Enter what you still owe, the rate you pay and how many months are left, then the rate of the new deal. The calculator compares both mortgages to their last payment: monthly payments, interest, any early repayment charge (ERC) and remortgage fees. You get the net saving and the number of months before the switch pays for itself. It works for repayment mortgages at a fixed rate, and the calculation is the same anywhere in the UK.
Find your balance and term
Your annual mortgage statement or online account shows the balance outstanding and the remaining term. Enter the rate you pay now and the number of monthly payments left.
Describe the new deal
Enter the new rate and, if you are changing the term, the number of months. A longer term lowers the monthly payment but can raise the total interest, so the calculator compares the full cost of each mortgage, not just the payments.
Early repayment charge and fees
Your mortgage offer says whether you pay a charge to leave early. UK rules set no fixed cap: the FCA requires the charge to be expressible as a cash value and to be a reasonable pre-estimate of the lender's costs. Enter it as a percentage of the balance or as an amount. Then add the remortgage fees (arrangement, valuation, legal) and say whether you pay them upfront or add them to the new mortgage, where they accrue interest.
Rounding and the result
Interest is worked out monthly at the yearly rate ÷ 12; a lender that charges interest daily may differ slightly. Monthly payments and each month's interest are rounded to the nearest penny, with halves rounded up, and the last payment clears the balance. Copy the workings, export them as CSV or save them to Priostack.
Monthly payment = B × r ÷ 12 ÷ (1 − (1 + r ÷ 12)^−n), where B is the balance, r the yearly interest rate and n the number of months.
Net saving = interest on your current mortgage − (interest on the new mortgage + early repayment charge + fees).
The switch breaks even in the first month in which the interest saved since switching covers the charge and the fees.
Example: £180,000 over 18 years, from 5.75% to 4.49%
You owe £180,000 with 216 monthly payments left at 5.75%, and no early repayment charge applies. A new deal at 4.49% over the same term, with £999 of fees added to the mortgage, takes the monthly payment from £1,339.51 to £1,223.18. Keeping your mortgage leaves £109,332.33 of interest to pay; switching costs £84,208.41, fees included. The net saving is £25,123.92, and the switch breaks even after 6 months.
