You're standing in the cereal aisle looking for the best deal on a box for breakfast. With so many sizes, packages, and competing brands to choose from, which do you buy? If you're like most people you grab the 'Value Size' – or the biggest box – because buying in bulk is always a deal, right? Maybe not.

Comparing prices at the grocery store can be tricky. Whether you're shopping for food, shampoo, or laundry detergent there's an easy way to see which brand and size really saves you the most money.

It's called unit pricing, and it's a shopping trick you need in your life. In many stores, an item's unit price is printed next to the product price on the shelf label, and gives you the cost of the product for a standard unit of measure – for example, per ounce or per 100 grams. Having a price to compare between different sizes helps you choose the better deal without being swayed by familiar brands or nice packaging.

So let's say you want to buy some yogurt. Good, dairy is delicious. Now which is the better price: the big 24-ounce tub of yogurt for $3.79 or the smaller 16-ounce container for $2.19?

This is tricky to answer if the unit price isn’t listed on the price label. So here's the money-saving equation for finding the unit price and how to use it:

TOTAL PRICE ÷ SIZE = UNIT PRICE

Unit pricing takes the price of the item ($2.19 for a 16oz container of yogurt) and divides it by its size (for example, 16 ounces) and then gives you a price comparison ($2.19 divided by 16oz = $0.14 per ounce). You can now use the unit price to choose the better deal. Is the larger yogurt tub a better price per ounce? When you do the math ($3.79 divided by 24 ounces = $0.16 per ounce), you can see that the answer is nope! You'll save two cents per ounce by buying the smaller container.

Now, two cents per ounce on a small tub of dairy doesn't sound like a lot, and it's not. But over time you'll notice similar products with 50-cent unit price differences, sale items that are more expensive than the value-sized option, and brand name products that retail for less than generic or store brand products.

A few unit price gotchas to watch for:
  • The biggest jar is not always the best deal. Watch for it!

  • Many retailers use the same base units between brands for easy price comparison, but that's not always the case! Check shelf labels to see if the unit price lists the price per 100g or something else.

  • Sale items boasting special discount labels don’t always reveal the unit pricing. So you'll need to do the math yourself.

Bottom Line: By taking a quick peek at a product's unit price you'll find the better value and save considerable cash by being a more informed consumer.

Kerry K. Taylor is a consumer expert at Squawkfox.com.