How to calculate a discount (and spot a fake deal)
"30% off!" — but what do you actually pay? Discount math is simple once you have the two formulas, and knowing them helps you spot when a "deal" isn't one.
The two things people want to know
1. What's the sale price?
Multiply the original price by (100 − discount) ÷ 100.
$80 at 30% off = 80 × 70 ÷ 100 = $56. Quick mental version: 30% of 80 is 24; 80 − 24 = 56.
2. What percent off is this?
If you know the original and sale price, the discount is (original − sale) ÷ original × 100.
Was $80, now $56: (80 − 56) ÷ 80 × 100 = 30% off.
Stacked discounts don't add up
This is the one that fools people. "30% off, then an extra 20% off" is not 50% off. The second discount applies to the already-reduced price:
$100 → 30% off → $70 → 20% off → $56. That's 44% off total, not 50%.
Stacked percentages always come out to less than their sum, because each one is taken from a smaller base.
Retailer tricks to watch
- Inflated "original" prices — a higher list price makes the discount look bigger than the real saving.
- "Up to 70% off" — the headline number usually applies to a handful of items; most are discounted far less.
- Tax and shipping — a discount before a big shipping fee can be worse than a smaller discount with free shipping.
Don't forget tax
The advertised discount comes off the pre-tax price; sales tax or VAT is then added to the reduced amount. To see the true out-the-door number, apply the discount first, then add tax.