Prefixes to Units...

We normally write:

Average weight of a newborn baby is W = 3.5 kg
Planes fly at an altitude H = 8,000 m
An hour has 3600 s

Value of a physical quantity is expressed as a comparison to a unit of that quantity. For example, the value of a physical quantity Z is written as the product of a unit [Z] and a numerical factor n: Z = n[Z]

SI syatem is the metric system used internationally in science and engineering

In the SI system, kilogram (kg), metre (m) and second (s) are the base units of mass, length and time respectively.

Other commonly used base units in the SI system are

Energy - Joule (J)
Power - Watt (W)
Electric potential - Volt (V)

Units may be combined to express complex quantities

Speed is distance travelled in unit time - so its base unit will be
metre per second or m/s or ms^-1

Frequently one needs to deal with very large or very small values and it is common to use a prefix with the base unit to obtain a derived unit. For example

Radius of the Earth is 6,450,000 m or 6,450 km {1 km = 1000 m}
Size of a virus is 0.0000001 m or or 0.1 microm or 100 nm
{1 micrometre = 0.000001 m; 1 nm = 0.000000001 m or 10^-9m}

Standard Prefixes are in steps of 1000 - they are listed in the following for distance measurements. Others are similar.

Large numbers:

1,000 m = 10³m or 1 km kilometre
1,000,000 m = 10⁶m or 1 Mm megametre
1,000,000,000 m = 10⁹m or 1 Gm gigametre
1,000,000,000,000 m = 10¹²m or 1 Tm terametre
1,000,000,000,000,000 m = 10¹⁵m or 1 Pm petametre
1,000,000,000,000,000,000 m = 10¹⁸m or 1 Em exametre
1,000,000,000,000,000,000,000 m = 10²¹m or 1 Zm zettametre

Small Numbers:

0.001 m = 10^-3m or 1 mm millimetre
0.000,001 m = 10^-6m or 1 microm micrometre
0.000,000,001 m = 10^-9m or 1 nm nanometre
0.000,000,000,001 m = 10^-12m or 1 pm picometre
0.000,000,000,000,001 m = 10^-15m or 1 fm femtometre
0.000,000,000,000,000,001 m = 10^-18m or 1 am attometre
0.000,000,000,000,000,000,001 m = 10^-21m or 1 zm zeptometre