Reverse Percentage Calculator

How it is calculated

The tool undoes a percentage change. If a value went up or down by a percent and you only know the result, then original = final ÷ (1 + percent ÷ 100), and the change that was applied is change = final − original. Enter an increase as a positive percent (20 for a rise of 20%) and a decrease as a negative percent (−20 for a fall of 20%). The percentage must be above −100, because a fall of 100% leaves nothing to work back from, and the field cannot be empty; 0 gives back the same number. Results are shown to two decimals and the units are whatever you typed, so the tool works for prices, salaries or quantities. For a price with tax included, the percent is the tax rate, which differs by country and product.

Common mistakes

• Subtracting the percent from the final value (249.90 − 20% = 199.92). The percent was added to the original, not to the final figure, so you must divide by 1.20.
• Entering a decrease as a positive number. A 25% discount is entered as −25, and 25 would be treated as an increase, giving a very different original.
• Mixing up the final value with the original. The first field is the value you see after the change, not before it.
• Using the reverse of a percentage point change. A rate going from 5% to 7% is 2 points, not 2%; this tool only handles a relative percent change of a number.
• Applying several successive changes at once. Reverse each one separately, from the last back to the first, because percents do not simply add.