UK mortgage calculators
Mortgage Calculator Input Checklist
A reliable illustration begins with current figures from documents, not remembered numbers from the original mortgage.
Outstanding balance
Use a recent lender statement or authenticated account. The original advance and an estate-agent estimate are not substitutes for the capital currently outstanding.
Remaining term
Record years and months remaining. A rounded original term can materially distort monthly repayment and total-interest illustrations.
Rate and rate period
Use the rate applying to the scenario and state whether it is fixed, variable or an assumed stress rate. Do not present an illustrative rate as a product quote or forecast.
Fees and charges
Include unavoidable product, arrangement, valuation, legal, broker and exit costs where relevant. Record whether a fee is paid now or added to the loan, because financing it can generate interest.
Affordability context
A repayment calculation is not an affordability assessment. Income stability, committed expenditure, tax, insurance, maintenance and emergency savings remain outside the formula.
Decision record
Save the source date, figures, scenarios and questions for the lender or adviser. Re-run the illustration when a rate, balance, term or fee changes.
Sources and review standard
Calculations are independently implemented illustrations and were reviewed on 4 September 2026. We check factual boundaries against MoneyHelper's mortgage cost guidance, FCA consumer mortgage information and relevant lender documentation. Rates and product terms are never presented as live offers.
Editorial responsibility
Published by Acerville Sparks Limited. Software assists research, drafting and automated formula testing. Primary sources, assumptions and limitations are shown so readers can inspect the work. No independent mortgage professional or financial adviser review is claimed. Reviewed 4 September 2026.