➗ Math
How to Calculate a Discount (Including Stacked Deals)
Working out a sale price is simple for one discount and sneaky for two. Learn how to calculate a percentage off, why stacked discounts don't add up, and how to find the original price.
Updated 24 August 2026 · 2 min read
"30% off" sounds simple until there's a second discount on top, or you're trying to reverse-engineer the original price from a sale sticker. The single-discount case is easy; the traps are in the extensions. Here's how to handle all of them.
A single discount
To take a percentage off a price, work out the discount and subtract it — or, faster, multiply by what's left:
Multiplying by 0.70 in one step is quicker than finding 30% and subtracting. The discount calculator shows both the price you pay and the amount you save.
The stacked-discount trap
This is where people lose money in their heads. "30% off, then an extra 20% off" is not 50% off. The second discount applies to the already-reduced price, not the original:
The general rule: multiply the "what's left" factors together. 30% off then 20% off is 0.70 × 0.80 = 0.56, i.e. 44% off. The discount calculator has a field for the extra discount so it does the stacking correctly — the same principle we cover in the percentages guide.
Finding the original price
Sometimes you know the sale price and want the original — to judge whether a "deal" is real. Reverse the multiplication by dividing:
Discount vs the everyday percentage
A discount is just a percentage decrease, so everything here is a special case of general percentage maths. If you find yourself doing these often — shopping, invoicing, quoting — the discount calculator handles single and stacked deals, and the percentage calculator covers the rest.
Frequently asked questions
- Is 30% off then 20% off the same as 50% off?
- No. The second discount applies to the already-reduced price, so 30% off then 20% off works out to 44% off, not 50%. Multiply the remaining fractions (0.70 × 0.80 = 0.56) to get the true combined discount.
- How do I find the original price before a discount?
- Divide the sale price by one minus the discount as a decimal. If something costs £45 after 25% off, the original was 45 ÷ 0.75 = £60. This reverses the discount instead of applying it.