Reverse Percentage Calculator
Solve any percentage problem: find the whole from a part and percent, find what percent one number is of another, find a part, or reverse a percentage change. Runs in your browser.
25 is 20% of the result
About this tool
Percentage problems come in several shapes, and the tricky ones run 'backward' — you know the result and need the starting number. This calculator covers all four common cases in one place. Find the whole when you know a part and what percent it is (25 is 20% of what? → 125). Find the percentage one number is of another (25 is what % of 125? → 20%). Find a part (20% of 125 → 25). And the one that trips people up most: reverse a percentage change, such as recovering the original price before a markup or the pre-tax amount from a total — because adding 20% and then removing 20% does not return the original. Each mode shows the answer and the relationship it solved. The math is exact and runs in your browser, so it is handy for pricing, discounts, tax back-calculation, statistics, and everyday 'what was it before?' questions.
How to use it
- Pick the mode that matches what you're solving for.
- Enter the two values you know.
- Read the result.
- Use 'Reverse a % change' to undo a markup, discount, or tax to find the original.
Frequently asked questions
- How do I find the original number before a percentage was added?
- Divide the final amount by (1 + percent/100), not by subtracting the percent. If a price is $120 after a 20% markup, the original is 120 ÷ 1.20 = $100 — the "Reverse a % change" mode does this.
- Why isn't removing 20% the same as the 20% that was added?
- Because the percentages are taken of different bases. Adding 20% to 100 gives 120; removing 20% of 120 gives 96, not 100. To truly reverse it you divide by 1.20. This is the single most common percentage mistake.
- How do I find the whole from a part?
- Divide the part by the percentage as a decimal: if 25 is 20% of the whole, the whole is 25 ÷ 0.20 = 125. Use the "Find the whole" mode.
- How do I find what percent one number is of another?
- Divide the part by the whole and multiply by 100: 25 of 125 is 25 ÷ 125 × 100 = 20%. Use the "Find the percentage" mode.
- Can I use this to back out pre-tax price?
- Yes — use "Reverse a % change" with the tax rate. A $108 total at 8% tax came from 108 ÷ 1.08 = $100 before tax. (The dedicated tax tool shows the tax amount too.)
- Is anything uploaded?
- No. All calculation runs in your browser with no network request.