Rates & sources
Standard amortisation formulas used across UK lenders. Interest rates move daily — confirm with your lender or broker.
Source: Bank of England — Statistics — figures refreshed at the start of each tax year.
When to use this calculator
- Before comparing lenders, brokers, or repayment options.
- When you want to test how a different deposit, rate, or term changes affordability.
- When you need a quick estimate before using a formal quote or agreement in principle.
- When you are stress-testing your budget against a potential rate rise to see the impact on monthly payments.
- When you want to understand the full cost of borrowing — not just the monthly figure — before you commit.
A realistic New Zealand planning example
Use these sample inputs as a quick scenario test, then change one variable at a time to compare outcomes.
Loan Amount (NZ$)
NZ$600,000
Interest Rate (%)
5%
Loan Term (years)
NZ$600,000
Repayment Frequency
Fortnightly
After entering these figures, review repayment per period, monthly equivalent and total repayment together rather than in isolation — each metric tells a different part of the story. Then rerun the tool with one input adjusted to see which variable has the biggest effect on all three outputs before you settle on a plan.
How to read your results
Repayment per Period
Use this metric to compare scenarios side by side and understand how changes in the key inputs drive the final outcome. If the figure surprises you, isolate one variable at a time and rerun the calculation to identify which assumption is responsible.
Monthly Equivalent
Use this metric to compare scenarios side by side and understand how changes in the key inputs drive the final outcome. If the figure surprises you, isolate one variable at a time and rerun the calculation to identify which assumption is responsible.
Total Repayment
This is the headline outcome of the calculation, but it is most useful when read alongside the supporting metrics below it rather than in isolation. Try changing one input at a time and watching how this total moves to understand which driver has the biggest impact.
Total Interest
This shows the long-run cost of borrowing beyond the original principal, which is especially useful when comparing terms or weighing up overpayment options. A shorter term usually cuts this figure significantly even if the monthly payment rises.
LVR
Use this metric to compare scenarios side by side and understand how changes in the key inputs drive the final outcome. If the figure surprises you, isolate one variable at a time and rerun the calculation to identify which assumption is responsible.
Method & assumptionsAuthoritative sources
This calculator uses the standard mortgage amortisation formula applied at your chosen repayment frequency — weekly (52 payments per year), fortnightly (26), or monthly (12). New Zealand borrowers commonly choose fortnightly repayments because the extra payments each year reduce the principal faster and cut total interest considerably over a 25–30-year term. The periodic interest rate is derived by dividing your annual rate by the number of payment periods, then applying the compound interest formula. The LVR figure is relevant to RBNZ lending restrictions: owner-occupiers generally need at least a 20% deposit, meaning an LVR at or below 80%, to access mainstream lending without additional conditions.
Note that this calculator assumes a constant interest rate for the full loan term and does not model refix decisions, rate changes, or lump-sum repayments. In practice, most New Zealand mortgages are fixed for one to five years at a time, after which you renegotiate with your bank. If you are comparing lenders or rate types, model each scenario separately using the interest rate for that product. The results here are indicative — your bank is required to provide a full disclosure statement and credit contract before any mortgage offer becomes binding under the Credit Contracts and Consumer Finance Act (CCCFA).
Common mistakes
- !Mixing up loan amount and property value, which can distort affordability and LTV.
- !Using a headline rate but forgetting fees, insurance, taxes, or repayment type.
- !Testing only one term length instead of comparing the payment and total cost together.
- !Forgetting that a repayment mortgage and an interest-only mortgage produce very different monthly figures and total costs.
- !Not accounting for the impact of a rate revert after an introductory fixed period ends, which can sharply increase payments.
What to do next
- Run a second scenario with a higher rate or shorter term so you can see the downside clearly.
- Compare the result with an affordability or overpayment calculator before applying.
- Use the related guides below to understand trade-offs before you request live quotes.
- Note down the monthly payment and total interest for your two or three strongest scenarios so you have a clear comparison ready when you speak to a broker.
- Check whether making a modest overpayment each month would reduce total interest significantly — run the overpayment calculator next to find out.
Frequently asked
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End-of-article next steps
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