Mortgage · New Zealand
See how much of an amortising mortgage scenario is principal and how much is interest.
Guide reviewed 20 August 2026 · Interactive calculator in development
KiwiTools is building and testing the mortgage interest calculator. Everything below—the method, required information, worked example, assumptions, limitations and sources—is available now.
No estimated result is shown until the calculation is ready.
The tool will provide a payment schedule and totals for an unchanged-rate scenario.
Use the result to compare clearly defined scenarios rather than as a prediction. Keep every other input unchanged when testing one decision so the effect of that change remains visible.
Each period applies interest to the opening balance, subtracts that interest from the payment to find principal repaid, and carries the reduced balance forward.
The finished calculator will show the assumptions beside the result and will not silently substitute a personal rate, price, eligibility decision or future forecast.
Using your own current figures will normally produce a more useful scenario than relying on a generic default. Prepare these inputs:
Use a recent statement, bill, agreement, quote or measured figure where one is available. Keep the same basis when comparing alternatives.
Use a recent statement, bill, agreement, quote or measured figure where one is available. Keep the same basis when comparing alternatives.
Use a recent statement, bill, agreement, quote or measured figure where one is available. Keep the same basis when comparing alternatives.
Early payments normally contain more interest because the balance on which interest is calculated is still relatively high.
This example demonstrates the method only. It is not a recommended setting, current market quote or estimate for a particular person.
The finished tool will provide a general planning estimate, not financial, legal, tax, lending or eligibility advice. Confirm decisions with the relevant provider or authority.
Read the result as an estimate produced from the values entered. Compare a realistic base case with a cautious alternative, and check which input causes the largest change.
A useful result should help frame the next question. It does not verify an application, contract, tax position, product suitability or future price.
These primary and reputable sources provide context for the method. Changeable rates and rules will be checked again before the interactive calculator launches.
The interactive calculator is still being built and tested. The method, inputs, example and limitations on this page are available now.
No. Future rate changes and lender rounding are not predictable The finished calculator will show its assumptions and provide a planning estimate only.
No. KiwiTools intends to add the calculator to this permanent page so saved links and the supporting guide remain in one place.