Is a 3-for-2 Deal Better Than a Discount on the Second Unit? The Supermarket Math

Walk a Spanish supermarket aisle and you'll see a "3x2" tag (buy 3, pay for 2) right next to one offering "the second unit at 70% off" — and it's tempting to just grab whichever one has the bigger number on it. That instinct is exactly what the tag is designed to trigger; the number that actually matters is hiding one step behind it.

The number that actually matters: price per unit

Instead of comparing taglines, there's one calculation that always works: divide the total you'll pay by the number of units you're getting. That per-unit price is directly comparable across offers, brands, and pack sizes — the percentage on the tag isn't.

This isn't a comparison trick we invented: Spanish law (Real Decreto 3423/2000) requires shops to display the price per unit of measure (€/kg, €/L...) next to the sale price, specifically so shoppers can compare offers without doing the math themselves. Look for that small print on the shelf tag before you trust the big number on the promotion.

Doing the math: 3-for-2 vs. 70% off the second unit

By this math, "second unit at 70% off" (a real 35% discount per unit) edges out the 3-for-2 (a real 33.3%) — and only requires buying 2 units instead of 3. But notice the gap between the tag and what you actually save: a "70%" that sounds dramatic on paper turns into a 35% real discount once it's spread across both units.

Why the tag on the shelf can mislead you

The "70%" on the tag applies only to the second unit, not to the whole purchase. Plenty of people read "70% off" and assume the entire purchase is 70% cheaper, when the real discount, spread across both units, is 35%. That gap between what the tag says and what you're actually paying on average is the part of the trick worth keeping in mind — it isn't false information, it's incomplete information.

Practical habits to avoid the trap

• Always divide the total price by the number of units (or just check the price-per-unit-of-measure figure already required on the label) before trusting the percentage on the tag.
• Ask yourself whether you'll actually use all the units before they expire or you stop needing them — a 3-for-2 on something perishable that you end up throwing out isn't a saving, it's a disguised expense.
• Compare price per unit across brands, not the advertised percentage — two "second unit at 70% off" tags from different brands can start from very different list prices.

Why supermarkets run these deals

It isn't just seasonal generosity. Two specific reasons explain most of these tags:

• Encouraging a buying habit. The more you take home at once, the faster you consume it, and the sooner you're back needing (and buying) that same product or brand again — the deal builds repeat purchasing, not just today's unit sales.
• Releasing stock that has a cost. Inventory takes up shelf space, ties up capital, and — for perishable goods — carries a real risk of expiring unsold. It's usually more profitable for the store to sell now at a discount than to write off expired stock later. That's the same logic — holding something has a real cost even when it never sends you a monthly bill — that we cover from the buyer's side in our total cost of ownership guide: here, the supermarket is the one "owning" the stock, and it's expensive for them to leave it sitting too.

Quick answers

Is 3-for-2 always worse than a discount on the second unit? Not necessarily — it depends on the exact percentage of the second-unit offer. As a quick reference: a 3-for-2 works out to roughly a 33% real discount per unit; to check whether a "second unit at X% off" beats it, run the per-unit math for that specific case rather than comparing the two percentages directly.

Why does the tag advertise "70%" instead of the real 35%? Because 70% is the more eye-catching number, even though it's technically accurate (it only applies to the second unit). It isn't false information, it's incomplete information — which is exactly why the law also requires showing the price per unit of measure, the number that can't be dressed up.

Do these deals make sense if I live alone? It depends on whether you can genuinely use the extra units before they lose value. A saving per unit is worthless if you end up throwing out whatever's left over.

This article offers general, educational information about how to compare per-unit offers and does not constitute financial advice. The effective discount percentages are calculated generically and may not match a specific retailer's exact pricing policy — always run the numbers with the real prices of whatever you're comparing.