Discount calculator
Stack as many discounts as you like. They do not add up — each one comes off what is left — and this shows both the real figure and the one people expect. Nothing you type here leaves your browser — it is worked out on your own device, so there is nothing to send and nothing to log.
“20% off, then a further 10%” is 28% off
Not 30%. The second discount applies to what is left after the first, so it is 10% of £80 rather than 10% of £100. The shop is not being dishonest — that is what “a further 10%” means — and almost nobody works it out.
| 20% then 10% | 28% off, not 30% |
|---|---|
| 10% then 10% then 10% | 27.1% off, not 30% |
| 50% then 50% | 75% off, not free |
| 30% then 30% | 51% off, not 60% |
The gap grows with the size of the discounts. Two small ones are nearly additive; two large ones are nowhere near.
Up 20% then down 20% is not where you started
It is 4% lower. Always, whatever the price, and whichever way round you do it: £100 up 20% is £120, and £120 down 20% is £96.
The shortfall is the percentage squared, divided by a hundred — so 10% gives 1%, 20% gives 4%, and 50% gives 25%. The starting price cancels out entirely, which is why the figure is the same for a paperback and for a house.
This is worth knowing outside shops too. A portfolio that falls 50% needs to rise 100% to get back, not 50%.
The order does not matter
20% then 10% and 10% then 20% both give 28%, because multiplication does not care about order. So a shop that lets you choose which voucher to apply first is offering you nothing — unless one of them is a fixed amount off rather than a percentage, in which case it matters a great deal and you want the fixed amount taken off last.
Some shops add them anyway
“30% off” applied as a single discount is more generous than “20% and then a further 10%”, and some retailers do the first while describing it as the second. The receipt is the authority, not this page and not the sign in the window.
What it will not do
- It will not handle fixed amounts off — “£10 off” mixed with percentages. The order matters there and the answer depends on the shop's rules.
- It will not know about minimum spends, excluded lines or whether the vouchers combine at all.
- It will not do VAT. That is the VAT calculator, and whether a discount comes off before or after VAT is a question for the seller.