How to convert between nm, um, mm, m and km

 The: nm, um, mm, m and km.

Are all units of measurement, depicting size. Nm for nanometers, um for micrometre, mm for millimetres, m for meters, and km for kilometre. 

Now each of these numbers is either 1000 times bigger or smaller than the previous one.
To convert each number, simply divide or multiply it by 1000, so if you want to convert 2 kilometres into meters, all you have to do is multiply 2 * 1000, and you get 2000 meters. Then if you want to convert it back to km do 2000 meters and do
2000/ 1000, and then you will get 2 kilometres.

Now let's take an example number of 6mm or 6 millimetres.
In order to increase the 6mm in size, you have to divide it by 1000, so if you want to convert the 6mm to meters then it would be 0.006 meters or 6x10^-3 meters, it is still the same number of 6mm but since the unit are larger the number is smaller. 

Now if you want to convert the 6mm to um or micrometers, you would multiply by 1000.
So, 6mm * 1000 = 6000. Or 6000um or micrometres, or 6x10^-3um. 

Now let's put these all into scale, atoms are 0.1 - 0.5 nanometers big, viruses are around 100 nanometers across, bacteria might be a micrometre, and plant and animal cells are around 10-100 micrometre range.
But not even the best eyesight can see past nanometers in size, as the smallest thing we can see is the width of the human hair, light microscopes allow you to see around 500 nanometers, but if you have a selection microscope you can see until 0.1 nanometers which is the size of atoms.