Discount formulas
Example: 25% off a price of 80. Sale price = 80 × 0.75 = 60, so you save 20. Working backwards, 60 ÷ 0.75 = 80. And a drop from 80 to 60 is (80 − 60) ÷ 80 = 25% off.
Stacked discounts don't add up
When a shop offers "20% off, plus an extra 10% at checkout", the second discount is taken from the already reduced price. On a price of 100:
- After 20% off: 100 × 0.80 = 80.
- After a further 10% off: 80 × 0.90 = 72.
You pay 72, a total discount of 28%, not 30%. The combined discount is 1 − (1 − d1) × (1 − d2). The order of the two discounts doesn't change the result.
| Discounts | Looks like | Real total discount |
|---|---|---|
| 10% + 10% | 20% | 19% |
| 20% + 10% | 30% | 28% |
| 30% + 20% | 50% | 44% |
| 50% + 50% | 100% | 75% |
Two 50% discounts don't make an item free: you still pay a quarter of the price.
Adding sales tax or GST
In most places tax is charged on the price you actually pay, after discounts. With 25% off 80 and an 8% sales tax, the tax is 8% of 60, so you pay 64.80. Mathematically, taking the discount before or after the tax gives the same final figure. What matters is whether the tag price already includes tax.
- US and Canada: shelf prices usually exclude sales tax, so add it here.
- India, UK, EU, Australia: consumer prices usually include GST or VAT already. Leave the tax field empty. To split the tax out of a price, use the GST calculator or VAT calculator.
A larger example: a 2,499 item with 20% + 10% off costs 1,799.28 (you save 699.72). Adding 18% GST on top brings it to 2,123.15.
Percentage off vs a fixed amount off
A coupon for a fixed amount, such as 10 off, is worth more on a cheaper item. On a price of 40 it is 25% off; on a price of 200 it is only 5% off. To compare it with a percentage deal, enter both prices in the percent off calculator above. When a shop lets you combine a fixed coupon and a percentage discount, using the percentage first and the fixed amount second gives the lower price.
Smart shopping tips
- Compare the final price, not the percentage. 30% off an inflated "original" price can cost more than a smaller discount elsewhere.
- "Up to 70% off" usually applies to only a few items.
- "Buy one, get one 50% off" is 25% off across both items, and only if you wanted two.
- Include delivery charges before deciding which deal is better.
For other percentage questions, like percentage increase or what percent one number is of another, use the percentage calculator.
Frequently asked questions
How do I calculate a discount?
Is 20% off plus 10% off the same as 30% off?
How do I find the original price before a discount?
How do I work out the percent off from two prices?
Is tax charged before or after the discount?
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