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.
What "price per unit" means
Price per unit is the total price divided by the total quantity. A 500 g jar at $8.99 is about $18 per kilogram. A 1 kg jar at $14.99 is about $15 per kilogram — the bigger jar is genuinely cheaper, but you only know once you divide.
Why bigger is not always cheaper
Retailers know shoppers assume the large size is the better deal, and sometimes price accordingly. The medium size is often cheaper per unit than the large. The unit price ignores the box and compares the contents.
Watch the units
The most common mistake is comparing grams against kilograms, or millilitres against litres. Always convert everything to the same base first.
Real examples
Two coffees: a 200 g bag at $6 is $30/kg; a 500 g bag at $13 is $26/kg — the bigger wins. But cereals: a 750 g box at $4.50 is $6/kg, while a 1.2 kg box at $7.80 is $6.50/kg — here the smaller box is better.
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 you throw half away is more expensive than the small size.
Quick shopping tips
- Compare price per unit, not per package.
- Convert to the same unit first.
- Check whether an on-offer small size beats the everyday large size.