New Zealand Calculator Methodology
Last updated 26 July 2026
Every New Zealand calculator on Fiscalgrove is built from a documented formula and the specific lending or statutory data that applies to it. This page summarises the shared method; each calculator's own page also works through its formula and a New Zealand example in context. This section grows as each calculator ships.
Table-loan repayments
New Zealand home loans are almost always table loans: a standard reducing-balance loan where each repayment covers the interest charged since the last payment, and the rest reduces the principal. Interest accrues daily, but for a table loan that is mathematically the same as the textbook per-period reducing-balance formula, M = P ร r ร (1+r)^n รท ((1+r)^n โ 1), where P is the loan amount, r is the annual rate divided by the number of payments a year, and n is the total number of payments. We compute on full unrounded figures and only round to the cent for display, so the schedule does not drift.
Repayment frequency
Because New Zealand borrowers routinely pay weekly, fortnightly, or monthly, each frequency is modelled as a genuine table loan on that basis (52, 26, or 12 payments a year), rather than simply halving or quartering the monthly figure. Paying more often lowers total interest slightly, because the balance falls sooner. The headline figure is the payment for your chosen frequency, with a monthly-equivalent shown alongside so options compare like for like.
Interest-only loans
An interest-only loan is modelled in two phases: during the interest-only period the payment covers interest only and the balance holds at the original amount, then the loan amortises that full amount over the remaining term. Because no principal was repaid earlier, the later repayment is higher than an equivalent loan that was principal and interest from the start.
Editable defaults
Every default in a calculation is visible and editable in the calculator itself. Where a figure is set by regulation (for example the Reserve Bank's loan-to-value and debt-to-income limits, used by the borrowing and deposit calculators), it reflects the published rule as at the verification date shown on that calculator.