Mortgage protection calculator
The mortgage keeps falling due whether or not the income behind it does. This calculator works out your monthly repayment at your current rate and at a stressed rate, how much of that repayment has no funding if the insured person’s income stopped, how long your emergency reserve bridges the gap, and an indicative monthly benefit as a starting point for an adviser conversation. It never calculates a premium, and it never recommends a product.
Your mortgage and household figures
Three fields are required — balance, remaining term and interest rate — because a repayment cannot be computed without them, and this tool never substitutes a made-up default for a number it does not have. Everything else is optional and defaults to the most conservative reading: no other income toward the repayment, and no reserve.
Step 1 — the loan
The amount still owing, from your banking app or latest statement.
Years left until the loan is scheduled to be repaid in full.
The rate you are actually paying now. If the loan is split, use the rate on the largest portion.
Leave blank to use the computed table repayment. Enter your real figure if you pay above the minimum or have a different structure.
Step 2 — the stress scenario
A QuoteHub planning assumption (v1, 2026-08-15), not a bank's published test rate — adjust it to any margin you want to test.
Step 3 — if the insured income stopped
After tax, per month — the part of a partner's take-home pay, rent or board that could actually go to the mortgage. Leave blank if none.
Savings you could actually reach, measured in months of repayments. Three months of a $2,500 repayment is $7,500.
How long a monthly benefit would need to keep paying. Capped at the remaining term — cover cannot usefully outlast the loan.
Enter the mortgage balance, remaining term and interest rate above to see your repayment, the stress scenario and the monthly gap. Nothing is estimated until all three are in — this tool never invents a figure for a field you have not filled.
How the repayment maths works
The calculator uses the standard table (amortising) repayment method that New Zealand home loans ordinarily follow: equal monthly instalments over the remaining term, with interest calculated each month on the reducing balance, so early payments are mostly interest and later payments mostly principal. The monthly repayment M is computed as the loan balance multiplied by the monthly interest rate, divided by one minus the compounding discount over the remaining months:
M = P × r ÷ (1 − (1 + r)−n)
In words: P is the current balance, r is the annual interest rate divided by twelve, and n is the number of monthly repayments left (remaining years multiplied by twelve). At an interest rate of exactly zero the formula collapses to the balance divided by the number of months, and the calculator uses that straight-line division directly rather than dividing by zero. All money figures are rounded to the cent.
The formula produces the scheduled minimum for a single amortising loan at one rate. Real repayments often differ — voluntary extra payments, split loans, offset accounts and interest-only periods all change the figure — which is why the calculator lets you enter your actual monthly repayment. When you do, every downstream number (the gap, the reserve bridge and the indicative benefit) runs on your real figure, and if your figure is below the computed table repayment the tool says so rather than silently averaging the difference away.
Why a stress-rate scenario is built in
A mortgage repayment is only fixed until the fixed term ends: at each refix the repayment is recalculated on the remaining balance and remaining term at whatever rate then applies, so cover sized precisely to today’s repayment can be undersized the day a refix lands at a higher rate. The stress scenario recomputes the table repayment with a margin added to your current rate — the same balance, the same term, only the rate changed — so you can see the repayment and the monthly gap as they would look after an unfavourable refix, not just as they look today.
The default margin of +2.00 percentage points is a QuoteHub planning assumption (version 1, dated 2026-08-15, unsourced and listed for licensed review): banks apply their own servicing test rates when they lend, but those internal margins are not published as a single citable figure, so this tool does not pretend to know them. The margin is fully adjustable — set it to zero to switch the stress scenario off, or higher to test a harsher refix.
What the monthly gap means — and what it leaves out
The monthly gap is the repayment minus the continuing household income you said could actually go to the mortgage if the insured person’s income stopped — the part of a partner’s take-home pay, rent or board that would genuinely be available for this bill rather than absorbed by everything else. A gap of zero means the household could keep servicing the loan without the insured income on the figures entered; anything above zero is repayment with no funding behind it, month after month, for as long as the income is missing.
