How to Calculate a Discount (Fast Mental Math + the Formula)
Every discount is two numbers: what you save, and what you pay. Shops lead with the percentage because it’s the biggest-sounding of the three — knowing how to turn it into pounds in your head is the whole skill.
The formula
Saving = price × discount ÷ 100. Then sale price = price − saving — or in one step, price × (1 − discount÷100). Thirty percent off £60: saving 60 × 30 ÷ 100 = £18, so you pay £42.
The 10%-block shortcut
Ten percent is just the decimal moved one place — build everything from it. 10% of £85 is £8.50; so 20% is £17, 5% is £4.25, and 25% is £21.25 (two blocks plus a half). This handles nearly every shop discount without paper.
| Discount | On £40 | On £65 | On £120 |
|---|---|---|---|
| 10% off | pay £36 | pay £58.50 | pay £108 |
| 20% off | pay £32 | pay £52 | pay £96 |
| 25% off | pay £30 | pay £48.75 | pay £90 |
| 50% off | pay £20 | pay £32.50 | pay £60 |
| 70% off | pay £12 | pay £19.50 | pay £36 |
Stacked discounts multiply, not add
“Extra 10% off sale prices” applies to the already-reduced price. 20% off then 10% off leaves 80% × 90% = 72% of the original — a 28% total discount. Retailers know 20+10 reads like 30; now you know it isn’t.
Judging the deal
Percentages sound bigger than they are on small prices — 40% off £12 saves less than 10% off £60. Compare savings in money, not percent. For quick checks in the aisle, the discount calculator shows both numbers live, and the percentage formulas guide covers the arithmetic behind it.
Questions people ask
What’s the formula for a discount?
Saving = price × discount ÷ 100; sale price = price − saving. 25% off £48: save £12, pay £36.
Is 20% off then 10% off the same as 30% off?
No — stacked discounts multiply. 20% then 10% leaves you paying 80% × 90% = 72% of the price: a 28% total discount, not 30%.