Everyday maths
How to calculate percentages
Percent means “per hundred.” The right formula depends on whether you need a percentage of a number, the percentage represented by a part, or the original whole.
Open the percentage calculator1. Find a percentage of a number
Result = percentage ÷ 100 × number
To find 18% of 250, convert 18% to 0.18 and multiply: 0.18 × 250 = 45.
A quick reasonableness check helps: 10% of 250 is 25 and 20% is 50, so 45 is sensible.
2. Find what percent one number is of another
Percentage = part ÷ whole × 100
If 42 of 60 questions are correct, 42 ÷ 60 × 100 = 70%. The denominator must be the whole being compared. Reversing the numbers gives a different and usually meaningless result.
3. Find the whole from a percentage
Whole = part ÷ (percentage ÷ 100)
If 30 is 12% of a total, the total is 30 ÷ 0.12 = 250.
| Question | Calculation | Answer |
|---|---|---|
| 15% of 80 | 0.15 × 80 | 12 |
| 24 is what % of 30? | 24 ÷ 30 × 100 | 80% |
| 45 is 20% of what? | 45 ÷ 0.20 | 225 |
Percentage points are not percent change
If a rate rises from 20% to 25%, it rises by 5 percentage points. Relative to the original 20%, that is a 25% increase. State which measurement you mean.
Percentages can be above 100% or below 0% when the context allows it. For currency, grades and measured data, keep full precision during calculation and round only the final answer.
Use ToolPixels for common percentage questions
The free Percentage Calculator handles each of these forms. Choose the question that matches what is unknown, enter the other two values, and check that the whole is not zero.