Percentages: the three calculations to know

A discount, a share of a budget, a price increase: the symbol is the same, but the calculation changes. Here is how to choose the right one.

Calculate a share of a number

A percentage means “per hundred”. To find 15% of 200, first convert 15% to 0.15, then multiply 200 by 0.15. The result is 30. In a practical example, 15% of a €200 bill therefore represent €30. This method also works with a decimal percentage: 2.5% of 80 give 2. The starting point is always the number of which you want to find a fraction.

Express a part relative to a total

This time, you know the part and the total. If 18 of 24 seats are occupied, divide 18 by 24, then multiply by 100: occupancy is 75%. Check that the denominator really represents the whole. Dividing 24 by 18 would answer a different question. A zero total prevents this calculation: the tool then displays a message instead of a misleading result.

Measure a change

Going from 80 to 100 gives a difference of 20. Relative to the starting value, that difference represents 20 ÷ 80 × 100 = 25%. Going back from 100 to 80 is a 20% decrease, because the reference is now 100. An increase and a decrease of the same amount therefore do not necessarily give the same percentage.

Distinguish percentage points from percentages

A rate going from 10% to 15% increases by 5 percentage points. Its relative increase is 50%, since 5 is half of 10. State the unit of change in a report or presentation to avoid ambiguity. To check your result, restate the question aloud: are you looking for a quantity, a share or a change? Then choose the corresponding calculator.

Put it into practice

• Percentage calculator
• Percentage of a total
• Percentage change

Educational text prepared with AI assistance. The examples use adjustable assumptions and are not personal advice.