Calculating Percent Difference

You can use the percent difference formula when you want to know the difference between two values in terms of percentage. It is used to calculate the percent difference between two values of the same category or class. For example - calculating the percent difference of budget increase from one year to the next, or the population growth over a period of 10 years. Scientists often use percent difference while comparing two observed values of a particular experiment.

We’ll have a look at how the percent difference formula works, along with a couple of examples that show how simple this calculation really is.

Percent Difference Formula

Percent difference is calculated as the difference between two values, divided by the average of the two values, expressed in terms of percentage.

Percent Difference = [First Value - Second Value] ÷ [(First Value + Second Value) ÷ 2] × 100

Using Absolute Values
While calculating the percent difference, always use absolute values, i.e. ignore the minus sign before any of the values. For example, a subtraction of 5 minus 10 gives -5. But the absolute value would be taken as 5, without any minus sign.

When to Use and When Not to Use Percent Difference Formula?

Some people may get confused about using the percent difference formula, and other related formulae for calculating percentage change or percentage error.

Use percentage change formula when you have one old and one new value, and you want to know the relative change in percentage, with reference to the old value.
Example: Percent increase in the price of a product, or percent increase in the population of a country over the last year.

Percent Change = New Value - Old Value ÷ Old Value × 100

If you have one exact or theoretical value, and one approximate or observed value, use the percent error formula.
Example: Percent error in the actual and observed values in a scientific experiment.

Percent Error = Approximate Value - Exact Value ÷ Exact Value × 100

How to Find Percent Difference

Example 1

Sam weighs 160 pounds, while Ron weighs 120 pounds. What is the percent difference between the weights of Sam and Ron?

Find the percent difference

Percent Difference = [First Value - Second Value] ÷ [(First Value + Second Value) ÷ 2] × 100

= [160 - 120] ÷ [(160 + 120) ÷ 2] × 100

Percent Difference = 40 ÷ 140 × 100

Percent Difference = 28.57%

Example 2

Here is a problem for you to try out. To view the solution, click on the ‘show answer’ link provided below.

» What is the percent difference between the population of city A, with a population of 2.5 million, and city B, with a population of 1.2 million?

As you see now, calculating percent difference is quite an easy task, if you just remember the formula, which again, is not a very complicated task either!

Getting comfortable with the percent difference formula is mostly a matter of core math skills and repetition.