Discount Calculator
Work out the sale price from a percent off, stack two or more discounts on top of each other, add or remove tax, or check whether a Buy One Get One deal is as good as it looks. The maths is simple. The retailer marketing is the bit that needs decoding.
Explain like I'm 5 (what this calculator does)
Type a price and a percent off, and it tells you what you actually pay and how much you save. There are four modes: a single discount, several discounts stacked together (which never add up the way the signs suggest), a price with tax added or removed, and Buy X Get Y Free, which is rarely as generous as the poster claims. Currency does not matter; the sums work the same in pounds, dollars, euros or yen.
Calculate
Enter values and press Calculate.
Single discount
- Original price—
- Discount %—
- Amount saved—
- Sale price—
Running price after each discount
Effective discount
- Final price—
- Total amount saved—
- Effective combined discount—
Stacked discounts do not add. 30% then 20% is 1 minus (0.7 times 0.8) = 0.44, so 44% off, not 50%.
Tax breakdown
- Net (excluding tax)—
- Tax amount—
- Final (including tax)—
Buy X Get Y Free
- Total items received—
- Total paid—
- Total saved—
- Effective price per unit—
- Effective discount %—
Prove it
Single: sale = original × (1 − d/100). Stacked: final = original × product of (1 − di/100); effective % = (1 − that product) × 100. Tax-exclusive: final = price × (1 + r/100). Tax-inclusive: net = price ÷ (1 + r/100). BOGOF: effective unit price = (buy × price) ÷ (buy + free); effective % = free ÷ (buy + free) × 100.
Useful? Press Ctrl + D to bookmark.
Why stacked discounts never quite add up
The signs in the window say "30% off, plus an extra 20% off at the till". The brain reads "50% off" and gets excited. The till disagrees, because each discount is applied to the running price, not the original.
A £100 jumper at 30% off is £70. Take 20% off £70 (not off £100) and you land on £56. That is 44% off the original, not 50%. The maths: 1 minus (0.70 times 0.80) = 0.44. The order does not matter; 20% then 30% gets you to the same £56. What does matter is that the second discount only ever attacks what is left after the first.
This is the friendly version. The cynical version is that retailers know perfectly well a "30 plus 20" sign feels bigger than "44% off". They are not lying. They are also not quite telling you the whole sum.
Buy One Get One Half Price is not 50% off
You buy two items. One is full price, one is half price. Total cost: 1.5 times the unit price, for two items. Per-unit price: 0.75 times the original. That is a 25% effective discount across the pair, not 50%.
Buy One Get One Free is the cleaner version: two items for the price of one means an effective unit price of half the original. 50% off if (and only if) you wanted two of them in the first place. Buying a second one you would not have bought anyway is not saving money. It is spending money.
Was-and-now pricing, and how to spot a fake "original"
The "was" price on a tag is supposed to reflect what the item recently sold for at full price, for a meaningful length of time. UK rules (the Chartered Trading Standards Institute pricing practices guide) suggest 28 days at the higher price as a rough benchmark, with caveats. In practice, that benchmark gets stretched. An item priced at £200 for two weeks in a quiet stock room and then "reduced" to £100 for the next six months is technically half price, in a way that means almost nothing.
Sense-checks that help: see if the same product is on Camelcamelcamel (for Amazon) or another price-history tracker. Compare against the original RRP from the manufacturer's site. If the "was" price only ever shows up on this retailer and nowhere else, treat it like fan fiction.
Quick-glance reference: what £100 becomes after a discount
Useful for back-of-an-envelope sense checking. Replace 100 with any starting figure and the proportions hold.
| Discount | £100 becomes | $100 becomes |
|---|---|---|
| 10% off | £90 | $90 |
| 15% off | £85 | $85 |
| 20% off | £80 | $80 |
| 25% off | £75 | $75 |
| 30% off | £70 | $70 |
| 33% off | £67 | $67 |
| 40% off | £60 | $60 |
| 50% off | £50 | $50 |
| 60% off | £40 | $40 |
| 70% off | £30 | $30 |
| 75% off | £25 | $25 |
Working backwards: from sale price to original
You see a £45 jumper marked 25% off and want to know the "was" price. The maths is just division: 45 divided by (1 minus 0.25) = 45 divided by 0.75 = £60. Save the difference, £15.
The general formula: original = sale price divided by (1 minus discount/100). Useful when you want to compare a sale price against typical RRPs to see if the deal is real.
Related calculators
The maths next door.
Frequently asked questions
How do I calculate a discount?
Multiply the original price by (1 minus discount/100). 25% off £80 is 80 times 0.75 = £60. The amount saved is the difference, £20.
How do I work out the original price from a sale price?
Divide the sale price by (1 minus discount/100). A £45 item at 25% off had an original price of 45 divided by 0.75 = £60.
Do stacked discounts add up?
No. Each discount applies to the running price. 30% then 20% is 1 minus (0.7 times 0.8) = 0.44, so 44% off, not 50%.
How do I work out percentage saved?
Divide the amount saved by the original price, then multiply by 100. Save £15 on a £60 item: (15 divided by 60) times 100 = 25%.
Is Buy One Get One Half Price really half off?
No. Across the pair, you pay 1.5 times the unit price for two items, so the per-unit price is 0.75 times the original. That is 25% off, not 50%.