The Metric System
All measurements in scientific work are recorded in the metric system.  The metric system consists of certain agreed upon names for the basic units of measure (litre, metre, gram, degree, lux, Pascals, etc.) and certain prefixes which designate various powers of ten. (milli-, deci-, kilo-, etc.)  The most recent set of rules for the use of the metric system is called the Système Internationale (SI).
The Système International definitions can be seen here.
Example_#1 Convert a measurement of 731 mm to its equivalent value in m.
Solution:
Conversions of this kind can be made in many ways.  One of the most convenient is by means of dimensional analysis.  Dimensional analysis means to use the units to decide what to multiply and divide so that unnecessary units cancel out.

Step 1:
Select the base unit.   This example uses 'mm' and 'm'; millimetres and meters.    The base unit is the part that is the same in both cases, the "meters."   It is the 'milli' that is important.

Step 2:
Go to the Metric Conversion Table and find 'milli'.  Write down what it means.

                                        1 milli = 0.001 or 10-3

Step 3:
Using the base unit, create two conversion possibilities.
               #1                                           #2
      1 millimetre           or           0.001 meter    
      0.001 meter                             1 millimetre

Step 4:
Decide on which one to use.  The desired answer is in meters.  So use the conversion with meters on the top (#2).

Step 5:
Set up the solution

731 mm  X    0.001 meter
                          1 mm

Step 6:
Cancel any units that are both above and below the dividing line.  

731 mm  X    0.001 meter
               1 mm
Step 7:
Multiply out

731  X    0.001 meter  =  0.731 meters
               1


Example_#2
Convert a measurement of 438 mm to km.
Solution:
Sometimes two or more unit factors may be required to make the necessary conversion.
Step 1:
There are two conversions needed, 'mm' to 'm' and 'm' to 'km'

Step 2:
1 mm = 0.001 m        and    1 km = 1000 m

Step 3:
    1-1                  1-2                     2-1               2-2

  1 mm  
   or      0.001 m               1 km    or    1000 m
0.001 m               1 mm               1000 m           1 km
Step 4:
The mm needs to be converted to the base and then the base to km.  The base is 'm'. So use conversions 1-2 and 2-1.

Step 5:
438 mm  X   0.001 m    X    1 km  
                      1 mm            1000 m

Step 6:
438 mm  X   0.001 m    X    1 km  
                      1 mm            1000 m

Step 7:
438  X   0.001   X    1 km      =  0.000438 km
                1              1000


The Metric System Practice Problems Level 1
The Metric System Practice Problems Level 2
The Metric System Practice Problems Level 3