Many people rely heavily on credit cards to pay for necessities as well as extravagances. When the credit card payment is due, however, it can sometimes be a shock to see how much the minimum payment is. Knowing how to calculate that minimum payment yourself can not only reduce that shock, but can also help you to budget more accurately. There are two main ways credit card companies calculate payments: by the Office of the Comptroller of Currency's formula or by a percentage of the outstanding balance.

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Call your credit card company or read the back of your last statement to see whether your lender calculates minimum payments by the Office of the Comptroller of Currency's 2003 recommended formula or by percentage of the balance.

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Determine the amount of any transaction fees you've incurred during the month. Typically, transaction frees are placed on cash advances and balance transfers. Other fees may include fraud insurance or, for some credit cards, monthly membership fees.

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Confirm the APR (Annual Percentage Rate) on your card. Since the APR can change during the lifetime of a credit card, you'll probably need to check your last bill to make sure it's what you remember. This is especially true if you opened the account during a zero percent APR promotion, as the APR increases after 6 months to a year.

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Find the outstanding balance on your card. This is how much you currently owe, not your credit limit.

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Calculate the minimum payment on a credit card using the OCC's formula. First, multiply your outstanding balance by your APR. If, for example, you owe $1,500 on a card with 15 percent APR, the equation would be 1,500 x 0.15 = 225. Set this number aside to plug into the formula.

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Multiply your outstanding balance by 1 percent. In the same example, the equation would be 1,500 x 0.01 = 15.

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Add this number to the number obtained when you multiplied your balance by your APR. Then add the sum to your transaction fees. This is your minimum payment. So if you owed $20 in transaction fees, the equation would look like this: $225 (APR times outstanding balance) + $15 (1 percent times outstanding balance] + $20 (transaction fees) = $260 (minimum payment).

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Calculate the minimum payment on your credit card based on the lender's percentage rate if they do not use the OCC formula. In this case you will need to confirm, either by reading a current statement or by contacting the issuer, what percentage of the balance is use to determine payment. It's usually between 2 and 4 percent.

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Multiply your balance by that percentage. For example, a $1,500 balance at 4 percent would be 1,500 x 0.04 = 60.

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Note that a good portion of that payment is going toward interest. You can calculate how much by dividing your APR by 12 (for months) and multiplying it by your outstanding balance. So the interest payment on a card with a $1,500 balance and a 15 percent APR would be $18.75. That means that of your $60 payment, only $41.25 of it is helping to reduce your balance.