You stand in front of the same milk at the store. One is BOGO (buy one, get one), one is 20% off, one is a 2L bulk pack. Your mind goes blank for a second. Which is actually cheapest? If this question trips you up every time, the problem isn't your arithmetic — it's the basis of comparison. You're comparing apples to oranges. The fix is one move: convert every price to the same ruler, the unit price (per 100g / 100ml / each).

This piece is hands-on math, not psychology. Why we're weak to discount signs is covered in Shopping Psychology and Price Perception; here we only look at "so how do you calculate it?" Note that every unit figure below is an illustrative made-up number.

Unit price — unify into per 100g / 100ml / each

Unit price is simple: price ÷ quantity. That's it. A 1,000-won, 200g snack has a unit price of 1,000 ÷ 200 = 500 won per 100g. A 1,500-won, 350g snack is 1,500 ÷ 350 ≈ 429 won per 100g. The second looks pricier by sticker, but per 100g it's cheaper. Even with different pack sizes, bundles, and discount rates, line them up by the single unit price and it becomes an apple-to-apple comparison.

The key is escaping the "total price" trap. Our eyes react first to the big number (2,000 vs 3,000 won), but the real comparison is "how much for the same quantity." Unit price answers exactly that.

Measuring BOGO and % off with the same ruler

Now line up BOGO, the discount, and bulk in one row. Say a 1L carton of milk normally costs 2,000 won (illustrative).

Laid out as a table, the answer is clear. By per-1L price: BOGO (1,000) < 20% off (1,600) < bulk (1,700) < regular (2,000). BOGO wins by a mile. But there's a trap: BOGO is cheapest only on the assumption you use all 2L. If you can only drink 1L before it expires, you have to divide by the wasted amount too, and the real unit price climbs back to 2,000. Remember: unit price assumes you use it all.

The "bulk is always cheaper" trap

A common belief is "buy big, save big." Usually true, not always. Even in the table above, the 2L bulk (1,700) had a higher unit price than the 20%-off 1L (1,600). Stores sometimes exploit the belief that "bulk must be cheaper" by nudging the unit price of the bulk pack up. So grabbing the big pack without checking the unit price means paying more while only feeling like you saved.

There's a reverse trap too. Pick bulk purely on unit price, fail to use it, and throw it out — and no matter how cheap the per-unit figure, you've lost. The cheapest-unit-price choice and the right-for-my-consumption choice can differ. Read both together.

Shrinkflation — catch it with unit price

Keeping the price the same while shrinking the quantity is called shrinkflation. If a snack bag's tag stays 1,500 won while the content drops from 200g to 180g, anyone reading only the sticker feels no hike. But by unit price it went from 750 won per 100g to about 833 — roughly an 11% increase (illustrative). The sticker sits still while the unit price honestly reveals the hike.

So the easiest weapon against shrinkflation is unit price. Instead of the big number on the tag, glance habitually at the small "○○ won per 100g," and you won't miss a hike disguised as a smaller package.

How to read the in-store unit-price label

Helpfully, to cut down on your own math, big-store tags often print the unit price too. That's thanks to the price-labeling rule (unit-price display), designed so consumers can easily compare differently sized products. On the tag, next to or below the large selling price, you'll find small text like "○○ won per 100g" or "○○ won per 100ml."

Reading it is simple. (1) First check whether the items use the same unit (one may be per 100g, another per each, which can confuse). (2) If the unit matches, just compare the small numbers. (3) If the units differ, recompute to a common unit. The intent and scope of the rule are in guidance from the Ministry of Trade, Industry and Energy and the Fair Trade Commission (see Sources).

✍️ Operator's note — The thing I saw most while building price-guessing games is that people get swayed by the "total price." 3,000 looks pricier than 2,000, so they grab the 2,000 — yet divide by quantity and the 3,000 one is often cheaper. At the store I look at the small "per 100g" in the corner before the big number. Just carrying unit price puts BOGO, discounts, and bulk ads all on the same ruler. Ads mix apples and oranges; unit price is the tool that turns them back into apple-to-apple.

At a glance

• Unit price = price ÷ quantity. Unify into per 100g/100ml/each and every offer lines up in one row.
• BOGO is cheapest only if you use it all. Waste it and the per-unit cost climbs back.
• Bulk ≠ always cheaper. Always check the unit price, and weigh it against your own consumption.
• Shrinkflation is caught by unit price. Watch "per 100g," not the sticker.
• Use the in-store unit-price label, but first confirm the units match.

Calculation becomes instinct — guess for yourself

Unit-price math feels fiddly at first, but after a few tries the per-unit cost jumps out at you. Use PriceGuess's Shopping Price Quiz to gauge a product's fair price, and the AI Price Sense Battle to quickly judge which of two is truly cheaper. For why the sticker differs from the final charge, read Why the Final Price Differs From the Sticker, and for how grocery prices split from perception, continue to Why Your Grocery Bill Feels Higher Than the Official Index.

Every unit figure in this article is a made-up number to show the method, not a real market price. For the concrete scope of the unit-price labeling rule, check official guidance.