Percentages
Reverse percentage calculator
Enter a percentage and the result of taking that percentage of a number, to find the original number — the reverse of a normal percentage calculation.
How to calculate it
A normal percentage calculation starts with a number and finds a percentage of it. This works backwards: starting from the result of that calculation and the percentage used, it finds the original number. For example, if 15% of a number is 30, dividing 30 by 0.15 gives an original number of 200.
Worked examples
| Example | Working | Answer |
|---|---|---|
| 15% of ? = 30 | 30 ÷ (15 ÷ 100) | 200 |
| 20% of ? = 17 | 17 ÷ (20 ÷ 100) | 85 |
| 7.5% of ? = 18 | 18 ÷ (7.5 ÷ 100) | 240 |
| 125% of ? = 100 | 100 ÷ (125 ÷ 100) | 80 |
| 50% of ? = 400 | 400 ÷ (50 ÷ 100) | 800 |
| 3% of ? = 45 | 45 ÷ (3 ÷ 100) | 1,500 |
Frequently asked questions
How is this different from the percentage calculator?
The percentage calculator finds a percentage of a known number. This tool works backwards: it finds the original number when you already know a percentage of it and the result. For example, if 20% of a number is 17, this finds that the number is 85.
Can I use this to find a price before a discount?
This tool finds a number from a percentage of it, not a percentage off it, so it needs care with discounts: if a price after a 20% discount is 80% of the original, enter 80 as the percentage. For discounts specifically, the discount calculator’s FAQ explains finding an original price directly from a discount percentage.
Can the percentage be more than 100?
Yes. For example, if 125% of a number is 100, the original number is 80 — smaller than 100, because 125% is more than the whole original number.
What if the percentage is zero?
Zero percent of any number is zero, so a result other than zero has no valid original number, and this tool cannot calculate one.
Do you round the results?
Results are shown with up to 12 significant digits, so long decimals are rounded slightly.
Method: standard arithmetic. Examples are illustrative. How our calculators work.