Use the How to Work Out A Discount in Your Head
To work out a discount, multiply the original price by the discount percentage, then subtract that saving from the original price.
Discount amount = original price × discount percentage. Sale price = original price − discount amount. For 20% off £50, the saving is £10 and the sale price is £40.
Want the sale price instantly?
The discount calculator works out savings, final price, stacked discounts and quantity totals.
The discount formula
A discount is just a percentage taken off an original price. The cleanest way to calculate it is to find the saving first, then subtract it.
Discount amount = original price × discount percentage ÷ 100
Sale price = original price − discount amount
Example:
20% off £80 = £80 × 20 ÷ 100 = £16 saving
Sale price = £80 − £16 = £64If you already know the percentage as a decimal, you can multiply directly: 20% is 0.20, 15% is 0.15 and 5% is 0.05.
Quick mental maths shortcuts
For common sale signs, you can often estimate the answer without a calculator. These shortcuts work because many discounts are built from 10%, 5%, 25% or half-price calculations.
| Discount | Quick method | Example on £60 |
|---|---|---|
| 10% off | Move the decimal one place left | £6 off, pay £54 |
| 20% off | Find 10%, then double it | £12 off, pay £48 |
| 25% off | Find a quarter of the price | £15 off, pay £45 |
| 50% off | Half the price | £30 off, pay £30 |
| 75% off | Take off three quarters, or pay one quarter | £45 off, pay £15 |
How to handle awkward discounts
For percentages such as 12.5%, 17.5% or 35%, break them into easier pieces. This is often faster than trying to do the full percentage in one step.
15% off
Find 10%, find 5%, then add them. On £80, 10% is £8 and 5% is £4, so 15% off is £12. Pay £68.
12.5% off
12.5% is one eighth. On £40, one eighth is £5, so pay £35.
35% off
Find 30% and 5%. On £100, that is £30 + £5 = £35 off. Pay £65.
2 for a discount
Work out the final price first, then divide by the number of items to compare the real price per item.
Stacked discounts are not usually added together
If a shop gives 20% off and then an extra 10% off, the total discount is not 30%. The second discount normally applies to the already-reduced price.
Original price = £100
First discount: 20% off = £80
Second discount: 10% off £80 = £8
Final price = £72
Total saving = £28
Effective discount = 28%This is why stacked discounts should be calculated step by step. Use the percentage calculator if you want to check the effective percentage saving after multiple reductions.
Adding stacked discounts together can overstate the saving. A 20% discount followed by a further 10% discount gives 28% off overall, not 30%.
Worked discount examples
20% off £49.99
£49.99 × 0.20 = £10.00 saving. Sale price = £39.99 after rounding to the nearest penny.
15% off £120
10% is £12 and 5% is £6, so the saving is £18. Sale price = £102.
25% off £36
A quarter of £36 is £9. Sale price = £27.
Buy 2 after 10% off
If one item is £18 and the discount is 10%, one costs £16.20. Two cost £32.40.
Discounts do not always mean best value
A sale price can still be poor value if the pack size, quality or quantity is different. For shopping comparisons, check the unit price as well as the headline saving.
Comparing two packs?
Use the unit price comparison calculator to see which option is cheaper per kg, litre, item or custom unit.
Rounding and money values
For pounds and pence, most discount results should be rounded to two decimal places. Small rounding differences can happen when a till calculates discounts line by line rather than on the whole basket.
- Round final prices to the nearest penny.
- For multiple items, calculate the discount method the retailer uses if you need an exact receipt match.
- For rough mental maths, round the starting price first, then check the exact result later.
- For VAT-inclusive sale prices, calculate the sale price first, then split out VAT if needed.
FAQs
How do I calculate 20% off?
Find 10% of the original price and double it. Subtract that saving from the original price. For £50, 10% is £5, so 20% is £10 and the sale price is £40.
How do I calculate 25% off?
25% is one quarter. Divide the price by 4 to find the saving, then subtract it. For £80, 25% is £20, so the sale price is £60.
Is 20% off then 10% off the same as 30% off?
No. If the second discount is applied after the first, 20% off then 10% off gives a 28% overall saving.
How do I find the original price after a discount?
If you know the sale price and discount rate, divide the sale price by one minus the discount rate. For example, if £80 is the price after 20% off, the original price was £80 ÷ 0.80 = £100.
Should I use discount percentage or unit price?
Use both. Discount percentage tells you how much the price dropped, while unit price helps you compare pack sizes and quantities fairly.