How to Calculate a Discount
Shopping guide · Updated July 2026
A discount is just a percentage taken off a price — but there are three questions people actually ask at the till: what will I pay, what was the original price, and how big was the saving really? This guide covers all three, plus the trap that trips up almost everyone in a sale: why a "30% off, then an extra 20% off" deal is not 50% off. If you only need the number, the percent-off calculator gives it instantly — but understanding the method means you will never be fooled by a headline again.
What "percent off" actually means
A discount reduces the original price by a share of itself. "25% off" means you keep 75% of the price and hand over that instead. So there are always two useful figures: the amount you save (the discount) and the amount you pay (the sale price). They add back up to the original. Keeping both in mind is what makes reversing a discount later so easy — the sale price is never the whole story, it is a fraction of the original.
The fast method: multiply by what's left
The quickest way to find a sale price is to multiply by the fraction you still pay, not to work out the saving and subtract it. For a 25% discount you pay 75%, so multiply by 0.75.
The one-step version saves a mistake: doing it in two steps (find 25%, then subtract) works, but it is where slips happen under time pressure. A discount is simply a percentage decrease, so the same logic drives the percentage decrease calculator.
Worked examples
Example 1 — a £60 jacket at 20% off.
- You pay 80%: £60 × 0.80 = £48
- You save 20%: £60 × 0.20 = £12
Example 2 — a $250 phone at 15% off.
- You pay 85%: $250 × 0.85 = $212.50
- You save 15%: $250 × 0.15 = $37.50
Notice the pattern: for any discount, the multiplier is one minus the rate as a decimal. A 10% discount uses 0.90, a 40% discount uses 0.60, and a 70% clearance uses 0.30. Memorise that and most sale maths becomes a single multiplication you can do in your head.
The stacking trap: why 30% + 20% isn't 50% off
This is the single most valuable thing to understand about discounts. When a shop offers "30% off, then take a further 20% off at the checkout," the second discount applies to the already-reduced price — not the original. So the percentages do not add.
A £100 item, 30% off then 20% off:
- After 30% off: £100 × 0.70 = £70
- After a further 20% off: £70 × 0.80 = £56
- Real discount: £44 saved on £100 = 44% off, not 50%
The shortcut: multiply the fractions you keep. 0.70 × 0.80 = 0.56, so you pay 56% and save 44%. Stacked discounts always come out lowerthan the two rates added together — the more you layer, the wider the gap. It is why "buy one get 50% off the second" is only a 25% saving across both items.
Working backwards: the original price from a sale price
Sometimes you see the sale price and the discount and want the original — to check a claim, or to know the "real" value. Here you divide by the fraction you paid, you do not add the percentage back.
A jumper is £48 after 20% off, so the original was £48 ÷ 0.80 = £60. The tempting mistake is to add 20% to £48, which gives £57.60 — too low, because the 20% was taken off the bigger original figure, not the smaller sale price. This "undo a percentage" step is exactly what the reverse percentage calculator is built for.
Finding the discount percentage from two prices
If a ticket shows both the old and new price but not the percentage, work out the saving and express it as a share of the original.
An £80 coat reduced to £60 saves £20, and £20 ÷ £80 = 0.25, a 25% discount. This is the same question as asking what percent one number is of another, which the "X is what percent of Y" calculator answers directly. Always divide the saving by the original price — dividing by the sale price inflates the figure and makes the deal look better than it is.
"Up to 70% off" and other marketing wording
"Up to" means the number quoted is the maximum, applied to at least one item, not the average across the sale. A rail advertised as "up to 50% off" may hold plenty of pieces reduced by 10% or 20%. Treat the headline as a ceiling and check each ticket. The same scepticism helps with "was/now" pricing: a genuine discount needs a genuine original price, so a large saving from an inflated "was" figure is not the bargain it appears to be.
Discounts and sales tax: which comes first
When tax is involved, the discount comes first and tax is charged on the reduced price. A £100 item at 20% off is £80, and 8% tax on £80 adds £6.40, for £86.40 in total — not the £8 you would pay if tax were charged on the full price. For the full breakdown across state and local rates, the sales tax calculator handles the second step once you have the discounted subtotal.
Quick reference
| Discount | You pay (multiplier) | £100 becomes |
|---|---|---|
| 10% off | ×0.90 | £90.00 |
| 15% off | ×0.85 | £85.00 |
| 20% off | ×0.80 | £80.00 |
| 25% off | ×0.75 | £75.00 |
| 30% off | ×0.70 | £70.00 |
| 50% off | ×0.50 | £50.00 |
| 70% off | ×0.30 | £30.00 |
For odd rates, awkward prices, or a full receipt with several items, let the discount calculator do the arithmetic. And if you want the wider picture of how increases and decreases behave — including why a price cut and an equal-sized rise never cancel out — the guide on percentage increase and decrease picks up where this one leaves off.
Frequently asked questions
Is 30% off then 20% off the same as 50% off?
No. The second discount applies to the already-reduced price, not the original. A £100 item at 30% off is £70; another 20% off £70 is £56 — a real discount of 44%, not 50%. Stacked percentages always come out lower than their sum.
How do I find the original price from a sale price?
Divide the sale price by (1 minus the discount as a decimal). A £48 jumper at 20% off came from £48 ÷ 0.80 = £60. Don't add 20% back to £48 — that gives £57.60, which is wrong, because the 20% was taken off the larger original figure.
How do I work out what percentage discount I got?
Divide the amount saved by the original price, then multiply by 100. If a £80 coat was reduced to £60, you saved £20; £20 ÷ £80 = 0.25 = a 25% discount. This is the same as asking what percent £20 is of £80.
Does the discount or the sales tax come first?
The discount is applied first, then tax is calculated on the reduced price — that is standard retail practice almost everywhere. A £100 item at 20% off is £80, and 8% sales tax on £80 is £6.40, for a total of £86.40. Taxing before discounting would overcharge you.
What does "up to 70% off" really mean?
It means 70% is the single biggest discount in the sale, not the typical one. Legally at least one item must be reduced by that amount, but most of the range is usually discounted far less. Always check the individual ticket price rather than trusting the headline.