How this extra repayment estimate works
The calculator starts with the standard amortising repayment for the loan amount, annual rate, term and selected payment frequency. It then models each scheduled period with any recurring extra repayment, one-off lump sum and offset balance entered.
The baseline and strategy use the same starting loan and constant annual rate. That keeps the comparison focused on the entered extras and offset rather than an assumed future rate path.
For an offset balance, the model applies periodic interest to the loan principal less the available offset amount, with the effective balance limited to zero. Extra repayments and lump sums reduce principal. Monthly offset contributions build the modelled offset balance as the schedule progresses.
Important timing limitation
This is an indicative periodic model. Many Australian home loans calculate interest daily and charge it monthly. A lender can also apply payments, offset deposits, fees and rate changes at different times from this simplified schedule.
The result can therefore differ from a lender statement even when the starting balance and annual rate match. Check the loan contract and lender ledger before relying on the estimated interest or payoff date.
Illustrative example
Assume a $500,000 principal-and-interest loan at 6.5% over 30 years with monthly repayments. Compare the baseline with a scenario that adds $200 to each monthly payment, starts with $10,000 in offset and adds $200 to the offset each month.
The strategy should produce less modelled interest and a shorter term because less principal is exposed to interest and more principal is repaid earlier. The calculator quantifies that difference using the assumptions entered. It does not predict future rates or reproduce a particular lender product.
Common mistakes
- Treating an offset as a repayment. Offset money can remain accessible, subject to the product terms. It does not reduce the legal loan principal in the same way as a repayment.
- Ignoring fixed-loan restrictions. A fixed-rate loan may cap extra repayments or charge break costs. Check the contract before making a large payment.
- Assuming every offset is 100%. Product rules, account links, fees and rate premiums can change the benefit.
- Comparing different payment amounts as if frequency alone caused the saving. Paying half a monthly amount every fortnight creates 26 half-payments each year, equivalent to 13 monthly payments.
- Relying on a constant rate for a long forecast. Test more than one interest-rate scenario and update the inputs when the lender changes the rate.
ASIC Moneysmart explains the practical differences in paying off a mortgage faster and mortgage offset accounts. Sources checked 1 September 2026.
This calculator provides a general estimate based on the assumptions entered. It is not a lender statement, product quote, approval, recommendation or financial advice.
