A conversion factor expresses the relationship between the unit in the numerator and the unit in the denominator. The quantities are actually the same, but the numbers are different because the units are different. Any time you can express a fixed relationship between two quantities, you can make two conversion factors out of that relationship. (Flipping the original conversion factor upside down gives you the second one!)

Examples

- 2.54 cm = 1 in. so the conversion factor is:

- 1.00 cal = 4.184 J, so the conversion factor is:

- 1.00 mole O
_{2}has a mass of 15.999 g, so the conversion factor is:

- In the production of ammonia, N
_{2}+ 3 H_{2}→ 2 NH_{3}, two moles of ammonia are produced for every 3 mol of H_{2}consumed, so the conversion factor is: