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Personal Loan issuer online service
It is pretty easy to setup a spreadsheet for calculating interest payments and remaining balance.  Do a quick search online.  You may want to put it in something like Google Docs, where brother can view the status, but only you can edit it.  When you get a payment, a portion goes to interest and another to principle.  The formulas will do the work for you. However, I feel that there is a bigger issue.  The math may seem like a good deal for the both of you, but I would be very hesitant to loan a family member money. What if he does not pay?  What if he is late with a payment and goes on a vacation himself?  What if his significant other resents the payment that you collect which precludes her from  buying a new TV, etc... People come to hate/resent big corporations that they have to make payments.  How much more so one that has a face....that comes over and eats?  While this loan is outstanding holidays may never be the same.   Is the loan a real need?  Are you in a position to give them the money?  You may want to consider the latter. Is there a reason he can't just borrow the money from the bank?