My brother's trying to remortgage his family home at the moment and for various reasons can't, so is having to go onto the higher rate, which is v high in her case. I want to lend him the money. He is insisting we do it on a proper interest rate basis (tbd, but probably about 4% as that is what I am getting in savings at the moment). Does anyone know if there is any software anywhere that would track what he "owes". Please don't worry about warning me about the risks of lending to my brother. I am absolutely aware of them, and I am prepared to write off the money if necessary. He just will only borrow on this basis. Is there any online system that will track what the eg £100,000 would earn? I know I could just add 4% a year, but we're keen to do it more accurately than that because he may well be able to remortgage in about six months time.