The second method is to use composite units or units representing a combination of other units. The third method used to derive equivalent units is to substitute units from an alternate system. English units can be substituted for metric units, or SI units can be substituted for cgs units. Regardless of which method is used, the strategy is the same. Conversion factors must be constructed that can be multiplied by or be divided into the original unit to derive the equivalent unit.
Conversion factors are ratios that describe the relationship of one unit to another.
A conversion factor has two essential properties. First it must mathematically be an alternate expression for the value 1. This allows either multiplication or division of an expression by the conversion factor without altering the value of the expression. The second property is that it contains units of both the original and desired equivalent expression. Conversion factors are constructed from identity equations that define the units involved.