Percentage change calculations appear in the AS or A-Level Biology exams pretty much every year and a lot of students struggle with them.

For most of the Biology calculations, there isn’t really a formula or a method you can learn, you just have to rely on your problem-solving skills on the day.

Fortunately, percentage change calculations are the exception to that.

The question won’t usually say ‘Calculate the percentage change’ – it’s more likely that they’ll ask you to calculate the percentage increase, percentage decrease or percentage difference.

The good thing is, these calculations all have the same formula, which we’ll call the formula for percentage change to keep it simple.

The formula for percentage change

Percentage change can be calculated using the following formula:

To calculate the difference between the 2 values just subtract the smallest value from the largest value.


Examples of percentage change calculations

In 2010 the United Kingdom had a population of 61.77 million. In 2011 this had increased to 63.26. Calculate the percentage increase in the size of the population of the United kingdom.

The difference between the 2 values is 63.26 – 61.77 = 1.49

The original value is 61.77

Therefore the % increase = 1.49/61.77 x 100

                       = 2.41%


In 2002 the number of confirmed cases of Malaria in Azerbaijan was 506. By 2012 this had fallen to 3. Calculate the percentage decrease in the number of confirmed cases of Malaria in Azerbaijan from 2002 – 20012.

The difference between the 2 values is 506 – 3 =  503

The original value is 506

Therefore the % decrease = 503 / 506 x 100

= 99.4%