Interest Calculation Question

Uncertain

I wonder if anybody can help me....

I'm trying to help a friend work out some figures.....

She has acquired a dollop of cash and realises it would be better to pay off some credit card debt rather than put it in the bank.

Obviously it is easy to work out the monthly saving in CC interest if she just pays off that amount of capital.

The question is this:

If she then uses the monthly interest saving to pay off further capital (i.e continues to pay what she had been paying each month) the effect obviously snowballs. Is there an easy formula to work this out?

Or, is their a recognised rough and ready estimating figure for this? For example, when I was into monthly savings accounts the annual interest was almost exactly the interest rate applied to seven twelfths of the capital at the end of the year?

Or do I have to do this the hard way with Excel?

Many thanks