The Math Behind "70% Off": How Stores Manipulate Discount Numbers

📅 May 11, 2026 ⏱️ 7 min read ✍️ By Lu Shen
Shopping discount concept with percentage tags, calculator and retail elements

Last Black Friday, I almost bought a jacket that was "70% off." The original price tag said $300, and the sale price was $89. I felt like I was stealing it. Then I did the math and realized the discount was actually 70.3% — rounded down to look more impressive on the sign. But that's not even the worst trick stores pull.

Retailers have turned discount math into an art form of deception. The numbers they show you are designed to make you feel smart for "saving" money while you're actually spending more than you should. Let me break down the most common tricks.

Trick #1: The Fake Original Price

This is the foundation of almost every discount scam. The "original price" listed on the tag isn't necessarily what the item ever sold for — it's often an inflated number that makes the discount look huge.

Here's how it works: A store plans to sell a jacket for $89. But instead of just pricing it at $89, they create a fictitious "original price" of $300 and slap a "70% OFF" sign on it. You see "70% off" and your brain says "deal." You would have walked past the same jacket priced at $89 with no discount sign.

The FTC has rules against this — the "original price" is supposed to be a price the item was actually sold at for a reasonable period. But enforcement is weak, and many retailers stretch the definition of "reasonable period" to absurdity. I've seen "original prices" that the item was sold at for maybe two days six months ago.

Trick #2: Stacking Discounts That Don't Stack

"Take 40% off, then an additional 30% off!" Sounds like 70% off, right? Wrong.

When stores stack percentage discounts, they apply them sequentially, not additively:

Original price: $200
First discount (40% off): $200 × 0.60 = $120
Second discount (30% off): $120 × 0.70 = $84

You paid $84, which is 58% off — not 70%. The store got to advertise two big numbers, but you saved significantly less than you thought.

This trick is everywhere: "Buy one, get 50% off the second item" combined with "20% off your entire purchase." People estimate their savings by adding the percentages, but the real savings are always less because each discount applies to an already-reduced price.

Trick #3: The "Was/Now" Illusion

Some stores use a clever anchoring technique: they show the "was" price so prominently that you focus on the difference rather than whether the "now" price is actually good.

A TV "was" $1,200, "now" $599. You see $601 in savings and your brain stops calculating. But was that TV ever worth $1,200? A quick search shows the same model selling for $580 at three other stores. You're not saving $601 — you're paying $19 more than the best available price.

The discount number is a distraction. The only number that matters is: "Is this a good price for this item right now compared to other sellers?"

Trick #4: The Per-Unit Price Shell Game

Grocery stores are masters of this one. They'll show you a "price per ounce" or "price per unit" to make comparison shopping easier — except the units don't match between brands.

Brand A shows price per ounce. Brand B shows price per pound. Most people don't notice the unit difference and just compare the numbers directly. I've caught myself doing this multiple times.

Even worse: the "family size" or "bulk" option that costs more per unit than the regular size. It happens more often than you'd think. Always check the per-unit price yourself using a percentage calculator or basic math.

Trick #5: The Minimum Purchase Threshold

"Free shipping on orders over $75!" or "Get $20 off when you spend $100!"

These thresholds are carefully calculated. The store knows their average order is, say, $60. By setting the threshold at $75, they get you to add a $20 item you didn't want — and now you've spent $80 instead of $60. The "free shipping" cost them maybe $6, but they made an extra $20 in revenue.

The "$20 off $100" trick is even more insidious. If you were going to spend $75, you now add $25 of stuff you don't need to save $20. Net result: you spent $80 instead of $75. The store made $5 more, and you're stuck with items you didn't want.

The Compound Discount Trap

Here's one that gets people with store credit cards: "Save 15% today when you open a store card!"

On a $200 purchase, that's $30 saved. Nice. But the card has a 29.99% APR. If you carry a balance for just two months, you've already paid more in interest than you saved. And the data shows that store credit card holders carry balances at higher rates than regular credit card holders — the stores know this.

The discount isn't a gift. It's customer acquisition cost. They're paying $30 to get you onto a high-interest credit card that makes them far more over time.

How to Actually Calculate Your Real Savings

After years of falling for these tricks, I've developed a simple system:

  1. Ignore the original price — It's probably fake. Focus only on whether the current price is fair for the item.
  2. Check the final price, not the percentage — A 70% discount on an overpriced item might still cost more than a 20% discount at a fairly-priced competitor.
  3. Calculate stacked discounts correctly — Use a discount calculator to figure out what you'll actually pay after multiple discounts.
  4. Compare across stores — One minute of price comparison usually saves more than any single discount.
  5. Set a buying budget before you shop — Decide what you'll spend before you see the "deals." Thresholds and flash sales can't manipulate you if you've already decided your limit.

The Psychological Numbers Game

Why do stores use 70% instead of just showing a good price? Because of something called "anchoring." When you see "$300 crossed out" next to "$89," your brain anchors on $300 and evaluates $89 relative to that number. Without the anchor, $89 is just $89 — maybe good, maybe not.

Studies show that anchoring works even when people know about it. Knowing the trick doesn't fully immunize you against it. The best defense is to always calculate the actual price you'll pay and compare it to what you'd normally spend — not to the fake original price.

Quick Reference: Common Discount Math

Here are some discount combinations and their actual total discounts (not what the signs imply):

Next time you see a too-good-to-be-true discount, pull up a discount calculator or percentage calculator and run the real numbers. It takes 10 seconds and has saved me hundreds of dollars in impulse purchases I didn't actually need.