A bond repayment calculator is sometimes called an amortization calculator because this is the kind of calculator that is used to figure out the principal and interest payment needed to retire a loan over a certain period of time.
A mortgage is a loan that is most often amortized over a period of 15 or 30 years. You use the bond repayment calculator to determine what level payment will be needed to pay both the interest and the principal on the loan so that the principal is completely paid off by the end of the term of payments. In the early years of a loan, most of the payment goes to interest, with a smaller amount going to reducing the principal. Towards the end of the loan period most of the payment will be going to the principal with a lesser amount being applied to interest.
The reason you want to use a bond repayment calculator when discussing loan options with a lender is that you can enter a loan amount and length of time then vary the interest rates to see the difference it will make in payments. You can also enter the payment amount you want and the loan amount to make a determination of what interest rate over what period of time you will need to get the payment amount you want.
Many people leave all of the calculations to the lender, but if you have your own bond repayment calculator you are able to discuss different options based on the figures you put in. The lender will see that you are a much more aware buyer than most and can discuss all of the different options available. Once you know what the payment is for a 30 year fixed rate loan at 4.5% , you can compute what interest rate you would need to get to finance the same loan for 15 years and compare that difference quickly. You may find the difference is so slight that it is to your benefit to take the shorter term loan.
A shorter term loan can save you thousands in interest compared to the same loan for longer term. By using your own bond repayment calculator you can quickly figure out what is the best rate and term for the loan you are considering.