How to calculate a discount
- Enter the original price before any reduction.
- Enter the discount percentage advertised.
- Add a second discount if a voucher or member offer stacks on top.
- Read what you pay, what you save, and the true combined discount.
- Copy the breakdown if you need to check it against a receipt.
What this calculator handles
- Sale price and cash saving from any percentage discount.
- Two stacked discounts, applied in the order shops actually apply them.
- The effective discount, which is always less than the two added together.
- The percentage of the original price you end up paying.
- Any currency symbol, since the arithmetic is the same everywhere.
- Live updating as you change any figure.
The stacking trap
"20% off, plus an extra 10% for members" sounds like 30 per cent off. It is not. The second discount is applied to the already-reduced price, not to the original, so the real reduction is 28 per cent.
The arithmetic: £100 less 20 per cent is £80. Ten per cent of £80 is £8, leaving £72. You saved £28 on £100, which is 28 per cent. The missing two per cent is the 10 per cent that was never applied to the £20 you had already saved.
The gap widens with larger discounts. Fifty per cent then a further 50 per cent is not free. It is 75 per cent off, because the second half is taken from what remains. Anyone who has been surprised at the till by a smaller reduction than expected has met this.
Order does not matter, despite appearances
A common worry is that applying the larger discount first gives a better result. It does not, multiplication is commutative, so 20 per cent then 10 per cent produces exactly the same price as 10 per cent then 20 per cent.
What does change the outcome is whether a discount is a percentage or a fixed amount. "£10 off then 20 per cent off" and "20 per cent off then £10 off" give different prices, and shops are not always consistent about which they apply first. If both are on offer, it is worth checking the receipt.
Reading a sale honestly
- "Up to 70% off" means at least one item is 70 per cent off. Most will not be.
- "Was £200, now £120" is only a saving if £200 was a real price people paid.
- "Buy one get one half price" is 25 per cent off the pair, not 50.
- "Three for two" is 33 per cent off, and only if you wanted three.
That third one catches people regularly. Two items at £10 each, with the second at half price, costs £15 rather than £20, a saving of £5 on £20, which is 25 per cent.
Working backwards from a sale price
If you know the original and the sale price but not the percentage, divide the saving by the original and multiply by 100. A £200 item at £150 is a saving of £50, which is 25 per cent off. The percentage calculator has a mode for exactly this, and a reverse mode for recovering the original price when only the discount and the final figure are known.
Everything is calculated in your browser, so nothing about what you are buying is transmitted anywhere.
Working out whether the sale is real
A discount is only a saving if the original price was one people actually paid. Consumer protection rules in many countries require a reference price to have been charged for a meaningful period before it can be advertised as reduced, precisely because inflating a "was" price is an easy way to manufacture a discount that never existed.
The practical check is to look up the item's price history rather than trusting the ticket. Several browser extensions and price-tracking sites do this for major retailers. If an item has spent most of the year at the "sale" price, the reduction is presentational rather than real, and the arithmetic on this page will happily tell you what 40 per cent off a fictional price comes to.