Reverse Percentage Calculator
Work backwards from a final total to discover the original starting number. Perfect for identifying the net cost of an item before VAT (20%) was added, or finding the pre-sale price of a discounted product.
Find Original Value
Original Value
Working Backwards
Most people know how to calculate 20% of a number. However, working backward—trying to figure out what the original price was before that 20% was added—trips up many people, including small business owners trying to do their VAT returns.
The Golden Rule of Reverse Percentages
You cannot simply subtract the percentage to find the original value. You must divide using decimals.
Reversing an Addition (e.g. VAT)
Divide by: 1 + (Percentage ÷ 100)
Example: For 20% VAT, divide the final total by 1.20 to find the net price.
Reversing a Deduction (e.g. Sales)
Divide by: 1 - (Percentage ÷ 100)
Example: For a 25% discount, divide the final sale price by 0.75 to find original.
The Most Common Mistake
The Wrong Way
"I bought a laptop for £120. That means it includes 20% VAT. So to find out how much the VAT was, I'll just calculate 20% of 120 (which is £24), and subtract it to get the original £96." WRONG!
The Right Way
The original price was £100. 20% of £100 is £20. So £100 + £20 VAT = £120 final price. You find this using the formula: 120 ÷ 1.20 = 100.