Welcome to the Total Credit Card Interest Calculator API!

Did you know that about half of all credit card users carry a balance from month to month? (source) Little do they realize how much that can cost in the long run. With some store credit cards charging upwards of twenty percent, those small monthly payments can really add up.

That’s why I wrote this API. It calculates exactly how much total interest you’ll pay on your balance, given a certain monthly payment. It’s flexible enough that you can also see what happens when you only use a minimum percentage, without a dollar amount, and vice versa.

How to Use the API

The API is really easy to use. It has only 1 endpoint: a GET request so simple that even a browser can make it!

There are a total of 4 query parameters as follows:

• balance: the outstanding balance on the credit card. This parameter is required and can be any value that is standard currency from 1 to 25,000, such as “5000”, “5,000”, “5000.00”, or “$5000”.
• rate: the annual interest rate. This parameter is also required and can be any value from 1 to 100, with an optional one decimal place of precision. For example, “18”, “18.9”, and “18%” are all valid.
• minPaymentPercent: the minimum payment to make each month, expressed as a percentage from 0 to 100. Being a percentage, “2”, “2.9”, and “2%” are all valid.
• minPaymentDollars: The minimum payment to make each month, expressed as currency from 0 to 25,000. As currency, values of “25”, “1,000”, “25.00”, and “$50” are all valid.
• showMonthlyData: An optional flag that shows monthly calculations. It includes the Balance before Payment Applied, Interest accrued, the Portion Applied To Reducing the Balance, and a few more fields.

Both the minPaymentPercent and minPaymentDollars parameters are optional, but at least one must be provided. You can (and should!) include both.

Some Things to be Aware Of

The API imposes few constraints, which can yield some unusual results. For example, including a small Minimum Payment Percent can take many months to bring the balance down to zero. In fact, it’s the reason that credit cards have both a minimum percentage and dollar amount. The minimum payment is whichever is greater. Typically, the percentage kicks in first. Then, as the balance decreases, the dollar amount eventually determines the minimum payment.

As the developer of the API, I made a conscious decision to payoff the balance whenever the accrued interest falls below one penny. After all, what would be the point of making payments where the credit card company doesn’t make any interest?

You’ll also notice that, if the balance is paid off with the very first payment, there is no interest charged, just as with any credit card purchase.

Support

If you have any thoughts on possible new features, or find a bug, you can contact me at raylsstr(AT)gmail(DOT)com.

Enjoy the API!

Ray Lassiter