If you carry a cell phone, you also have a calculator. Pretty much every mobile phone on the market has one, so you have no excuse not to do some intelligent per-unit math while you're out shopping.

Companies love that their customers assume economy of scale, and they want you to assume that buying more stuff means you're getting a better deal for it. Too often, though, it's not true.

This summer, WalletPop made the papers when we wrote about the "great McNugget caper," which proved that McDonald's popular bird chunks can actually cost more if you buy them in bulk.

The photo above is another case of a store that actually gives the shaft to customers who buy more. We were at a Sports Authority and spotted these hand warmers sitting side-by-side at wildly different rates: $1.79 for two (or 90¢ each) or $14.99 for 10 (or $1.50 each -- 60 cents more per unit). The larger package has a seal screaming "Great Value!" but in fact, the great value is for the Sports Authority if you neglect to actually do the math.

Most supermarkets list a "per-unit" price along the shelf that gives you a clue to how much you're actually paying for quantity of product that you're buying. If you're like me, though, you don't completely trust numbers offered by someone with an interest in getting you to buy something.

That's where your cell phone comes in. Take it out and open its calculator. When you're comparing two sizes of the same product, divide the full price by the number of ounces or units in each package to find out the true rate you're paying.

You've had the tool in your pocket all the time.

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