Compare two repayment rates on the same outstanding balance and remaining term. Use the balance you expect when your fixed deal ends, rather than the original loan amount.
By QuidCast · Published 15 September 2026 · Educational estimates
Explore your numbers
All starting figures are examples. Replace them with your own assumptions. Calculations run on your device; inputs are not sent to QuidCast.
A worked example
With £180,000 remaining over 25 years, a 2% rate gives an estimated payment of £762.94 a month. At 5%, it is £1,052.26: an increase of £289.32 a month. Both rates are illustrative, not available deals.
How the calculation works
The calculator uses the standard capital-and-interest repayment formula: balance × monthly rate ÷ [1 − (1 + monthly rate) raised to the negative number of payments]. Monthly rate is the annual percentage divided by 1,200. At a zero rate, the balance is divided evenly across the remaining monthly payments.
Assumptions and limits
This compares capital-repayment mortgages only. It excludes fees, early repayment charges, insurance, overpayments and rate changes after the comparison date. It assumes monthly payments and equal remaining terms. A Bank of England rate change does not necessarily change your mortgage by the same amount.
This tool provides general educational estimates, not personal financial advice.
Common question
Does this tell me which mortgage to choose?
No. It compares the monthly cost of two rates you enter. Eligibility, product fees, term, loan-to-value and other conditions also matter. Use actual lender illustrations when comparing offers.