Supermarkets are full of choices that look like bargains: bigger boxes, multipacks, "value" sizes. But the only way to know which is truly cheaper is to compare the price per unit — the cost of a single gram, litre or item, stripped of the packaging and the marketing.
What "price per unit" means
Price per unit is simply the total price divided by the total quantity. A 500 g jar at $8.99 works out to about $18 per kilogram. A 1 kg jar at $14.99 is about $15 per kilogram. Same product, and the bigger jar is genuinely cheaper per kilogram — but you only know that once you do the division.
Why bigger is not always cheaper
Retailers know shoppers assume the large size is the better deal, and sometimes price accordingly. It is common to find the medium size cheaper per unit than the large, or a multipack that costs more per item than buying singles on offer. The unit price is the great equaliser — it ignores the box and compares the contents.
Watch the units
The most common mistake is comparing different units — grams against kilograms, millilitres against litres. Always convert everything to the same base first. 500 g and 1 kg only compare fairly once both are in the same unit. Shelf labels sometimes help by showing a "price per 100 g," but not always, and not consistently.
Real examples
Two coffees: a 200 g bag at $6 is $30/kg; a 500 g bag at $13 is $26/kg — the bigger bag wins. But two cereals: a 750 g box at $4.50 is $6/kg, while the "family" 1.2 kg box at $7.80 is $6.50/kg — here the smaller box is actually the better buy. You cannot tell without checking.
When the bigger pack really is worse
Even when the large size is cheaper per unit, it is only a saving if you use it all. Buying a huge pack of something perishable that you throw half away is more expensive than the small size, whatever the unit price says. Best value assumes you actually consume it.
Quick shopping tips
- Compare price per unit, not price per package.
- Convert to the same unit before comparing.
- Check whether an on-offer small size beats the everyday large size.
- Factor in whether you will use it before it spoils.