I, like many other wet-behind-the-ears college students trying to make their way in the world, applied for and received my first credit card about a year and a half ago. At first, I wondered how on Earth my credit card company could make money off of someone like me. Sure, I used the card for practically everything: groceries, textbooks, clothes. But I paid off the balance religiously practically every month. So, they weren't making anything off of me, except for a few piddly cents of interest every year from the times when I actually carried a balance.

Then it hit me, just a few nights ago, as my boss explained the finer points of managing the massive overhead that is the liability of every restaurant: basically, credit card companies don't make money off their customers. Instead, they make money off merchants. Even if the customer is a goody-two-shoes like me, who almost never carries a balance, they still make money off of me, however indirectly.

You see, in order to accept credit cards as payment at their businesses, merchants have to open an account with the credit card company in question. For every transaction made by credit card, the merchant must pay a transaction fee, either a flat fee of a few cents, or a specific percentage of the charge (usually in the neighborhood of 3%). It's not unusual for both to be used simultaneously — shaogo kindly informed me that the credit card companies of which he is a client charge 15 cents across the board. On top of that, American Express charges him 3.5%, and Mastercard and Visa both charge him 1.8%.

So, to give you an example: I go to a gas station that accepts my particular credit card, and purchase $11.00 worth of goods. The credit card company charges the merchant, say, 30 cents for the transaction, and by that, makes the bulk of its income. Even if I carry the balance of the $11.00 over to the next month, only a wickedly usurious interest rate will match the transaction charge applied to the merchant's account.

This is often why many businesses (in violation of many card companies' contracts) require a minimum purchase to pay by credit card, and why some businesses have eschewed credit cards altogether; all those transaction fees add up, forcing businesses to raise prices (or absorb the loss). So, indirectly, despite my not being charged interest for my purchases, the credit card companies still make money off me.

Now, sometimes, credit cards will offer suspiciously good deals to customers, like cash back programs, interest grace periods, frequent flyer miles or 'points'. They can afford to do so, because they don't make the bulk of their money directly off of you, the consumer; they make most of their money by charging merchants for the privilege of using their cards. They can afford to give you a few dollars back for every couple of hundred you spend, because the transaction fees are more than enough to put them in the black.

Thanks to TenMinJoe for pointing out that my original example of a merchant charge of $3.00 was off by a factor of ten and suggesting a far more reasonable fee of 30 cents, or roughly 3% of the total transaction. Also, thanks to shaogo for pointing out that many credit card companies, such as American Express, specifically forbid their clients to require customers to make a minimum purchase if paying by credit.