Need Help Calculating Credit Card Payments

Question

Okay, let's say that I owe my credit card company $2000.00, and they're charging me 20% APR, based on my average daily balance. If I send them $30/week, then how many weeks will it take me to pay them off?
It seems like there should be a Financial Function for stuff like this, but if there is, I can't figure out which one to use or how it would work. I'm currently doing something like this:
=(Current Balances :: Balance Chase*(1+Current Balances :: APR Chase/365)^7)-30
… to figure out what I still owe at the end of the first week, then then doing a Fill Down with this formula:
=(A2*(1+Current Balances :: $APR $Chase/365)^7)-30
Then I do a:
=COUNTIF(Payment Schedule :: Chase,">50")
… figuring that if the balance is below $50, I'll just pay it off.
Anyway, not only is this an incredibly tedious process, and probably wildly inaccurate, it's also hard to work with it in the rest of my spreadsheet. Isn't there some Function that will figure this out for me? It doesn't have to be super accurate, as long as it can give me a ballpark idea of when I'll be paid off.
Thanks in advance,
surfer

Answer 1

Surfer,
There's a handy financial formula that was designed to do just what you want. It's not really that difficult to find, as it's named NPER for Number of Periods.
Here's an example using your data:
I converted the APR to weekly using 365.25 days in a year and 7 days in a week. There may be some error in this calculation, but it should get you within a payment.
In this case, your Period is a Week, since that's how often you are paying.
Your Payment is negative since it's paying down, not up.
Future Value is zero.
This should put you in the ballpark.
Jerry