What is the excell formula to calculate interest rate

[talib225]

I need to create a fomula in excell that will give me the dollar amount of interest added monthly based on a monthly interest rate

[factor-fiction]

If average balance in the first month is $A and the monthly interest rate is m% then the balance at the end of the month including interest will be :
A * (100+m)/100
The interest will be A*m/100

So if average balance is $100 and monthly interest is 1% then interest in month will be 100*1/100=$1

Next month the interest will be 1% of the new balance of 101 so it will be 101*1/100=$1.01