Mortgage · New Zealand
Compare interest-only payments with principal-and-interest payments and the balance left behind.
Guide reviewed 20 August 2026 · Interactive calculator in development
KiwiTools is building and testing the interest-only mortgage 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 result will highlight that an interest-only payment does not reduce principal during the selected period.
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.
Interest-only payment = outstanding balance multiplied by the periodic interest rate; principal-and-interest payments use the amortising-loan formula.
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.
A user can compare two years of interest-only payments with the higher payment required to repay the same balance over the remaining term.
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. Eligibility and rollover terms are lender decisions 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.