Unit price calculator
Retailers rely on the assumption that it is. Roughly a quarter of the time the larger pack costs more per unit, especially when a smaller size is on promotion. Unit pricing on shelf labels exists precisely to expose that, and it is legally required in most of Europe, but only in the units the retailer chooses, which is why a comparison between a 500 g box priced per kilo and a 6-pack priced per item is still work you have to do yourself.
Unit price is the pack price divided by the quantity it contains. A 3.00 pack of 500 g is 0.006 per gram; a 5.00 pack of 1,000 g is 0.005. The larger pack is 16.7% cheaper per gram — but that is not always the way it falls.
How to compare unit prices
Unit price is necessary but not sufficient. A bigger pack that is cheaper per gram is only a saving if it gets used. Food that spoils at 30% remaining was not cheaper at all, it was 30% more expensive than the small pack. The same logic applies to bulk buying anything with a shelf life or a storage cost. The other trap is unit mismatch: comparing a price per kilo against a price per item is not a comparison, and supermarket shelf labels do it constantly.
Questions
Divide the pack price by the quantity. Use the same unit for both options or the comparison is meaningless.
No. Roughly a quarter of the time it is not, particularly when the smaller size is on promotion.
Convert one first. Comparing a price per kilo against a price per item tells you nothing.
It should. A pack that is a third cheaper per unit but a third goes off is exactly break-even at best.
Regulations require a unit price but often leave the unit to the retailer, so a per-kilo and a per-item label can sit side by side.