Credit card and effective interest rate

Suppose you own a credit card that charges an interest rate of 3.1% per month for revolving credit. What would be the yearly interest rate you end up paying the bank that have issued you the credit card? Is it 3.1 x 12 = 37.2%? Well, no!

Let’s see why.
Consider that you have made a purchase of Rs. 50,000 in your credit card having 3.1% monthly interest and have paid only 20,000 on the due date. The bank will take forward the remaining amount (30,000) to the next month's bill with an interest charge of Rs. 930 (3.1% of 30,000), making the total amount due to be Rs. 30,930.

Now suppose once again you couldn’t pay the entire amount and you paid only 20,000 out of the total due amount of 30,930. The bank will charge an interest of 3.1% on the remaining 10,930 (not 10,000). Thus the bank charges interest on the previous interest amount also or simply, the interest charged is compounded! Due to compounding, the effective annual interest rate will be higher than 3.1% x 12.

The effective annual interest rate, when monthly interest rate is quoted can be found out using the following method.

Effective annual rate = (1 + i/m)^m – 1
where i is the nominal yearly interest rate (3.1% x 12 = 37.2%) and m is the total number of compounding periods in a year (12, since monthly).

Effective annual rate = (1 + 0.372/12)^12 – 1 and that comes out to be 44.25% instead of 37.2%!

Think about a lender who charges 44.25% for the money that you borrow from him. That’s exactly the reason why we should keep our credit card spending to the minimum with absolutely no revolving credit.