Often measurements are given in one set of units, such as feet, but are needed in different units, such as meter. A factor that is used to change between units, and therefore gives the relationship between two units is called a conversion factor. It is the number or formula you need to convert a measurement in one set of units to the same measurement in another set of units. A conversion factor uses your knowledge of the relationships between units to convert from one unit to another. For example, if you know that there are 2.54 centimeters in every inch.