The rate of decrease measures a decline as a percentage of the original amount. You might want to know the rate of decrease to find out how quickly a population is shrinking or how much money is being lost on an investment. To calculate the rate of decrease, you need to know the original amount and the final amount.

Look up the initial amount and the final amount for what you are calculating the rate of decrease. For example, if you were calculating the rate of decrease of a bacteria population, you would need to know the initial amount of bacteria and the final amount of bacteria.

Subtract the initial amount from the final amount to determine the amount of decrease. For example, if you started with 1 million bacteria and ended with 900,000, you would subtract 900,000 from 1 million to find a decrease of 100,000.

Divide the amount of the decrease by the original amount to calculate the rate of decrease expressed as a decimal. In this example, divide 100,000 by 1 million to get 0.1.

Multiply the rate of decrease by 100 to convert from a decimal to a percentage. In this example, multiply 0.1 by 100 to find the rate of decrease of the bacteria is 10 percent.