How our mortgage repayment calculator works.

What it calculates

You enter four things: a loan amount, an interest rate, a term in years, and how often you’ll pay (weekly, fortnightly or monthly). Optionally, an extra amount per repayment. The calculator returns your scheduled repayment, the total interest over the loan’s life, the total you’d pay, and — if you add extra — how much sooner you’d be debt-free and the interest saved.

The method, step by step

1. Your repayment comes from the standard amortisation formula. The annual rate is divided by your repayment frequency to get a per-period rate, and the repayment is set so the loan reaches exactly zero at the end of the term: Repayment = Loan × r ÷ (1 − (1 + r)⁻ⁿ) where r is the annual rate ÷ repayments per year, and n the total number of repayments (years × frequency). This is the same “table loan” structure NZ banks use for standard home loans: early repayments are mostly interest, later ones mostly principal.
2. Total interest comes from simulating the loan, period by period. Each period the balance earns interest at the per-period rate, then your repayment is subtracted. The interest charges are added up across the whole term. If a repayment wouldn’t even cover the interest, the calculator tells you rather than showing a misleading number.
3. Extra repayments re-run the same simulation. Your extra amount is added to every scheduled repayment from the first one. The calculator compares the two runs and reports the months saved and interest avoided.

A worked example

$600,000 loan, 5.5% p.a., 30 years:

Adding $50 to every fortnightly repayment: debt-free about 2 years 2 months sooner, saving roughly $54,405 in interest.

The assumptions — read these before relying on any number

• The interest rate stays the same for the whole term. Real loans refix or float — over 30 years your rate will change many times, and the totals are highly sensitive to it. The rate you enter is an illustration, not a rate anyone is offering you.
• The per-period rate is the annual rate ÷ frequency. This is the standard calculator convention. Lenders actually accrue interest daily and charge it at your repayment frequency, so a bank’s figures will differ slightly.
• Repayments never change and are always made. No missed payments, holidays, refixes, restructures or lump sums (other than the regular extra amount, which is assumed from day one for the life of the loan).
• No fees are included. Application, monthly service, valuation, legal or break fees would all add to the true cost. Lenders may also round repayments up.
• The loan is a standard table (fully amortising) loan. Interest-only periods, revolving credit and offset structures aren’t modelled.
• The final repayment is treated as a full one. In reality the last payment is usually slightly smaller.

What this calculator is — and isn’t

This tool does the maths on the numbers you give it. It doesn’t recommend a loan, lender, rate or structure, and it doesn’t consider your circumstances, so it isn’t financial advice under the Financial Markets Conduct Act 2013 — and it certainly isn’t an offer of finance. Any lender will apply their own credit criteria, responsible lending checks, rates and fees.

Whether to fix or float, how to structure a loan across terms, and whether extra repayments beat investing the difference — those are advice questions, and they’re what we do. Book a free chat and we’ll work through your situation properly.

Sow Financial Limited (FSP1008822) is a licensed financial advice provider. Our public disclosure statement sets out our licence status, how we’re paid, our complaints process, and our dispute resolution scheme.

A note on privacy: numbers you enter stay in your browser and aren’t sent to us or stored.

Calculator methodology last reviewed: July 2026. If we change the method or assumptions, we’ll update this page.