Discount calculator
Work out a sale price and what you actually save, from either a percentage or a fixed reduction. Fast enough to use standing in a shop.
Built and checked by Mubashir, who works in accounting and finance. Formula and sources shown below. Not financial advice.
The price
You pay
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Original price
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You save
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Effective discount
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Show the working
—Stacked discounts do not add up
The single most useful thing to know when shopping. An extra 20% off an item already reduced by 30% is not 50% off. It is 44%.
The second discount applies to the already-reduced price, not the original. Starting at 100: down 30% gives 70, then 20% off 70 gives 56. You paid 56, which is 44% off, not 50%.
The gap widens as the discounts get larger, and it is always in the retailer's favour. Signage advertising "30% off plus an extra 20%" is technically accurate and reliably read as more generous than it is.
Working backwards from the sale price
To find what something originally cost, divide rather than subtract. An item marked 25% off and priced at 75 originally cost 75 divided by 0.75, which is 100. Subtracting 25% from 75 gives 56.25, which is the answer to a different question.
This is the same reversibility trap that appears in tax arithmetic, and it catches people just as often.
Percentage off against a fixed reduction
Which is better depends entirely on the price, and the crossover is worth knowing when a retailer offers a choice.
Twenty off a fixed price beats 20% off whenever the item costs less than 100. Above that, the percentage wins and keeps winning. On a 500 item, 20% saves 100 while the fixed reduction saves 20.
Retailers know this, which is why fixed-amount vouchers usually carry a minimum spend. The voucher exists to raise the size of your basket, not to save you money.
Reading a sale honestly
Two things are worth checking before treating a discount as a saving. First, whether the reference price was ever the real price — many jurisdictions require that an item was genuinely sold at the higher price for a period before it can be advertised as reduced, precisely because inflated reference prices were widespread. Second, whether you wanted the item at all. A 70% discount on something you will not use is a 30% loss, not a 70% gain.
Frequently asked questions
How do I calculate a discount?
Multiply the price by the discount percentage and divide by a hundred to get the saving, then subtract it. Or multiply the price by one minus the discount as a decimal to get the final price directly.
What does 20% off mean?
You pay 80% of the original price. On a 50 item that is 40, saving 10.
Do stacked discounts add together?
No. Thirty percent off followed by twenty percent off is 44% off in total, not fifty, because the second reduction applies to the already-reduced price.
How do I find the original price from a sale price?
Divide the sale price by one minus the discount as a decimal. An item at 75 after 25% off originally cost 75 divided by 0.75, which is 100.
Is a percentage or a fixed amount off better?
A fixed amount is better on cheap items and a percentage on expensive ones. Twenty off beats 20% off on anything under 100, and loses above it.
Should I trust the original price shown on a sale tag?
Check it. Many consumer protection rules require the higher price to have been genuine for a period beforehand, precisely because inflated reference prices were a common practice.