How our UK mortgage calculations work

Methodology version 1.0. Evidence reviewed 12 Aug 2026. Designed for UK residents in England, Scotland, Wales, and Northern Ireland.

Scope

Calculator models residential capital-and-interest repayment mortgages with one initial rate, one follow-on rate, and optional product fee added to mortgage balance. It supports total-term and known-monthly-repayment modes.

Model excludes:

Interest-only, buy-to-let-specific, shared ownership, equity-loan, and offset mortgages
Overpayments, early repayment charges, cashback, and lender-specific incentives
Broker, legal, valuation, insurance, tax, and property costs
Eligibility, affordability, lender quotation, mortgage offer, and personal recommendation

Inputs and opening balance

Mortgage amount means cash advanced to borrower. Product fee is financed when entered, so opening balance is mortgage amount + product fee. Initial and follow-on rates are quoted nominal annual percentages. Initial period and total term use exact whole months internally.

Entered annual rate converts to monthly decimal rate as annual rate ÷ 1,200. Model uses equal monthly periods with payments at month end. It does not use calendar dates or day-count conventions.

Payments and schedule

For balance P, monthly rate r, and n remaining payments, annuity payment is P × r ÷ (1 − (1 + r)⁻ⁿ). At zero rate, payment is P ÷ n.

Initial payment assumes initial rate continues across entered term. At initial-period boundary, follow-on payment recalculates from remaining balance and months. In known-payment mode, entered payment applies during initial period; calculator finds smallest whole-month term that repays balance, capped at 1,200 months.

Each month allocates interest as opening balance multiplied by monthly rate; remaining payment repays principal. Scheduled calculated payments round upward to nearest penny. Final payment settles exact remaining balance, so it can differ from regular payment.

Cash flows and effective rates

Initial-period cash flows start with mortgage cash received less financed product fee, include monthly payments, then settle remaining balance at boundary. Remaining-period cash flows start with boundary balance and include later payments. Full-term cash flows include original borrower advance and every payment.

Bisection solves monthly internal rate of return from those borrower cash flows. Annualisation uses (1 + monthly rate)¹² − 1. These fee-inclusive results are custom effective rates for comparison. They are not regulatory Annual Percentage Rate of Charge (APRC) figures. APRC uses prescribed regulatory assumptions and can include costs or timing outside this model.

Lender regression evidence

Static fixtures captured 12 Aug 2026 test generic behaviour; tests make no runtime network calls. Published APRC reconstruction exists only as regression evidence and never becomes product output.

Published lender examples covered by static regression fixtures
Lender	Fixture-backed behaviour	Evidence date
Barclays	Representative calculated-schedule effective rate vs published APRC	12 Aug 2026
Halifax	Repayment and interest-only monthly payments	12 Aug 2026
HSBC	Published-cash-flow APRC and residential calculated schedule	12 Aug 2026
Lloyds	Repayment, interest-only, and borrowing-rate-increase payments	12 Aug 2026
Nationwide	Representative phase payments and published-cash-flow APRC	12 Aug 2026
NatWest	Published payments, representative phases, and published-cash-flow APRC	12 Aug 2026
Santander	Representative phase payments and published-cash-flow APRC	12 Aug 2026

HSBC worked comparison

Dated 12 Aug 2026: £175,485.41 advance, 336 months, 5.48% for 24 months, then 6.24%. HSBC publishes £1,023.22 initial payment, £1,102.57 follow-on payment, £1,103.79 final payment, £193,074.93 total interest, and 6.3% representative APRC.

Comparison	Accepted difference
Initial payment	≤ £1
Follow-on payment	≤ £1
Final payment	≤ £10
Total interest	≤ 0.2% of HSBC value
Full-term effective rate	Rounds to 6.3% at one decimal

HSBC total-amount-payable statement is internally inconsistent with its payment sum and stated components, so it is excluded from validation and not reproduced as site truth.

Limitations

Real lenders may accrue interest daily, add it monthly, use specific payment dates, apply extra fees, or adjust payments. These differences explain small gaps from lender illustrations. Test suite protects rate conversion, zero-rate payment, phase transition, fees, inferred term, cash flows, effective rates, rounding, and final settlement alongside seven lender fixture families.