Last updated 31 August 2026
Why this exists
Almost every mortgage calculator in New Zealand answers one question: what is the repayment on this loan? That number is easy to produce and close to useless on its own, because nobody's budget breaks on the repayment alone. It breaks on the repayment plus council rates, plus house and contents insurance, plus the power bill that is bigger than it was in a rental, plus the car, the groceries, the childcare and the maintenance nobody warned you about.
This calculator was built for our own budgeting while working through exactly that problem, and it is still used that way. It asks for the whole picture (every person's take-home pay, every recurring cost, and your day-to-day spending) and then shows what is actually left over each month, how much you would have saved after a year, and how far interest rates could rise before the surplus disappears. The tool is free, needs no sign-up, and stores nothing on a server.
Who runs it
Own Your Home is published and maintained by Franklin Consulting Ltd, a New Zealand company. Questions, corrections and data disputes go to contact@ownyourhome.co.nz and are read by the person who maintains the tool. We are not a bank, not a mortgage broker, and not a licensed financial advice provider; see terms of use.
How the calculations work
Mortgage repayments
Repayments use the standard amortisation formula for a table loan, which means equal payments of principal and interest over the full term:
payment = P × r / (1 − (1 + r)−n)
Where P is the total amount borrowed, r is the annual interest rate divided by 12, and n is the term in months. The total borrowed includes any "additional mortgage" you enter, because money borrowed on top for renovations, appliances or furniture is still debt you service. Interest is charged monthly and the rate is held flat for the entire term, which is a simplification: in practice New Zealand loans are fixed in one to five year tranches and then refixed at whatever rate applies at the time.
The three scenarios
The scenario table runs the same household budget at your quoted rate, that rate plus one percentage point, and plus two. This is a stress test, not a forecast. It exists because the single most common way a first home budget fails is not a job loss; it is a refix at a materially higher rate two or three years in, with every other cost also higher than it was on settlement day.
Costs, periods and totals
Every cost line can be entered weekly, monthly or annually and is normalised to a monthly figure. Weekly amounts are multiplied by 52 and divided by 12 (not by four, which understates annual cost by about 8%), and annual amounts are divided by 12. Day-to-day living is entered as a daily rate and multiplied by 365 / 12. Household income is the sum of each person's take-home pay, so it should be after PAYE, ACC levies, KiwiSaver contributions and student loan deductions, which is the figure that actually lands in your account.
Runway
Runway divides your accessible savings by your total monthly outgoings, including the mortgage repayment. It answers "if all income stopped, how many months could this house be paid for?" The six and twelve month versions add the surplus you would have accumulated by then, assuming the surplus is saved rather than spent.
Where the default figures come from
The pre-filled numbers are starting points for an Auckland household, not recommendations, and every one of them is editable. They are drawn from published sources:
- Council rates default to the Auckland average residential rates bill. Auckland Council adopted a 7.9% average residential increase for 2026/2027 and forecasts increases of up to about 3.5% a year after that. Rates are struck every July and vary enormously by property, so look up the actual figure for the property you are buying.
- Grocery, food and general cost movements are checked against the Stats NZ food price index and consumers price index.
- Interest rates shown as defaults reflect indicative advertised retail rates at the time of the last update. They are not quotes. Your rate depends on your lender, term, equity position and negotiation.
- Insurance, power, internet and transport defaults are representative mid-range figures for a household of two. Use your own bills wherever you have them, because a downloaded CSV of three months of bank transactions will beat any default in this tool.
What this calculator does not do
Being clear about the gaps matters more than looking comprehensive:
- It does not calculate tax. Income is entered net, so it does not model PAYE, secondary tax, self-employment or fluctuating income.
- It does not model a low equity margin or premium, which most lenders add when your deposit is under 20% and which can add a meaningful amount to your rate.
- It does not model fixed-rate tranches, refix dates, offset or revolving credit accounts, or interest-only periods.
- It does not include one-off purchase costs: legal fees, LIM report, building inspection, moving costs or Lenders Mortgage Insurance where it applies.
- It does not assume any capital gain, rental income, boarder income, or KiwiSaver first home withdrawal.
- It is not a lender's serviceability test. Banks apply their own test rate, typically well above the advertised rate, plus their own living cost floors and credit scoring. Passing this calculator does not mean you will be approved, and failing it does not mean you will be declined.
Corrections
If a default figure is out of date or a formula looks wrong, we want to know. Email contact@ownyourhome.co.nz with the number and, where possible, the source. Figures are reviewed when the underlying data is republished: Auckland rates each July, Stats NZ indices as they are released.
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