Bigger doesn't always mean cheaper per unit. Enter two options, even in different units, and see the real price per gram, per litre, or per item.
Supermarket shelf labels do show unit prices - but not on the same basis. One product's label reads per 100g, the one next to it per kilogram, a third per item, and imported goods sometimes per pound. Comparing "1.20 per 100g" against "9.80 per kg" in your head, mid-aisle, is exactly the kind of arithmetic that goes wrong. This tool converts both options to the same base unit first (grams for weight, millilitres for volume), so the comparison is real. It also refuses to compare across categories - a price per gram against a price per litre isn't a comparison, and pretending otherwise would just be a confident wrong answer.
Bulk sizes are usually cheaper per unit, which is exactly why the exceptions catch people: promotions on small sizes, "family packs" priced above two singles, and shrinkflation - where the pack shrinks but the price doesn't - all flip the usual rule. The per-unit number is the only honest comparison. One honest limit in the other direction: the cheaper-per-unit option is only the better buy if you'll actually use it before it expires. A 3L jug you pour half of down the drain cost more per used litre than the small bottle would have.