The percent decline is the ratio of the amount by which something has decreased compared with the original quantity. It can be used to compare any before-and-after quantities where the total amount has decreased over time. For example, if you start with a full box of chocolates, you can calculate the percent decline in the number of chocolate pieces left by the end of the week. Because this is a percentage, the highest possible value is 100 percent and the lowest possible value is 0 percent. Calculating the percent decline is easy when you know the original and final quantities.