Free Lump Sum Repayment Calculator for New Zealanders
See the impact of a one-off extra payment on your mortgage. The calculator returns how much sooner the loan is paid off and how much interest you save by keeping your scheduled payment unchanged.
Your loan and the lump sum
Based on a $650,000 loan at 6.50% over 30 years, with a one-off $10,000 extra payment.
About our Lump Sum Repayment Calculator
A $20,000 lump-sum on a $480,000 mortgage at 6% saves about $60,000 of interest and clears the loan two years sooner, provided regular repayments stay at the original level. The lump-sum effectively earns a return equal to the mortgage rate, which is hard to beat in low-risk savings.
The calculator runs the per-month iteration: apply the lump-sum to principal, keep repayments unchanged, watch the balance fall faster. The output shows the new payoff date, the months saved, and the total interest saved.
How to use it
Enter your current mortgage balance, contract rate, current monthly repayment, years remaining, and the lump-sum amount you are considering.
The result panel returns the original payoff date, the post-lump-sum payoff date, the time saved, and the interest saved.
Why use it
For an owner-occupier, lump-summing into the mortgage is one of the best low-risk uses of a windfall, with the interest saving acting as a guaranteed pre-tax-equivalent return at the contract rate. The calculator quantifies the saving for the specific lump-sum amount you are considering.
The maths behind it
New term = months until balance reaches 0 with payment M unchanged after lump sum A lump-sum payment reduces the principal immediately. With the regular repayment kept at the same dollar amount, more of each subsequent payment goes to principal (because there is less interest accruing on the smaller balance), so the loan clears sooner. The interest saving is the difference between total interest under the original schedule and total interest under the post-lump-sum schedule. The calculator iterates month by month to find the new payoff date.
Worked example
Heath, builder in Gore, putting a $20,000 inheritance toward his $480,000 mortgage with 22 years remaining at 6.0%.
Heath inherits $20,000 from a great-aunt. His mortgage balance is $480,000, contracted rate 6.0%, with 22 years (264 months) left of fortnightly $1,580 repayments.
Applying the $20,000 lump-sum directly to principal reduces the balance to $460,000. Keeping repayments at $1,580 a fortnight, the loan now clears in 240 months (20 years), 24 months sooner than the original schedule.
Interest saved: about $60,000 over the remaining life of the loan. The lump sum effectively earns a return equal to the mortgage rate (6%, after-tax-equivalent for an owner-occupier), which is hard to beat in any low-risk savings vehicle.
Things to keep in mind
- Fixed-rate loans have lump-sum limits. Most NZ banks allow up to 5% of the original loan balance per year as extra repayments without break-fee. Above that, partial break fees apply on a fixed-rate loan. Floating-rate loans accept any lump-sum without penalty.
- Lump-sum into offset has the same effect. An offset accountOffset accountSavings linked to a home loan; the balance reduces the interest charged without paying it down.View in glossary → reduces interest charged without paying principal down directly. The lump-sum effect is similar but reversible (you can spend the offset balance later); a lump-sum repayment is one-way.
- Tax-deductibility for rental loans. For owner-occupiers, mortgage interest is paid from after-tax income, so saving 6% interest is equivalent to a 6% pre-tax return. For rental-property loans, interest deductibilityInterest deductibilityPercentage of rental-mortgage interest a landlord can deduct against rental income.View in glossary → reduces the after-tax effective rate, so the saving is smaller in net terms.
- Cash flow vs interest saving. Lump-summing $20,000 into the mortgage is a permanent commitment. The cash is no longer available for emergencies, opportunities, or short-term needs. Some borrowers split lump sums between mortgage and emergency fund rather than applying the full amount.
NZ-specific notes
FAQs
Should I lump-sum my mortgage or invest instead?
The break-even depends on your mortgage rate, expected investment return, and tax treatment. For an owner-occupier, mortgage interest is paid from after-tax income; saving a 6% mortgage rate is equivalent to a 6% pre-tax return on a no-risk investment. Few easily-accessible investments beat that hurdle reliably. The calculator does not advise on the trade-off.
Will the bank allow a lump-sum on a fixed-rate loan?
Most NZ banks allow up to 5% of the original loan balance per year as extra repayments on a fixed loan, without break fee. Above that, a partial break fee applies. Floating-rate loans accept any lump-sum without penalty.
Does keeping repayments unchanged matter?
Yes. If you reduce the regular repayment to match the lower balance, the loan term stays the same and the interest saving is much smaller. Keeping repayments unchanged is what produces the dramatic time-and-interest saving.
How does this compare to refinancing?
Refinancing changes the rate; lump-summing changes the principal. The two can be combined: refinance to a lower rate, then lump-sum into the new loan. The break-fee calculator shows whether breaking is worth it; the lump-sum calculator shows the impact of the principal reduction.
Is lump-sum better than offsetting?
Both produce similar interest savings. Lump-sum is permanent (cash is gone). Offset is reversible (cash sits in a linked account, available if needed). Many borrowers prefer offset for emergency-fund flexibility, accepting a slightly lower effective rate of return because of the cash availability.
References & sources
- Reserve Bank of New Zealand, "Mortgage interest rates". rbnz.govt.nz
- Commerce Commission, "Lending decisions". comcom.govt.nz
- Inland Revenue, "Interest deductions on residential investment property". ird.govt.nz
- Te Ara Ahunga Ora Retirement Commission, "Sorted mortgage calculator". sorted.org.nz