Sure, you pay the finance charge on your bill, but do you understand how the credit card company arrives at the number?

Check out four of the most common calculation methods. But be warned: the method alone only tells part of the story. To make sure you’re getting the best deal on a card, ask how the company calculates the charges and whether interest is calculated on a daily or monthly basis. Find out if there is a grace period for new purchases. And have the company rep explain when and how your monthly payments are applied.

Calculation methods

Type of balance:  Average daily balanceHow it’s calculated:  The company averages your daily balance. For instance, if you charged $100 on the first day of June and charged an additional $200 on the 16th, your average daily balance would be $200. That number times roughly one-twelfth your annual percentage rate, or APR, equals your monthly finance charge. Interest may be calculated on a daily or monthly basis.

Type of balance:   Daily balanceHow it’s calculated:   The company calculates the actual balance you carried each day of your billing cycle and multiplies it by roughly 1/365th of your APR and adds it together.

Type of balance:  Two-cycle balanceHow it’s calculated:  A credit card practice where the consumer is charged interest on debt already paid. Here’s how it works: A cardholder begins a billing cycle with a zero blance and charges $500 on a credit card. They make an on-time payment of $450. With double-cycle billing, they would be charged interest on the $500 — instead of the $50 still owed — in the next billing cycle.

Type of balance:   Previous balanceHow it’s calculated:   The bill will show beginning balance and ending balance for your account. The finance charge is based on the outstanding balance at the beginning of the billing cycle.