I'm looking for an easy to use calculator so that I can keep a good record of repayments for a private loan to one of our sons. I've googled but there are so many, and most seem to want to show me how much my repayments would be on a certain loan. I need one where I can add the repayment made to us by our son each week/month and insert the interest rate we are charging, and it will reduce the total as each payment is entered. I'm looking for interest only. I have a copy of one my brother uses, but it seems fairly complicated and I need one for dummies .