tools for length
In the $$16th\ century$$, William Bedwell invented a ruler or metre scale that are used these days to measure the length of an object.
A measuring tape helps to measure the length of a particular distance.
An odometer measures the distance moved by an automobile. The device can be seen in the dashboard of vehicles like buses and cars.

Important!
The Bureau of Weights and Measures, Paris displays a standard metre rod made of an alloy of platinum and iridium. A model of this metre rod is held in the National Physical Laboratory, New Delhi.
Reference:
https://cdn.pixabay.com/photo/2015/05/29/22/18/measuring-tape-789899_1280.jpg