How to calculate percentage decrease between two numbers

Calculating percentage increase

Calculating percentage increase is an important skill for geographers to have. When geographers collect data over a period of time the results may increase. Calculating a percentage increase allows a geographer to see how much their data has changed. For example, it may be useful to find out how much the width of a river channel increases as you travel downstream.

1. work out the difference between the two numbers being compared
2. divide the increase by the original number and multiply the answer by 100
3. in summary: percentage increase = increase ÷ original number × 100

For example, the number of robins in a woodland area is counted over two different months. In December 15 robins were counted. In January 23 robins were counted. What is the percentage increase of robins in the woodland?

1. the difference between the two numbers is 8
2. 8 ÷ 15 × 100 = 53.3
3. the percentage increase of robins found in the woodland is: 53.3%

Calculating percentage decrease

Calculating percentage decrease is also a useful skill to have. For example, it may be useful to find out how much the load particle size decreases in a river as you travel downstream.

1. work out the difference between the two numbers being compared
2. divide the decrease by the original number and multiply the answer by 100
3. in summary: percentage decrease = decrease ÷ original number × 100

For example, the number of robins in a woodland in February and March is counted. In February 22 robins were counted. In March 12 robins were counted. What is the percentage decrease of robins in the woodland?

1. the difference between the two numbers is 10
2. 10 ÷ 22 × 100 = 45.4
3. the percentage decrease of robins found in the woodland is: 45.4%