Sales signs can be confusing. "30% off", "extra 10% at the till", "buy one get one half price". Knowing how to turn those offers into a real price helps you decide whether a deal is genuinely good. Here is the maths in plain steps.
The basic discount formula
Final price = original price × (1 − discount ÷ 100). A jacket costs 80 and is 30% off. 80 × 0.70 = 56, so you pay 56 and save 24.
Stacked discounts
When a shop gives a second discount, it applies to the price after the first one. For 20% off followed by an extra 10% off a 100 item: 100 × 0.80 × 0.90 = 72. That is a total saving of 28%, not 30%. The order does not change the result, but adding the percentages overstates the saving.
Buy-one-get-one deals
- Buy one, get one free is the same as 50% off when you buy two.
- Buy two, get one free is about 33% off when you buy three.
- Buy one, get one half price is 25% off when you buy two.
These only save money if you really need the extra items.
Finding the original price
If you only know the sale price and the discount, divide: original = sale price ÷ (1 − discount ÷ 100). A coat is 150 after 25% off. 150 ÷ 0.75 = 200, so the original price was 200.
Adding tax or VAT
In many places tax is added after the discount. If the discounted price is 72 and the tax rate is 8%, tax is 72 × 0.08 = 5.76 and you pay 77.76. Always check whether a displayed price already includes tax.
How to judge whether a deal is good
- Compare the sale price with other stores, not only the "was" price on the label.
- Add delivery fees, which can cancel a discount.
- Ask whether you would buy it at full price. If not, you are spending, not saving.
- Check return rules before buying heavily discounted items.
Use our free Discount Calculator to see the final price with a second discount and tax. For other percentage questions, the Percentage Calculator shows the formula step by step.
Frequently asked questions
Is 50% off then 50% off the same as free?
No. Two 50% discounts leave you paying 25% of the original price.
Which is better, 20% off or 20 off?
It depends on the price. On an item over 100, 20% saves more than a fixed 20. Under 100, the fixed amount saves more.
This guide is for general information only. See our disclaimer.