To work out a discount, multiply the original price by the percentage off (as a decimal) and take that amount away. 25% off £80 is £80 × 0.25 = £20, so you pay £60. The calculator above does this instantly, and the rest of this guide covers the cases a simple sum misses: working backwards, fixed amounts off, multi-buys, stacked offers and delivery.
How to calculate a percentage discount
There are two routes to the same answer. Pick whichever feels easier.
Method 1: find the saving, then subtract it
Saving = original price × (discount ÷ 100). Final price = original price − saving. For 20% off £45: £45 × 0.20 = £9 saved, so you pay £36.
Method 2: multiply by what you keep
If something is 30% off, you pay the other 70%. Multiply by 0.70 for the final price in one step: £45 × 0.70 = £31.50.
Quick reference table
| Discount | Multiply price by | You pay on £100 | You pay on £64 |
|---|---|---|---|
| 10% off | 0.90 | £90.00 | £57.60 |
| 20% off | 0.80 | £80.00 | £51.20 |
| 25% off | 0.75 | £75.00 | £48.00 |
| 30% off | 0.70 | £70.00 | £44.80 |
| 50% off | 0.50 | £50.00 | £32.00 |
Mental maths shortcut
Find 10% by moving the decimal point one place, then build from there. For 30% off £64, 10% is £6.40, so 30% is £19.20 and you pay £44.80. For 15%, add half of the 10% figure on top of it.
Working backwards from the sale price
Find the original price. If a jacket costs £63 after 30% off, you are paying 70%, so £63 ÷ 0.70 = £90. Do not add 30% to £63, because the percentage applies to the original price, not the sale price.
Find the percentage off. Subtract the new price from the old, then divide by the old. A pair of trainers cut from £120 to £84 saves £36, and £36 ÷ £120 = 0.30, so 30% off.
Turn a fixed amount into a percentage. £10 off a £40 basket is £10 ÷ £40 = 25%. This is handy for comparing a pounds-off code with a percentage code.
Multi-buys, BOGOF and stacked discounts
- Half price on the second item: two items at £20 cost £20 + £10 = £30 instead of £40, which is 25% off overall, not 50%.
- 3 for 2: three items at £6 cost £12 instead of £18. That is one third off, or £4 per item.
- Successive discounts: 20% off followed by a further 10% off does not equal 30%. On £100, the first step gives £80, the second gives £72, so the total saving is 28%.
For sale prices plus a voucher code, use the Stacked Discount Calculator to see the true combined saving.
Discounts and VAT in the UK
Shelf and website prices to consumers include 20% VAT, so a percentage discount simply reduces the price you pay. The order does not change the result for a standard-rated item: £100 + VAT is £120, and 20% off gives £96. Taking 20% off the £100 first (£80) and adding VAT (£16) also gives £96.
If you need to separate VAT from a price, the VAT Calculator does it in one step.
Is the deal really cheaper? Delivery and fine print
A bigger percentage is not always the better deal. Say a £50 order has two options: 20% off with £4.95 delivery, or 15% off with free delivery.
| Option | Item price | Delivery | Total |
|---|---|---|---|
| 20% off | £40.00 | £4.95 | £44.95 |
| 15% off, free delivery | £42.50 | £0.00 | £42.50 |
The smaller discount wins by £2.45. The Voucher Comparison Calculator and Free Delivery Calculator are built for exactly this check. Before relying on a code, also look for:
- Minimum spend requirements.
- Exclusions such as sale lines, certain brands or gift cards.
- Expiry dates and whether a code is single-use.
- Unit cost: for multipacks, compare with the Price Per Unit Calculator, since a "deal" pack can cost more per item.
Spotting a genuine saving
A big percentage only matters if the starting price was realistic. Check the item at two or three other retailers, and look at the price history if the shop shows it. If a "was" price looks inflated, judge the deal on what you would pay elsewhere today.
When you are ready to shop, browse the stores on PrimeVoucher or start from a category to find current codes, then run your basket through the calculator before you check out.