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Assuming I have a $1000 balance on a credit card with a 12% APR how can I calculate my total interest payments if I pay $300 a month? All the online calculators I've found are geared toward showing the interest if you make the minimum payments. Given an existing balance and interest rate I'd like to plug in numbers to see how total interest cost will very with monthly payments. An online calculator will work but I'd like to know the math behind this as well.

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At the end of each period, add the interest, in this case an easy 1%, and then subtract the payment. With less than 4 months to payoff, the interest here is about $21. Instead of trying to find credit card calculators, just use the more common mortgage calculator. The math is the same until the final month, when the credit card may handle accrued interest slightly differently.

Edit - A finance calculator indicates 3.407 payments, or total payment of $1022.12, $22.12 is interest. (from my initial guess of $21 above)

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