QUICKTOOLS GUIDE
How to Calculate Percentages
A practical explanation of percentages, percentage changes, discounts, and common mistakes.
In this guideA percentage is a ratio expressed out of 100.To find p% of a number n, use n × p ÷ 100.Percentage increase compares the change with the original value.Percentage points describe a difference between percentages themselves.
The key ideas
- A percentage is a ratio expressed out of 100.
- To find p% of a number n, use n × p ÷ 100.
- Percentage increase compares the change with the original value.
- Percentage points describe a difference between percentages themselves.
How the calculation or process works
p% of n = n × p ÷ 100
For example, 15% of 240 is 36. The same formula can be rearranged for other percentage questions when the unknown value is different.
Common mistakes
- Confusing a percentage with percentage points.
- Using the new value as the denominator when the question asks for change from the original.
- Forgetting that a 20% discount followed by another 20% discount is not the same as a single 40% discount.
Use the related QuickTools utility
Once you understand the method, the QuickTools Calculate Percentages page can handle the repetitive part of the task.
Important: A general guide explains a method; it does not know the rules, assumptions, or requirements of a particular business, contract, institution, or software system. Check those requirements when they matter.