Stacked discounts multiply, they do not add. A 30% sale plus a 10% voucher code takes 37% off the original price, not 40%, because the code only applies to the already reduced price. Use the calculator above for your own numbers, then read on for the formula, worked examples and the checkout snags that catch people out.
How stacked discounts work
Each discount is taken from the price left after the one before it. The second saving is therefore calculated on a smaller number, which is why the total always falls a little short of the two percentages added together.
Worked example on an £80 jacket with 30% off, then a 10% code:
- 30% off £80 is £24, leaving £56.
- 10% off £56 is £5.60, leaving £50.40.
- Total saving is £29.60, which is 37% of the original £80.
The formula for a combined discount
Turn each discount into what you keep, multiply them, then subtract from 1:
Total discount = 1 − (1 − first) × (1 − second)
For 20% then 15%: 1 − (0.80 × 0.85) = 1 − 0.68 = 0.32, so 32% off. A shortcut for two discounts is first + second − (first × second ÷ 100), so 20 + 15 − 3 = 32.
The same method works for three or more. Take £200 with 20%, then 10%, then another 10%: £200 → £160 → £144 → £129.60. That is £70.40 saved, or 35.2%, not the 40% you get by adding.
Quick reference table
| Discounts | Adding them (wrong) | Real total off |
|---|---|---|
| 10% + 10% | 20% | 19% |
| 20% + 10% | 30% | 28% |
| 20% + 15% | 35% | 32% |
| 30% + 10% | 40% | 37% |
| 25% + 25% | 50% | 43.75% |
| 40% + 20% | 60% | 52% |
| 50% + 20% | 70% | 60% |
The bigger the first discount, the less a second code is worth. After 50% off, a 20% code takes off only another 10% of the original price.
Does the order of discounts matter?
Not for two or more percentages. 30% then 10% ends at the same price as 10% then 30%, because multiplication does not care about order.
It does matter when one discount is a fixed amount. On a £100 item with 20% off and a £10 code:
- Percentage first: £80 − £10 = £70.
- £10 first: £90 × 0.80 = £72.
A percentage-first order is better for you, because the £10 comes off at full value. Retailers set the order in their checkout, so you rarely get to choose. It is still worth knowing why two similar offers can finish £2 apart.
Working backwards: what code do you need?
If you want a particular total saving, rearrange the formula. To reach 40% off with a 25% sale already applied, you need to keep 60% of the price overall. Divide 0.60 by 0.75 and you get 0.80, so you need a 20% code. Check: 0.75 × 0.80 = 0.60, which is 40% off.
Why a stacked saving may not work at checkout
The maths is the easy part. The terms decide whether you can stack at all.
- Sale exclusions. Many UK retailers exclude sale or clearance lines from voucher codes. Others allow only one code per order, or one offer per item.
- Minimum spend. A code may need £50 or more in the basket. A £70 item at 30% off becomes £49, which can drop it below the threshold and the code is refused.
- Delivery. A discounted basket may no longer qualify for free delivery. Compare the delivery cost against the saving using the Free Delivery Calculator.
- Loyalty and student discounts. These often cannot be combined with other codes, and the terms are usually on the retailer's offers page.
- Gift cards and credit. Paying with a gift card is not a discount, so it never counts towards the stack.
If a code is rejected, work out whether the sale price alone beats the full-price total with the code. Sometimes the plain code on a full-price item wins.
Adding cashback on top
Cashback is usually paid as a percentage of what you actually spend, so it sits after your discounts rather than inside them. On the £63 you pay in the 30% plus 10% example on £100, 5% cashback is £3.15. Counting it, you are £40.15 better off than the £100 starting price. Cashback is often confirmed later and can be declined if a code is used outside the cashback site's terms, so treat it as a bonus. The Cashback & Voucher Calculator handles this properly.
Other calculators to try
- Comparing a percentage code against a pounds-off code: Voucher Comparison Calculator.
- Working out one percentage on its own: Discount Calculator.
- Checking whether a bargain is cheaper per gram or per item: Price Per Unit Calculator.
To see what codes and deals are currently listed before you shop, browse PrimeVoucher's stores or categories, and always check the terms before you count on a stack.