Reverse Percentage Calculator
Find an original value before a percentage increase or decrease, including discounts and increases above 100%. See the change amount instantly.
Increases can exceed 100%; decreases must be less than 100%.
Your results
Calculation breakdown
- After an increase
- Original value = Final value ÷ (1 + Percentage ÷ 100)
- After a decrease
- Original value = Final value ÷ (1 − Percentage ÷ 100)
Worked example
A jacket now costs $90 after a 10% discount. Reversing this: $90 ÷ (1 − 0.10) = $100 was the original price.
Assumptions
- A 100% decrease always ends at zero, so the final value alone cannot identify the original; larger decreases are outside this everyday non-negative amount model.
- Percentage increases over 100% are valid and can be reversed.
- Enter the percentage as a positive number and choose the direction it moved.
How this calculator works
Work backwards from a final value and its known percentage increase or decrease to find the original amount. Unlike a standard percentage calculator that applies a change to a starting number, this tool divides by the relevant percentage multiplier. It also shows the positive amount of the increase or decrease.
Work backwards from the final value
A reverse percentage calculation divides the final value by the change multiplier. After an increase, divide by 1 plus the percentage as a decimal; after a decrease, divide by 1 minus the percentage as a decimal.
For example, $90 after a 10% discount came from $100 because $90 ÷ 0.90 = $100. A result of $300 after a 200% increase also came from $100 because the final value is three times the original.
- After an increase: original = final ÷ (1 + percentage ÷ 100)
- After a decrease: original = final ÷ (1 − percentage ÷ 100)
- The change amount is the absolute difference between final and original
Why subtracting the percentage gives the wrong answer
Taking 10% off the final value does not reverse a 10% increase, because the forward percentage was calculated from a different base. If $100 increases by 10% to $110, the original is $110 ÷ 1.10, not $110 minus 10%.
A 100% decrease cannot be uniquely reversed: every non-negative starting value ends at zero. The calculator therefore accepts decreases below 100%, while percentage increases above 100% remain mathematically valid.
Frequently asked questions
How do I use this for a discounted price?
Enter the sale price you paid, select 'decrease' and enter the discount percentage to find the original price before the discount.
Can I use it for GST-inclusive prices?
Yes, enter the GST-inclusive price, choose 'increase' and enter 10% to find the GST-exclusive amount.
What happens if the percentage is 100% decrease?
Every starting amount becomes zero after a 100% decrease, so a final value of zero cannot reveal which original amount was used. A non-zero final value is impossible under that assumption. The calculator therefore asks for a decrease below 100%.
Can I reverse an increase greater than 100%?
Yes. For example, a final value of 300 after a 200% increase came from an original value of 100, because 300 is three times the original.
Content responsibility
Maintained by CalcAussie and last updated 15 October 2026. Checks applied: calculation method, worked examples, Australian terminology, material assumptions, official references where an external rule applies, and user-facing limitations.
Calculator results remain estimates based on the figures entered. Check material decisions against current official information and report a suspected error so it can be reproduced and corrected.