Mortgage Amortization Calculator NZ
This calculator builds a full amortization schedule for a New Zealand home loan, showing exactly how every repayment splits between principal and interest from your first payment to your last. Enter your loan amount, annual interest rate, loan term and repayment frequency (fortnightly, monthly or quarterly), then add any extra repayment per period or a one-off lump sum and the year it is paid, and choose whether to view the schedule as a yearly summary or every individual payment. The calculator returns your regular repayment amount, total interest paid over the loan term, total amount repaid, and your actual term once any extra payments are included. A detailed breakdown then shows your total principal and interest, interest as a percentage of the loan, how much interest you pay in the first year compared with the last, and any interest saved from a lump sum payment. Below that sits the full schedule itself, listing the opening balance, repayment, principal, interest, closing balance and cumulative interest for every period or year. Use it to see how quickly your balance falls and how much extra repayments or a lump sum could save you in interest and shave off your term. Figures are indicative estimates based on a fixed interest rate for the full term; actual costs will vary as your rate changes at refix.
1. Loan Details
2. Extra Repayments (optional)
Loan Summary
Interest Breakdown
Amortization Schedule
| Period | Opening Balance | Repayment | Principal | Interest | Closing Balance | Cumulative Interest |
|---|
How Mortgage Amortization Works
When you take out a principal-and-interest home loan in New Zealand, your lender calculates a fixed regular repayment that will pay off the entire balance (including all interest) by the end of your agreed term. This process of gradually paying off a debt through regular payments is called amortization.
Each repayment covers two things: the interest that has accrued since your last payment, and a portion of the principal. In the early years, most of each payment goes toward interest because the balance is large. As you pay down the principal, less interest accrues each period, so more of each payment goes toward principal. By the final payment, almost the entire amount is principal.
The Amortization Formula
The standard repayment formula for a fully amortizing loan is:
| Variable | Meaning |
|---|---|
| M = P x [r(1+r)^n] / [(1+r)^n - 1] | The annuity repayment formula |
| P | Principal (amount borrowed) |
| r | Periodic interest rate (annual rate / payments per year) |
| n | Total number of payments (years x payments per year) |
For example, a $600,000 loan at 6.5% per annum repaid monthly over 30 years gives r = 0.065 / 12 = 0.005417 and n = 360. The monthly repayment is $3,792.41. Over 30 years you pay back $1,365,266.93 in total, of which $765,266.93 is interest.
Worked Example with Default Inputs
| Item | Value |
|---|---|
| Loan amount | $600,000 |
| Annual interest rate | 6.5% |
| Term | 30 years (360 monthly payments) |
| Monthly repayment | $3,792.41 |
| Total repaid | $1,365,266.93 |
| Total interest | $765,266.93 |
| Interest as % of principal | 127.5% |
| Interest in month 1 | $3,250.00 (principal: $542.41) |
| Interest in month 360 | $20.43 (principal: $3,771.98) |
How Extra Repayments Reduce Interest
Making extra repayments, even small ones, reduces your loan balance faster. Because interest is calculated on the outstanding balance, a lower balance means less interest accrues each period. This shortens your loan term and significantly reduces the total interest you pay. For example, paying an extra $200 per month on a $600,000 loan at 6.5% saves roughly $120,000 in interest and cuts the term by about four years.
A lump sum payment works in the same way: it directly reduces your principal on the day it is made, and every subsequent payment then accrues less interest than it would have otherwise.
Fortnightly vs Monthly Repayments
Choosing fortnightly repayments (26 per year) rather than monthly (12 per year) changes how often interest is charged and repaid. This calculator amortizes a fortnightly loan over the same term you select: it uses a periodic rate of the annual rate divided by 26 and a total of your term in years multiplied by 26 payments. Because interest is calculated and paid down more often, the total interest is marginally lower than the monthly equivalent, but the term stays the same and no extra 13th payment is created.
Some lenders offer an accelerated fortnightly option, where the fortnightly payment is set at exactly half the monthly amount. That does add the equivalent of one extra monthly payment each year and can shorten the term. This calculator uses a standard fortnightly schedule rather than the accelerated version, so to genuinely shorten your term here, use the extra repayment field to pay more than the scheduled amount each period.
Related Calculators
- Mortgage Calculators: the full hub of NZ home loan tools.
- Mortgage Repayment Calculator: calculate your regular payment amount quickly.
- Mortgage Extra Repayment Calculator: see how extra payments cut your term and interest.
- Mortgage Lump Sum Impact Calculator: the effect of a one-off lump sum payment.
- Mortgage Amortisation Schedule: detailed period-by-period payment schedule.
Method: Standard annuity repayment formula M = P x [r(1+r)^n] / [(1+r)^n - 1], where r is the periodic interest rate and n is the total number of payments. Interest per period = opening balance x periodic rate. Principal per period = repayment minus interest. Schedule terminates when the balance reaches zero (final period adjusted for rounding). This matches the method used by New Zealand banks for standard principal-and-interest home loans.
This calculator provides indicative estimates for educational purposes. It assumes a fixed interest rate for the full term. NZ mortgages are typically refixed at shorter intervals (one to five years), so your actual interest cost will vary as rates change. Consult your bank or a licensed mortgage adviser for advice specific to your circumstances.