Deliberately, the gap ignores what might partially replace the lost income, because each replacement has conditions this tool cannot verify from four inputs: ACC weekly compensation applies to injuries only and pays nothing when illness stops you working, employer sick leave runs out in weeks rather than months, and any income protection you already hold has its own waiting period and offsets. Our income protection calculator
The gap also frames a structure question this tool deliberately does not answer: mortgage repayments can be protected with a monthly repayment benefit, with income protection, or by lump-sum cover that clears the balance entirely on death or serious illness. Each structure behaves differently at claim time and each insurer applies its own maximum-benefit rules and offsets. Which one fits — or whether the honest answer is none — depends on your circumstances, and that is an advice conversation, not a calculator output. Our guide to life insurance versus mortgage protection
How the emergency reserve bridge is measured
The emergency reserve is entered in months of repayments because that is how most households actually hold and think about it, and its dollar value is that many months multiplied by the repayment in use. The bridge is then the reserve divided by the monthly gap — not by the full repayment — because when continuing household income keeps paying part of the repayment, the reserve only has to fund the remainder and therefore lasts longer than its stated months. Three months of repayments held against a gap that is half the repayment bridges roughly six months. When there is no gap at all, the tool reports that the reserve is not being drawn on at all rather than showing a meaningless number.
The bridge matters because it is the household’s real waiting period: it is the time you could self-fund before any cover would need to start paying, and insurance with a waiting period longer than your bridge leaves a stretch of repayments nothing funds. Comparing the bridge months against the protection period you selected shows whether savings alone carry the risk or merely soften its start.
The indicative benefit is a starting point, not a recommendation
The indicative monthly benefit is simply the monthly gap, capped so it can never exceed the repayment itself — a mortgage-repayment benefit exists to keep the mortgage paid, and this tool never sizes past that obligation. It is a starting point for scoping a conversation, not a recommendation to buy anything: insurers apply their own maximum-benefit rules, offsets against other cover, and full underwriting before any benefit is agreed, and none of that is visible to a calculator. No premium is calculated anywhere on this page, and no insurer is named or implied — what cover of any given size would cost is a question only an insurer can answer through an adviser at underwriting.
Every assumption and where it comes from
This calculator uses no market-fact constants at all: every dollar figure in the result comes from your own inputs, processed through the published repayment formula above. The full list of working assumptions, each with its status, is short enough to state completely.
- Table amortisation formula — standard published lending mathematics, stated in full in the methodology section above. Status: standard formula, not an estimate.
- Stress margin default of +2.00 percentage points — QuoteHub planning assumption v1, dated 2026-08-15, unsourced, user-adjustable, listed for licensed review. It is not a bank’s published test rate.
- Continuing household income stays constant — modelling choice: the amount you enter is assumed to keep arriving unchanged and to remain fully available for the mortgage. In practice a partner’s hours often change when they become a carer.
- Reserve drawn down evenly — modelling choice: the reserve funds the gap in equal monthly amounts with no interest earned or emergency top-ups.
- Benefit cap at the repayment — a deliberate design rule of this tool, not an insurer’s limit: insurers set their own maximum-benefit rules, which an adviser confirms against actual product terms.
Input guards (maximum balance, term, rate and margin accepted by the form) exist only to keep the arithmetic finite against typos — they are sanity limits of the calculator, and are never presented as limits on what any lender or insurer offers.
What to do with the number
A monthly gap figure is useful for exactly one thing: arriving at an adviser conversation already knowing the size and shape of the problem, so the conversation starts at structure and suitability instead of arithmetic. No quote is generated here and nothing on this page prices, recommends or applies for any product. A licensed adviser tests these figures against your loan structure, any existing cover, and real policy wording — including the waiting periods and offsets this tool deliberately leaves out — at no cost and with no obligation.
Book a free cover review, see the mortgage protection overview
This page is general information, not personalised financial advice. Financial advice is provided by Craig Smith Business Services Limited, trading as Smiths Insurance & KiwiSaver, a licensed Financial Advice Provider (FSP712931). Methodology last reviewed 15 Aug 2026.
Review your mortgage protection with a licensed adviser · free, no obligation.
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