The factor-label method was developed to keep track of units in multi-step conversion problems. In the method, equalities (i.e., conversion factors) are set up in fraction form. The equalities are then lined up sequentially and units used on the top and bottom of neighboring fractions are alternated so that units cancel.

For example, consider the conversion of inches to centimeters (1.00 in = 2.54 cm). How many centimeters are in 5.00 inches?

For this problem, the following equalities exist:

All three are equivalent, but only the last has inches in the denominator. By choosing the last equality, we can cancel units in the conversion.