A reverse percentage calculator works backwards to find the original value before a percentage was added or subtracted. For example, if a price includes 20% VAT, or an item is on sale with 20% off, this tool calculates the starting amount before the change.

To find the original price before VAT, divide the final price by 1 plus the VAT rate as a decimal. For example, if the final price is £120 and VAT is 20%, the calculation is £120 ÷ 1.20 = £100. The original price was £100, and the VAT added was £20.

To find the original price after a discount, divide the sale price by 1 minus the discount rate as a decimal. For example, if an item costs £80 after a 20% discount, the calculation is £80 ÷ 0.80 = £100. The original price was £100, and the discount was £20.

The formula depends on whether the percentage was added or subtracted. If added: Original = Final ÷ (1 + Rate/100). If subtracted: Original = Final ÷ (1 – Rate/100). These formulas reverse the standard percentage increase or decrease calculations.

Percentages are relative to the current base value. If you start with £100 and apply a 20% discount, the price drops to £80. If you then add 20% to £80, you are calculating 20% of £80 (which is £16), bringing the total to £96, not £100. The base value has changed.

Yes. You can use the ‘Added’ mode to calculate the original amount before tax, VAT, or a tip was added. Simply enter the final total and the percentage rate, and the calculator will separate the original amount from the tax or tip.

To calculate manually, convert the percentage to a decimal. If 20% was added, divide the final amount by 1.20. If 20% was subtracted, divide the final amount by 0.80. For example, to reverse a 5% increase on 105: 105 ÷ 1.05 = 100.

A normal percentage calculator finds the result after applying a percentage to a known starting value (e.g., what is 20% of 100?). A reverse percentage calculator starts with the final result and the percentage rate, and works backwards to find the unknown starting value.