Reverse Percentage Calculator
Undoing a percentage means dividing, not subtracting.
Work out Reverse Percentage. Undoing a percentage means dividing, not subtracting. Refuses out-of-range input instead of guessing.
Original amount
100
Before the 20% was added
Reversing a percentage means dividing, not subtracting. To undo a 20% increase you divide by 1.2; subtracting 20% instead lands 4% low, because the two percentages are taken from different bases. The fourth row shows exactly that error, and it is not a curiosity. It is how VAT gets miscalculated on inclusive prices, how discounts on marked-up figures come out wrong, and why a 10% pay cut followed by a 10% rise leaves you worse off than you started. The larger the percentage, the larger the gap. At 50% the naive reverse is 25% adrift, which is a difference nobody can afford to hand-wave.
How the Reverse Percentage Calculator works
Find the amount a percentage was applied to. It also shows what happens if you subtract the percentage instead of dividing by it — the mistake that costs 4% on a 20% markup and rather more on larger figures.
Also known as: price before discount · price before vat · work backwards from a percentage · original amount before increase
Frequently asked questions
How do I find the original price before a discount?
Divide by one minus the discount as a decimal. A price of £80 after 20% off came from £100, because 80 divided by 0.8 is 100 — not from £96, which is what subtracting 20% from £80 gives.
Why doesn't adding 20% then subtracting 20% return the original?
Because the two percentages are taken from different bases. The 20% added is 20% of the original; the 20% subtracted is 20% of the larger figure. You land 4% below where you started.
How do I find the price before VAT?
Divide the inclusive price by one plus the VAT rate. At 20% VAT, divide by 1.2 — a £120 inclusive price is £100 plus £20 VAT. Taking 20% off £120 gives £96 and is wrong.
Does the error get worse with bigger percentages?
Substantially. At 20% the naive reverse is 4% adrift; at 50% it is 25% adrift. The gap grows with the square of the percentage, which is why the shortcut passes unnoticed on small figures and fails badly on large ones.
Is a 10% pay cut then a 10% rise break-even?
No — you end up 1% down. The cut is 10% of your original salary and the rise is 10% of the reduced one, so the rise is worth less in absolute terms than the cut took.
Put this calculator on your own site
Free to use, on any site, commercial or not. Paste this where you want it to appear. It is a plain iframe, so it works in WordPress, Squarespace, Wix, Webflow, Ghost and anything else that accepts HTML.
<iframe src="https://www.thecalclibrary.com/embed/reverse-percentage-calculator" width="100%" height="640" style="border:1px solid #e2e8f0;border-radius:12px" loading="lazy" title="Reverse Percentage Calculator"></iframe>
<p style="font:13px/1.5 system-ui,sans-serif;margin:6px 0 0;color:#64748b">Powered by <a href="https://www.thecalclibrary.com/reverse-percentage-calculator" style="color:#64748b">Reverse Percentage Calculator</a> from The Calc Library</p>The only condition is that the credit line below the frame stays in place. That one line is what pays for the tool being free — it is how anyone else finds it.