Remaining Loan Term: The Mortgage Calculator Input That Changes the Answer

Short answer

For an existing mortgage, enter the time left to repay the loan, together with the current balance and the rate being tested. The original term describes the loan at its start. Reusing it years later can understate the monthly payment needed to clear today's balance by the agreed end date.

Try the Mortgage Repayment Calculator with the term from the loan statement or lender. Keep that term unchanged when comparing two interest rates. Change it only when deliberately testing a different repayment period.

Three dates that are easy to confuse

The third entry does not mean the whole loan must be repaid in 18 months. It describes the rate arrangement. A split loan can have different rates, end dates and repayment details for each portion.

Subtracting elapsed years from the original term is only a starting check. A refinance, approved term change or other alteration can make it wrong. Use the current contractual details, especially if payments were paused or the account was restructured.

Worked example: the same debt across three terms

Suppose a borrower is modelling $600,000 at a constant 6%, paid monthly as principal and interest. The figures below use the same balance and rate in every row. These are invented scenarios, not available loan offers.

If the actual remaining term is 25 years, entering 30 years produces a payment about $269 a month lower. The calculation has given the borrower five extra years to repay. Over those modelled periods, total interest is about $135,287 higher in the 30-year case.

That comparison does not mean every refinance increases interest or that a shorter term is suitable for a particular household. It isolates one input. Real choices can involve different rates, fees, repayment amounts, cash needs and later changes.

How the example is calculated

The monthly model uses M = P × i ÷ (1 − (1 + i)^−n), where P is the balance, i is the annual percentage rate divided by 100 and then by 12, and n is the number of monthly payments. At 6%, i is 0.005; 25 years gives 300 payments.

Total modelled interest is the unrounded monthly payment multiplied by the number of payments, less the opening balance. Monthly figures are shown to cents and interest totals to the nearest dollar. Differences are calculated before rounding, so subtracting displayed totals can differ by a dollar.

The example assumes a constant rate, equal monthly periods, no fees, no offset and no extra payments. Actual lenders may calculate interest daily and use different payment timing. This is a comparison model, not an amortisation schedule for a specific contract.

Check a refinance on two bases

First compare the existing and proposed rates over the same remaining term. This helps show the effect of the rate and fees without also extending the loan.

Then, if a different term is genuinely proposed, run that separately and label it. Record the monthly payment, total modelled interest, fees and repayment end date. A lower payment alone does not show a lower overall cost.

ASIC Moneysmart's mortgage switching calculator asks for the current balance and term remaining. Its published assumptions calculate payments to clear the loan over that period. The site's refinancing checklist helps organise the separate switching costs and documents.

Common input mistakes

• Using the original amount borrowed instead of the current outstanding balance.
• Resetting a 25-year balance to 30 years when the only intended change is the rate.
• Using a two-year fixed period as the amortisation term for a much longer loan.
• Subtracting offset savings from the balance when trying to reproduce a contractual minimum repayment without checking the lender's method.
• Calling constant-rate lifetime interest a forecast, even though later rates can change.

For a rate notice, use the home loan rate change checklist. For a dated policy example, the September 2026 cash rate update holds the term constant when showing a 25 basis point loan-rate scenario.

Sources and limitations

• ASIC Moneysmart: Mortgage switching calculator, term-remaining input and modelling assumptions checked 1 October 2026.
• ASIC Moneysmart: Mortgage calculator, principal-and-interest calculator scope checked 1 October 2026.
• The three-term table is original amortisation arithmetic. No tax threshold, lender eligibility rule or advertised product rate is used.

General information and estimates only, not financial, legal, tax or credit advice, a loan quote, approval or recommendation to refinance. Confirm the repayment period and account details with the lender. Seek appropriately licensed advice for personal circumstances.

Last updated: 1 October 2026.

Editorial illustration: a conceptual image, not a photograph or market chart.