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.