Reverse Percentage Calculator
You know the part and the percent it represents — this finds the whole. The "what number was this taken of?" tool.
Enter your values to see instant results:
Keyboard shortcuts
| Tab / Shift+Tab | Next / previous field |
| ↑ / ↓ | Step a value by 1 (Shift: 10, Alt: 0.1) |
| Enter | Copy the focused row's result |
| [ / ] | Fewer / more decimal places |
| Esc | Clear the field |
Working backwards
A forward percentage takes a whole and produces a part: 24% of 150 is 36. Reverse
percentage is the same equation solved the other way: original = value ÷ percent ×
100, so 36 ÷ 24 × 100 = 150. Any time you hold a slice and its
share — 36 points that were 24% of the grade, $210 commission that was 3.5% of the sale —
this one division recovers the whole.
How to use it
- Enter the value you know. Type the known part into the first field — 36 in the example.
- Enter what percent it represents. Type the percentage into the second field — 24 if 36 is 24% of the mystery number.
- Read the original number. The original appears instantly: 36 is 24% of 150.
After an increase or decrease: divide by the multiplier
The trickier household version starts from a value that already includes a percentage move. A restaurant bill of $86.40 that includes 8% tax wasn't taxed on $86.40 — divide by 1.08 to get the $80 pre-tax amount. A jacket ringing up at $63 after 30% off started at 63 ÷ 0.70 = $90. The rule in both cases: identify the multiplier that was applied (1 + p/100 for additions, 1 − p/100 for reductions) and divide by it. Subtracting the percentage from the final value is the classic wrong answer — it takes the percent of the wrong base, and the error grows with the rate.
Exam-style reverse problems ("a population grew 12% to 44,800 — what was it before?") are the same division: 44,800 ÷ 1.12 = 40,000. More worked examples live in percentages in real life.
Reverse percentage FAQ
How do I find the original number from a percentage?
Divide the known value by its percentage, then multiply by 100: original = value ÷ percent × 100. If 36 is 24% of something, the something is 36 ÷ 24 × 100 = 150. Division undoes what the percentage did.
How do I find the price before a discount or before tax?
Divide by the multiplier that was applied. Price after 20% off: divide by 0.80. Price after 8% tax was added: divide by 1.08. A $54 total at 8% tax started as 54 ÷ 1.08 = $50. The instinct to subtract 8% of $54 gives $49.68 — close, wrong, and the most common reverse-percentage mistake.
Why can't I just subtract the percentage to reverse it?
Because the percentage was taken of the original, and after the change you're no longer holding the original. Adding 25% then subtracting 25% multiplies by 1.25 × 0.75 = 0.9375 — you end up 6.25% short. Reversing a multiplication requires division, always.
What if the percent is more than 100?
It works the same. If 360 is 120% of last year's sales, last year was 360 ÷ 120 × 100 = 300. A value that is "more than 100% of" its base is simply bigger than the base.