Percentages are used in everyday life to compare values, measure changes and understand how one number relates to another.
You might use percentages to calculate discounts, price increases, exam scores, interest, profit margins or changes in value.
In this guide, you’ll learn how to calculate a percentage, understand the basic formula, work out percentage increases and decreases, and use simple examples for everyday calculations.
What Is a Percentage?
A percentage shows a number as a part of 100.
For example:
50% means 50 out of 100
25% means 25 out of 100
10% means 10 out of 100
Percentages make it easier to compare values, even when the original numbers are different.
For example, saying that a price increased by 20% is often more useful than only saying how many pounds it increased by.
How to Calculate a Percentage: A Simple Step-by-Step Guide
By ToolNestPro | Updated September 2026
How to Calculate a Percentage
The basic percentage formula is:
Percentage = (Part ÷ Whole) × 100
For example, if you scored 45 out of 60 on a test:
(45 ÷ 60) × 100 = 75%
So your score is 75%.
Another example:
If £30 is part of a total of £120:
(30 ÷ 120) × 100 = 25%
This means £30 represents 25% of £120.
How to Calculate Percentage Increase and Decrease
Percentages are also useful for measuring how much a value has increased or decreased.
To calculate a percentage increase, use:
Percentage Increase = ((New Value − Original Value) ÷ Original Value) × 100
For example, if a price rises from £80 to £100:
((£100 − £80) ÷ £80) × 100 = 25%
So the price increased by 25%.
To calculate a percentage decrease, use:
Percentage Decrease = ((Original Value − New Value) ÷ Original Value) × 100
For example, if a price falls from £100 to £75:
((£100 − £75) ÷ £100) × 100 = 25%
So the price decreased by 25%.
Common Percentage Calculation Mistakes
Percentage calculations are usually straightforward, but a few common mistakes can lead to the wrong answer.
Common mistakes include:
dividing the numbers in the wrong order
forgetting to multiply by 100
using the new value instead of the original value for percentage change
confusing percentage points with percentage change
applying a percentage increase or decrease to the wrong starting number
For example, if a value rises from 40% to 50%, that is an increase of 10 percentage points, but the percentage increase is 25%.
Checking the starting value and using the correct formula can help you avoid these errors.
Calculate Percentages Online
Need to work out a percentage, percentage increase or percentage decrease quickly?
Use our free Percentage Calculator to calculate percentages and percentage changes without doing the maths manually.
Frequently Asked Questions
How do you calculate what percentage one number is of another?
Divide the first number by the second number, then multiply the result by 100.
How do you calculate a percentage increase?
Subtract the original value from the new value, divide the difference by the original value, then multiply by 100.
What is the difference between percentage change and percentage points?
Percentage change measures the relative increase or decrease from the original value, while percentage points measure the direct difference between two percentages.