4.1
Unit Conversion
To convert length measurements is not that difficult if you know the following prefixes. 

From larger to smaller units
Example A
Express12 km in meters.
Hopefully, you know that kilo stands for thousand. 12 kilometers means then 12 thousand meters and we write this as 12,000 m.
12 km = 12,000 m 

Example B
Express 23.2 m in millimeter.
Milli means thousandth. This means that a thousand millimeters is one meter.
We get 1,000 · 23.2 m = 23,200 mm.
23.2 m = 23,200 mm 

From smaller to bigger units
Example C
Express 2,434 mm in meter.
Milli means thousandth. Millimeter means a thousandth of a meter.
1 millimeter = 1/1,000 meter
So 2,434 mm = 2 ,434/1,000 m = 2.434 m
2,434 mm = 2.434 m 

Example D
Express 19 cm in km.
Centi means hundredth. 19 centimeter means 19 hundredths of a meter.
It is written 19/100 m = 0.19 m.
1 meter is a thousandth of a kilometer, 1 m = 1/1,000 km.
So 0.19 m = 0.19/1,000 = 0.000 19 km
19 cm = 0.000 19 km 
