In chemistry and biology, a dilution factor is a measure of the change in concentration of a liquid substance when it is mixed with another liquid substance. It is often used as the denominator of a fraction to represent the amount of the original stock solution that remains in the entire solution after dilution. The dilution factor can be expressed as a ratio or an exponent, and it is calculated by dividing the final volume by the initial volume. For example, if you add a 0.1 mL aliquot of a specimen to 9.9 mL of diluent, you have diluted the sample by a factor of 100. The dilution factor is a convenient method of thinking about dilutions, and it is widely used in both science and everyday life. It is also employed in almost all chemical and biological research because the stock solution of a substance is frequently far more concentrated than needed.