Comparing on unit price, not package price
The only reliable way to compare products of different sizes is to reduce both to a common unit — price per 100 grams, per litre, per item. Shelf labels in many jurisdictions are legally required to display this, but the unit chosen is not always consistent between neighbouring products, and comparing price per 100 g against price per kilogram at a glance is exactly the error the requirement was meant to prevent.
Packaging is designed to obstruct this comparison. Sizes are chosen so that no two products divide neatly, and multipacks are priced to appear cheaper without necessarily being so. Working in a single unit removes the ambiguity in a few seconds.
The assumption worth checking is that the products are actually comparable. Drained weight against gross weight for tinned goods, concentrate against ready-to-drink, and dry weight against cooked weight are all cases where a unit price comparison is arithmetically correct and practically meaningless.
Bigger is not automatically cheaper
The belief that the largest package always offers the best unit price is widespread and frequently wrong. Studies of supermarket pricing repeatedly find a substantial minority of cases where the larger size costs more per unit, particularly on promoted small sizes and on premium ranges.
Promotions invert the ranking routinely. A multibuy or a temporary reduction on a medium size regularly beats the large one, and the shelf label's unit price may not reflect the promotion at all. The largest size is also where shrinkflation tends to be applied first — a reduction in package contents at an unchanged price, which shows up in unit price immediately and in the headline price not at all.
Buying larger only saves money if you use it. For perishables, a larger pack at a lower unit price that is partly thrown away is more expensive per unit consumed, which is the figure that actually matters. Storage space and the cash tied up in a bulk purchase belong in the calculation too.
Where unit price stops being the right measure
Unit price compares quantity, not value, and the two diverge whenever quality differs. Concentrated products are the clearest case: a detergent at twice the price per litre that requires half the dose is identical per wash, and the meaningful unit is the wash rather than the litre.
The same applies wherever a product's effectiveness varies — cleaning products, coffee, spices, batteries rated in different capacities. Where possible, compare per use rather than per unit of mass or volume, which sometimes requires reading the dosage instructions rather than the label.
Nutritional differences matter for food in the same way. A cheaper product per 100 g that is substantially water, filler or sugar is not the better value on any measure other than mass. Unit price is a tool for comparing like with like, and its usefulness depends entirely on whether the things being compared genuinely are alike.