Reverse Credit Card function, need formula to calc. current balance i am OWED

I have loaned someone money and they are paying it back sporadically ( not on a set basis) they have agreed to pay a fixed interest on the current balance.

I have a speadsheet made up of the current balance and their past payments, but HOW do I set it up so it calculates interest on the current balance and updates this sum each month, but takes into account payments made at any time?

In essence, I need a calculation formula like the credit card companies use, but with a simple fixed interest rate calculated each month

all the posts I read are about paying off loans, not receiving them, so any help would be great

thanks

Receiving the money is the flip side of owing/paying it.

If you put youself in the position of the debtor, all the number are the essentially the same.

Ed

yes, I know that, I can just reverse debits to be credits, but I still need a way to have a fixed interest rate being calculated on a changing balance on a monthly basis and then adding that amount to the balance. All the other formulas I have found tell "how many payments till 0 balance, etc" I am not concerned with # of payments, just the remaining balance after each payment.

any ideas?

Check out the NPV function,

Ed