Percentage Calculator Guide
How to calculate percentages, percentage change and percentage differences.
Percentages are a compact way to describe a part of a whole. They are used for discounts, salary changes, test results, budgets, business reports and everyday comparisons. The important first step is to identify what the percentage is being measured against.
Three common percentage questions
What is X% of a number?
Use this when you know the percentage and the original number. Convert the percentage to a decimal and multiply it by the number. For example, 15% of 200 is 0.15 × 200 = 30.
What percentage is one number of another?
Divide the part by the whole and multiply by 100. If a class has 18 students and 6 are absent, the absent share is 6 ÷ 18 × 100 = 33.33%.
What is the percentage change?
Compare the difference with the original value. The formula is (new value − original value) ÷ original value × 100. A move from 80 to 100 is a 25% increase because the change is 20 and the original value was 80.
Why the reference value matters
A common mistake is using the new value as the denominator when calculating a change. Percentage increase and percentage decrease are normally based on the starting value. If a price falls from 100 to 80, the decrease is 20%, not 25%.
Practical checks
- Write down the original or reference number first.
- Keep percentage and percentage-point changes separate.
- Round only at the end when precision matters.
- For financial or contractual decisions, verify the result against the relevant source.
Example: a price discount
A €120 item with a 15% discount saves €18, leaving a final price of €102. The same percentage can represent very different amounts depending on the starting price.
Use the calculator
Enter your values in the Percentage Calculator and choose the calculation that matches your question.