Web app The Real Damage calculates the true price of adding another purchase to a credit card that's already carrying a balance.
The best way to avoid throwing away money on credit card interest is simply to not carry a balance. (A good portion of Lifehacker readers pay off their cards every month.) But if you're in the process of digging out from credit card debt, it can still be tempting to put the occasional purchase on the card, particularly if it's not that expensive. The catch, of course, is that if you take the added interest that accrues on the increased balance of your debt into account, over time that price quickly grows — and that's the dollar amount The Real Damage focuses on.
Plug in the price tag of an item you're interested in, along with the APR, balance and your current monthly payment on your credit card, and it'll figure out the true damage. Once you've got a more accurate picture of the price, then you can decide if it's worth it.