As far as I can remember you calculate the interest based on the balance at the end of each year, or month, or whatever however often the repayments are being made. So long as you are paying the loan off faster than the interest is accumulating you will find that the overall amount, and hence the interest, reduces over time. Also Cityboy I think that's incorrect, you still use A*(1+r/100), with A being the amount outstanding, but since A reduces over time so does the interest. If it worked the way you stated you could theoretically leave a loan to its own devices without making repayments and it would reduce itself to an infinitessimally small amount, which is not the case.