Unit price: the number supermarkets would rather you ignored
Bigger packs are not always cheaper per unit. Here is how to compare sizes in seconds, and the packaging tactics that make comparison deliberately hard.
Two packs of the same coffee sit side by side. One is 400 g for $3.50, the other 750 g for $6.00. The larger looks like the sensible buy, and this time it is — but only just, and not for the reason most people assume. Working it out takes one division, and the habit pays for itself weekly.
One division settles it
Divide price by quantity and compare the results in the same unit:
| Pack | Price | Price per kg | Verdict |
|---|---|---|---|
| 400 g | $3.50 | $8.75 | — |
| 750 g | $6.00 | $8.00 | Better by 8.6% |
The bigger pack saves 75 cents per kilogram — real, but far less than the packaging implies. And it only works out if you actually use 750 g before it goes stale, which is the assumption bulk pricing quietly depends on.
The larger size is not automatically cheaper
This is the myth worth abandoning. Retailers know shoppers assume bulk means value, and they price accordingly. A 1.5 L bottle at $2.40 works out at $1.60 per litre; a 500 ml at $1.20 is $2.40 per litre — so the big one wins there. But reverse those prices slightly and the small pack takes it, with nothing on the shelf signalling the change.
Promotions muddy it further. "3 for $10" on an item normally $3.50 saves 4.8%, not the third the framing suggests. And a multi-buy on a size you would not otherwise choose can leave you paying more per unit than the plain single would have.
Where comparison is made deliberately hard
- Mismatched units. One brand priced per 100 g, the next per kg, a third per item. Converting is the only way to compare, which is exactly the friction intended.
- Odd pack sizes. 340 g against 400 g against 750 g. Nothing divides cleanly, so mental arithmetic fails and most people give up.
- Shrinkflation. The pack looks identical and the price has not moved, but the contents fell from 200 g to 180 g. That 10% less product is an 11.1% rise in unit price — invisible unless you are tracking it.
- Bundles. Boxes of individually wrapped portions often cost several times the unit price of the same product bought loose.
When the cheaper unit price is the wrong choice
Unit price answers one question well and should not be asked to answer others. Buying 5 kg of something you use twice a year is not a saving if half of it spoils — the effective unit price includes what you throw away. Storage space, freshness after opening, and whether you will genuinely get through it all belong in the decision.
The rule that holds up: compare unit prices to find the best value among options you would actually buy, then let practicality decide. The arithmetic tells you which is cheaper per gram; only you know which one gets used.
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