Four common percentage calculations — instantly. X% of Y, X is what % of Y, X + Y%, and reverse percentage (original before discount).
Runs entirely in your browser. Your data never leaves your device.
| Mode | Formula |
|---|---|
| X% of Y | (X / 100) × Y |
| X is what % of Y | (X / Y) × 100 |
| X ± Y% | X × (1 ± Y/100) |
| Original before discount | final / (1 − discount/100) |
| Original before markup | final / (1 + markup/100) |
| Mode | Input | Expected |
|---|---|---|
| X% of Y | 20% of 150 | 30 |
| X% of Y | 7.5% of 200 | 15 |
| X is what % of Y | 40 of 200 | 20% |
| X ± Y% | 100 + 15% | 115 |
| X ± Y% | 100 − 15% | 85 |
| Reverse (discount) | 85 after 15% discount | 100 |
| Reverse (markup) | 130 after 30% markup | 100 |
All formulas verified manually. Last checked: September 2026.
Reverse percentage (also called 'original before discount') lets you enter a final amount and a discount or tax percentage to recover the original amount before the change. For example: you paid $85 after a 15% discount — what was the original price?
No. All calculations run entirely in your browser using vanilla JavaScript. Nothing you type is transmitted anywhere.
Results are rounded to 2 decimal places by default. For very large or very small numbers we switch to 4 significant digits to keep the output readable.
A dash means a field is still empty or does not hold a readable number, so there is nothing to calculate yet. In reverse mode it also appears when the percentage makes the math impossible, such as a 100% discount, since no original price would leave you paying nothing.
It shows up whenever the second field is 0. You are then asking how much of nothing a number is, and that answer has no finite value, so the tool displays the infinity symbol instead of an error.
Keep the numbers moving: reduce any ratio with the Fraction Calculator, describe how spread a data set is with the Standard Deviation Calculator, and work out fast compounding growth in the Exponent Calculator.
If you just finished with Percentage Calculator, the natural next steps are Discount Calculator, GPA Calculator, Scientific Calculator, or browse every tool in Daily Tools.