Mortgage guide · Reviewed 27 August 2026

A mortgage payment is a useful starting point, not a full housing budget.

A principal-and-interest estimate answers one narrow question. Affordability requires a wider view of taxes, insurance, maintenance, debt, income stability and local lending rules.

The payment formula

The standard amortization formula converts a loan amount, periodic interest rate and number of payments into a level scheduled payment. Early payments usually contain more interest because interest is calculated on a larger outstanding balance. As principal falls, the interest portion generally falls too.

Costs outside the basic result

Property taxes, homeowners insurance, association fees, maintenance, closing costs and rate changes can materially change the cash required. A lender may also use income definitions, stress tests and debt limits that differ from a simple affordability ratio.

Make comparisons fair

Compare like-for-like terms. A longer replacement term can lower the monthly payment while increasing total interest. Extra payments may shorten the modelled schedule, but contract rules and prepayment penalties must be checked separately.

Calculate a mortgage payment ↗ Review affordability assumptions ↗

Source note: The mathematics is standard fixed-rate amortization. This page is not lending, tax, legal or financial advice and does not estimate approval.