Buying in bulk feels like saving money, and sometimes it is. But larger packages aren't always cheaper per ounce, and sales on smaller sizes can flip the math entirely. Without checking, you can end up paying more for the privilege of buying more.

The most useful number on the shelf tag is usually the unit price, which shows the cost per ounce, pound or count. Compare that figure across sizes and brands rather than looking only at the total price. If the store doesn't list it, the calculator on your phone can do the division in seconds.

Even when the bulk size is truly cheaper, it only saves money if you use it all. A giant bag of produce that spoils halfway through or a tub of sauce that expires in the back of the fridge can cost more than the small package would have. Buy large quantities of things you use steadily and that keep well.