Price per unit: how to compare properly
Two packs of the same product are almost never the same size. One is 750 grams, the other a kilo. Comparing their prices tells you nothing, because they are not the same amount.
The number that lets you compare is the price per unit: what a kilogram, a litre or 100 grams costs. It turns two different packs into two options you can actually choose between.
The rule, and its exception
This is the idea behind "buying big pays": a large pack is often cheaper per unit.
But not always. A promotion on a small pack can make it cheaper per kilo than the family pack next to it, and promotions on the big packs are the rarer kind. So look at the small number under the price instead of assuming.
The second sum: how much you will actually use
A pack that is cheaper per kilo but half of which ends up in the bin is no saving. For things that spoil, such as vegetables, dairy and bread, the real sum is the price per amount you will eat, not per amount you bought. For things that keep, like toilet paper or rice, there is no such problem and the big pack is fine.
Where this meets the comparison between chains
When you compare the same product across shops, the barcode already guarantees it is exactly the same pack, so the gap you see is real. The price per unit matters mostly within one shop, when you are choosing between different packs of the same thing.
Common questions
Where do I see the price per unit in the shop?
The law requires chains to mark a price on every product, and the shelf label also carries the price per unit, usually in small print under the main price.
Does it work for products sold by the piece?
Less well. In a six-pack it makes sense to compare the price per piece, but when pieces differ in size between brands you are back to weight or volume.
