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.
Write down the total number for the original quantity. We will call this "T".
Write down the total number for the final quantity. We will call this "Tf".
Subtract Tf from T. We will call this difference "D", because this is the numerical amount that the quantity has decreased.
T - Tf = D
Take D and divide it by the original amount T. We will call this amount R, because this is the ratio of decline.
D/T = R
Multiply R by 100 to convert this ratio into a percentage, "P". This is the percent decline.
R x 100 = P
TL;DR (Too Long; Didn't Read)
Before you start your calculations, make sure the final quantity is less than the original quantity. If the final quantity is bigger, then there has been an increase instead of a decline.
If your calculations result in a negative number, check to see if the final quantity is bigger than the original quantity. If so, disregard the negative sign and you have calculated the percent increase.