Traps & Dark Patterns
When two shelf labels refuse to use the same unit
Unit pricing only compares like with like when both labels measure in the same thing, and a surprising number of them do not.
Comparisons of inconsistent unit measures usually pick a winner. This one picks the circumstances, which is more useful.
The difference in one place
- Comparisons break when one label counts items and another counts weight.
- Concentrated and diluted products are not comparable by volume.
- Where units differ, converting by hand is the only reliable check.
Why unit pricing sometimes fails
A unit price is only useful when every product in the category is measured against the same denominator on the shelf. In practice you will find weight beside volume, items beside weight, and portions beside either of them.
Where the denominators differ, the numbers cannot be compared at all, however carefully each one has been calculated. A shopper comparing them anyway is doing arithmetic on incompatible quantities and reaching a confident wrong answer. The first step in any unit comparison is therefore checking that both labels are measuring the same kind of thing.
Where the mismatches cluster
Cleaning products are a common example, because concentrated and ready-to-use versions are both sold by volume. The concentrate delivers far more washes per unit of volume, so the comparison has to run on uses rather than on litres. Foods sold both loose and packaged frequently carry different denominators for reasons that are entirely practical.
Across a sale weekend, products with drained weights differ again, since the figure on the front and the amount you actually use are not the same. None of this requires bad intent to occur, and it occurs constantly in ordinary shops with ordinary labelling.
Counting the thing you actually consume
The most useful denominator is almost never the one on the label; it is the number of times you will use the product. Cost per wash, per portion, per application or per cup answers the question a household is actually asking.
Working that out requires the dose or portion size, which is usually printed somewhere on the packaging. One calculation per category, done once and written down, covers every future purchase you make in that category. This is the same reasoning behind unit pricing, applied to the unit that matters rather than the one that was convenient to print.
Where rules apply and where they do not
Many countries require unit pricing on certain categories, with rules about which unit must be used and how it is displayed. Those rules commonly cover groceries and household goods and often do not extend to other categories at all. Requirements about legibility, placement and consistency vary widely and are not always well enforced in practice.
Online listings are frequently outside the scope of such rules, or covered by different ones, depending on the jurisdiction.
The practical consequence is that you cannot assume a comparison has been made possible for you, especially online.
Multipacks and the arithmetic they invite
A multipack may show a unit price per pack while the single shows one per unit of weight, which invites a wrong comparison. Where the pack contains different sizes, an average unit price conceals a wide range within the pack itself. Promotional multipacks sometimes carry a worse rate than the singles beside them, which is easy to verify and easy to miss.
Checking takes seconds with a phone and it is the single most reliable saving available in an ordinary shop. The habit is worth building on the categories you buy every week and ignoring almost everywhere else.
Consumer protection rules are national, and what is unlawful in one market is routine in another.
A short routine that works
Read the denominator first, then the number, and only compare labels that share the same denominator. Where they differ, convert one of them by hand rather than guessing which product is likely to be better value. For anything dosed or portioned, work in uses rather than in weight or volume, because that is what you consume.
Write the resulting figure down for the handful of products your household buys constantly and stop recalculating. The point of the routine is to stop the label from choosing your denominator, which is what makes the comparison honest.
Side by side
| Consideration | What it means in practice |
|---|---|
| Why unit pricing sometimes fails | Comparisons break when one label counts items and another counts weight. |
| Where the mismatches cluster | Concentrated and diluted products are not comparable by volume. |
| Counting the thing you actually consume | Where units differ, converting by hand is the only reliable check. |
The takeaway
Check the denominator before you read the number, because two unit prices measured differently cannot be compared at all.
A discount is a claim about a price you were never asked to pay.
Questions readers ask
Why do shelf labels use different units for similar products?
Because rules cover some categories and units and not others, and because products are genuinely measured in different ways. Comparisons only work when the denominator matches.
What is the best unit to compare on?
The one you actually consume: cost per wash, portion, application or cup. That answers the household's question rather than the labelling requirement's.
Also by Rajat Oberoi
- Unit price is the only number on the shelf that compares like with likeShopping Tactics
- Price history turns a discount claim into something you can testShopping Tactics
- The same product under a different model numberShopping Tactics
- Splitting one purchase across two sellersShopping Tactics
