Calculator methodology
How the numbers are produced, what is assumed, and why a lender statement can differ.
Last checked UK · illustrative onlyNot a lender or brokerAboutDisclaimer
RLPublished by Rodway Labs — not a mortgage adviser
Published
Remortgage Lab calculators are maintained by Rodway Labs. They are illustrative estimates, not a lender quote and not FCA-regulated advice. Each tool has a How this calculator works section under the inputs.
Formula
Capital-and-interest payments use the standard amortising formula. Monthly payment equals balance times r(1+r)^n divided by ((1+r)^n − 1), where r is the annual rate divided by 12 and n is the number of months. Interest over a comparison window is summed from that schedule. Interest-only loans are not modelled.
ERC
An early repayment charge is estimated as a percentage of the balance you enter, or as months of interest at the rate you enter. A 5%, 4%, 3%, 2%, 1% step is something you type if your offer declines that way. It is not your contract. The offer wording wins.
Rounding and testing
Payments are shown in pounds and pence. Comparison totals may be rounded to the nearest pound. Formulae are checked against worked examples. A formula is not changed unless a worked example shows an error. Zero and negative balances are rejected rather than shown as a payment.
Why a lender figure differs
Day count, payment date, fees added to the loan, a different ERC balance date, and a valuation below the value you typed will move the result. A stress test or credit decision is outside these tools.
SDLT bands follow GOV.UK stamp duty land tax for England and Northern Ireland only. Process context follows MoneyHelper on remortgaging.
Related
Calculators and articles on Remortgage Lab are illustrative and not personalised financial advice. Always check current lender terms and, where appropriate, speak to an FCA-authorised adviser. England & Northern Ireland focus where tax rules are cited; Scotland and Wales differ.