The mortgage loan calculator can be used to work out how much you will have to pay for your mortgage with a given loan amount, interest rate and loan term. Of course, this monthly payment estimate will only apply if you take out a fixed rate mortgage for the entire term of the mortgage. If you choose to only fix the mortgage rate for a set period, your rate (and therefore your repayments) could fluctuate significantly.
The reason why many people choose to take out a fixed rate mortgage is so that they will be able to budget more easily each month. You take on the risk that the interest rates will fall during the fixed-rate period (as happened recently), but you may also find that you are protected against financial difficulty if the rates suddenly rise. The mortgage loan calculator should be used as a part of the overall research process, but should not be seen as an exact indication of the financial outlay required throughout the term of your mortgage.