"70% OFF — TODAY ONLY!" Your brain hears a deal; your wallet should hear a math problem. Retailers design sales to feel urgent and generous, and the only defense is quick percentage math. Once you can compute a sale price in your head — and spot the tricks — you'll never overpay for the thrill of a red sticker again.
Start with the basics in our percentage guide, or jump straight to answers with Utilo's free percentage calculator.
The one formula: sale price
Sale price = original price × (1 − discount)
A $90 pair of shoes at 40% off: 90 × (1 − 0.40) = 90 × 0.60 = $54. Remember tax comes after the discount in most US states, so that $54 becomes about $58–59 at the register depending on your local rate.
Quick mental trick: find 10% (move the decimal), then scale. 30% off $90: 10% is $9, times 3 = $27 off, so $63. Takes five seconds once it clicks.
Stacked discounts multiply — they don't add
This is the #1 discount mistake. A store offers 30% off, plus an extra 20% coupon at checkout. That's not 50% off. Each discount applies to the already-reduced price:
0.70 × 0.80 = 0.56 → you pay 56%, saving 44%.
Retailers know most shoppers add the percentages. Now you know better: multiply the "keep" fractions. Two 25% discounts? 0.75 × 0.75 = 0.5625 → 43.75% off, not 50%.
BOGO deals decoded
- Buy one, get one free: 50% off the pair — but only if you actually wanted two.
- BOGO 50% off: on two equal-priced items, that's 25% off the total. The discount always applies to the cheaper item, so with unequal prices your savings shrink.
- "3 for $10": check the unit price. If singles are $3.50, the deal saves you $0.50 — worth it only if you'll use all three before they expire.
Spotting fake sales
The oldest trick in retail: inflate the "was" price, then "discount" back to normal. A jacket "was $200, now $99" might never have sold at $200. Defenses:
- Check price history. CamelCamelCamel for Amazon, Google Shopping's price graph, or Keepa show what an item really sold for.
- Compare unit prices. Shelf tags show price per ounce/count — the "family size" isn't always cheaper.
- Ignore the countdown timer. Artificial urgency is a sales tactic, not information. The same "deal" usually returns next month.
- Set a target price. Decide what an item is worth to you before shopping, and buy only below that number.
When a deal is genuinely good
Real bargains share traits: the discount applies to the out-the-door price (watch for excluded brands and "doorbuster" fine print), the item has a stable price history below the sale price, and you'd buy it at full price anyway. A 40% discount on something you don't need is 60% wasted money, not 40% saved. Run the numbers with Utilo's free tools before the timer pressures you.
Frequently asked questions
How do you calculate a sale price from a discount?
Multiply the original price by (1 − discount as a decimal). $80 at 30% off = 80 × 0.70 = $56, plus sales tax.
How do stacked discounts work?
They multiply, not add. A 30%-off sale plus a 20% coupon = 0.70 × 0.80 = pay 56%, i.e., 44% off total — not 50%.
Is buy-one-get-one-half-off the same as 25% off?
Only on two equal-priced items. The half-off applies to the cheaper item, so unequal prices change the effective discount.
How can you tell if a sale is fake?
Check price history (CamelCamelCamel, Google Shopping), compare unit prices, and ignore countdown timers. Many "sales" just restore the normal price.