Comparing grocery prices is genuinely difficult because pack sizes vary continuously. The unit price on the shelf label solves it, and it is printed small and frequently in inconsistent units.
What unit pricing is
The price expressed per standard measure — per kilogram, per litre, per hundred grams, per item.
Which allows two packs of different sizes to be compared directly, and it is the only number on the label that permits that.
Many jurisdictions require it to be displayed, with rules on prominence and on which unit to use.
Compliance and legibility vary, and consumer organisations have repeatedly found errors and inconsistencies in practice.
The inconsistent unit problem
The most common obstacle.
Two comparable products displayed per hundred grams and per kilogram require mental arithmetic to compare, which is exactly the friction unit pricing was meant to remove.
The same happens with per item and per weight for products sold both ways.
Where rules specify a consistent unit within a category this is addressed, and enforcement is patchy.
The practical response is to do the conversion, which is a factor of ten and is easy once you notice it is needed.
The larger pack assumption
The belief that bulk is cheaper, which is frequently but not reliably true.
Larger packs are commonly better value and are not always, particularly where a smaller size is on promotion.
Multibuy offers can be worse per unit than the single item price, which sounds impossible and occurs regularly and has been documented by consumer groups.
Which is why checking rather than assuming is the whole discipline, and it takes seconds once you are looking at the right number.
Shrinkflation
The practice of reducing pack size while holding the price.
The effect is a price increase that does not appear as one, and it is widespread and well documented.
Unit pricing makes it visible, since the per-unit figure rises even though the shelf price has not.
Some jurisdictions have introduced requirements to disclose size reductions on packaging for a period, which is a direct response and is not widespread.
The practical detection is noticing the unit price rather than the pack price, which is the same discipline that solves everything else here.
The promotional structures
Worth understanding since they are designed to be difficult to evaluate.
Loyalty pricing, where a lower price requires a card, which is straightforward and produces a two-tier shelf.
Multibuy offers requiring a quantity, which are only a saving if you would have bought the quantity.
Percentage extra free, which is a unit price reduction and is straightforward once converted.
And time-limited price reductions, which are genuine and are the category where price history matters least since perishables cannot be stockpiled.
Where the savings actually are
Larger than unit price optimisation, in practice.
Waste, which for many households is a substantial proportion of the grocery bill and which no amount of price comparison addresses.
Buying to a list rather than browsing, which reduces the unplanned purchases that store layout is designed to produce.
Own-brand substitution, where the products are frequently made in the same facilities and the price difference is substantial.
And shopping less frequently, since each visit produces unplanned additions.
The layout point
Worth naming since it shapes what you buy.
Product placement in shops is a paid arrangement in many cases, and eye-level positioning is commercially allocated rather than reflecting value.
Higher and lower shelves frequently carry better unit prices, which is a straightforward consequence.
Looking up and down rather than straight ahead is a small habit with a measurable effect, and it is the physical version of reading the unit price.
The practical routine
Read the unit price rather than the shelf price.
Convert when units differ.
Check the single price against the multibuy.
Look at the shelf above and below.
And buy from a list, which addresses more of the total than everything else combined.
Online grocery
Where unit pricing is generally better presented and other problems appear.
Most online grocery interfaces display unit prices consistently and allow sorting by them, which is a genuine improvement on a shelf.
The offsetting problems are substitutions, where an unavailable item is replaced with something at a different price, and the tendency of a saved basket to perpetuate purchases nobody has reconsidered.
Reviewing a repeat order rather than accepting it is the equivalent of shopping to a list, and it is skipped for the same reason.
Loyalty schemes and the data
Worth a note since two-tier pricing has become widespread.
Member-only prices are now common enough that the shelf price is effectively the price for people not participating, which has attracted regulatory interest in some jurisdictions.
What is being exchanged is purchase data, which is valuable and is used for targeted offers and sold in aggregate form.
Whether that is a reasonable trade is a personal judgement, and it is worth making deliberately rather than assuming the card is simply a discount.