Whether you're shopping a sale, comparing offers or working out a trade price, calculating a discount accurately takes about ten seconds once you know the formula. This guide covers every discount scenario you're likely to encounter.
The Basic Discount Formula
A discount is simply a percentage subtracted from a price. The formula is:
So a 30% discount on a £120 item: 120 × (1 − 0.30) = 120 × 0.70 = £84.
Alternatively, you can do it in two steps: calculate the discount amount first, then subtract it.
Original price: £120
Discount rate: 30%
Discount amount: 120 × 0.30 = £36
Sale price: 120 − 36 = £84
Both methods give the same answer. The one-step version is faster when you just need the final price. The two-step version is useful if you want to see the saving amount separately.
How to Find the Discount Percentage
If you know the original price and the sale price and want to know the discount percentage, use this formula:
A jacket was £180, now priced at £126.
Saving: 180 − 126 = £54
Discount %: (54 ÷ 180) × 100 = 30%
This is useful when a retailer shows a "save £X" figure but doesn't tell you the percentage. It's also how you can compare discounts across different items to see which deal is actually better value.
Stacked Discounts: What to Watch Out For
Sometimes you'll see two discounts applied at once — a sale discount plus an additional code, for example. Stacked discounts don't add up the way you might expect.
Original price: £200
First discount: 20% off → £200 × 0.80 = £160
Second discount: 10% off the reduced price → £160 × 0.90 = £144
Total saving: £200 − £144 = £56
Effective discount: (56 ÷ 200) × 100 = 28%, not 30%
A 20% discount plus a 10% discount is not the same as a 30% discount. Each percentage is applied to a successively smaller number, so the combined effect is always less than the sum of the two rates.
How to Find the Original Price from a Discounted Price
If you know a price is already discounted and you want to find what it was before, you use a reverse percentage. Divide the current (discounted) price by (1 − the discount rate as a decimal).
Sale price: £75 (after 25% off)
Original price: 75 ÷ (1 − 0.25) = 75 ÷ 0.75 = £100
This is a useful sanity check when you see "now £75, was £120" — you can verify that £75 is actually 37.5% off £120, not whatever percentage the retailer claims.
Mental Shortcuts for Common Discounts
- 10% off: move the decimal one place left. 10% off £85 = £8.50 off → £76.50.
- 20% off: find 10% and double it. 20% off £85 = £17 off → £68.
- 25% off: divide by 4. 25% off £80 = £20 off → £60.
- 50% off: divide by 2.
- 15% off: find 10%, find 5% (half of 10%), add them. 15% off £80 = £8 + £4 = £12 off → £68.
Comparing "Was/Now" Prices Honestly
Retailers sometimes inflate the "was" price to make the discount look larger. The way to sense-check this: calculate the discount percentage yourself using the formula above, then consider whether that original price seems realistic. Was the item actually sold at that price for a meaningful period? UK consumer law requires that a "was" price must have been the genuine price for at least 30 consecutive days.
Work out any discount instantly — use the Subtract a Percentage calculator on PercentageHQ.
Discount Calculator →Quick Reference
- Sale price: Original × (1 − discount/100)
- Discount amount: Original × (discount/100)
- Discount percentage: ((Original − Sale) ÷ Original) × 100
- Original price from discounted: Sale price ÷ (1 − discount/100)
Examples in this article use illustrative prices. Always verify calculations before making financial decisions.