Mortgage calculator Enter the details for your mortgage below and press the calculate button. Mortgage amount (£): The mortgage amount required. Repayment period: The number of years for which the mortgage is taken out. Interest rate (%): The mortgage interest rate applicable. Calculate Note: These calculations are only for illustrative purposes and are not a substitute for professional advice. The NI calculation is annualised and therefore due to the change in the employee rate during the year, the amount is only an indication.