Interest-Only Loan Calculator Australia
See the payment runway from an interest-only period into principal-and-interest repayments. Model different rates for each phase, optional extra repayments, fees, total interest and an immediate-amortisation comparison.
Loan phase assumptions
Payment runway
The modeled principal is unchanged when the interest-only phase ends.
How this interest-only loan calculator works
During the interest-only phase, the calculator multiplies the starting balance by the annual rate and divides by twelve. The displayed monthly payment covers modeled interest only, so the principal does not fall. Interest paid in this phase is that monthly amount multiplied by the entered interest-only months. This assumes every payment is made and the rate remains unchanged throughout the phase.
When the interest-only period ends, the full starting balance enters the principal-and-interest phase. The calculator uses the post-interest-only annual rate and the shorter remaining term to find the required amortising monthly payment. It then adds the optional extra payment and simulates the balance month by month. Each month, interest is added and the planned payment is subtracted, with the last payment capped at the amount needed to clear the balance.
Reading the two-phase results
The interest-only monthly payment is not a fully amortising repayment and is not a recommendation. “Still owing” displays the principal at the phase change. The required principal-and-interest payment is the amount that would repay that balance over the entered remaining years at the second rate. The planned payment adds your extra amount. The simulated payoff time can be shorter than the contractual principal-and-interest term when extra repayments are entered.
Total modeled interest combines interest from the interest-only phase and the simulated principal-and-interest phase. Total entered fees adds the upfront cash fee and prorates annual fees across the interest-only years and simulated payoff months. Total loan cash outflow is starting principal plus total interest plus fees. It is a cash-flow summary, not a comparison rate calculation and not a present-value measure.
The immediate principal-and-interest benchmark assumes the same starting balance is amortised from month one over the combined entered term, using the initial interest-only rate for the whole benchmark. It reports the benchmark payment and interest. The difference compares modeled staged-loan interest with benchmark interest. Because your post-period rate may differ, this is a scenario comparison rather than proof of the cost caused only by the interest-only feature.
Why repayments can jump
Consider a 30-year loan with five years interest only. After five years the principal can still be the original amount, but only 25 years remain to repay it. Even if the rate were unchanged, the new payment must include principal over the shorter period. If the interest rate also rises, both effects lift the repayment. A borrower who budgets only around the opening payment can face a substantial cash-flow shock.
Use the page to run a same-rate scenario, a higher post-period rate and a shorter remaining term if the loan contract requires it. Then compare the later payment with after-tax household cash flow, essential expenses and emergency savings. A lender’s serviceability assessment does not eliminate the need for a personal buffer, and this calculator does not conduct a responsible-lending assessment.
Owner-occupiers and investors
For an owner-occupier, paying interest without reducing principal can delay equity building and leave greater exposure to a property price fall. Refinancing later is not guaranteed. Income, expenses, credit policy, valuation and interest rates can change. If the property must be sold, transaction costs and a lower sale price can affect the amount available to clear the debt.
Investors sometimes consider interest-only borrowing for cash-flow or tax reasons. Tax outcomes depend on purpose, use of funds, ownership, redraws and individual circumstances. The calculator does not estimate deductions, taxable income, capital gains or negative gearing. A tax benefit should not be entered as if it were guaranteed repayment capacity. Obtain independent tax advice and preserve records tracing borrowed funds.
Extra repayments, offsets and redraw
The extra payment field applies only after the interest-only phase. Some interest-only facilities permit principal reductions, but doing so may change scheduled interest and available redraw. Check the contract before assuming an extra payment is accepted without charge or remains accessible. If you expect principal reductions during interest only, model a lower balance in a second calculation after the planned reduction date.
An offset account can reduce interest when its balance is contractually linked to the loan. This page has no offset input because daily offset balances and product rules can be complex. A simple way to test a stable effective offset is to run a separate scenario using loan balance minus the assumed offset, while remembering that the legal principal and liquidity position remain different. Do not subtract an offset and also treat its interest saving as an extra repayment.
Interest-only loan scenario table
| Scenario | Input change | Risk or question exposed |
|---|---|---|
| Payment reset | Keep balance, shorten remaining term | How much does principal repayment add? |
| Rate stress | Increase post-period rate by two or three points | Can cash flow absorb rate and amortisation together? |
| Extra repayment | Add a realistic monthly amount in the P&I phase | How much sooner might the modeled balance clear? |
| Immediate P&I | Use the benchmark shown | What opening payment and total interest provide a reference? |
| Fee comparison | Enter quoted upfront and annual fees | Does a lower headline rate still have the lower cash cost? |
Before selecting an interest-only product
Moneysmart recommends comparing home loans and examining features, fees, rates and repayment types. Ask how long the interest-only period lasts, whether the overall loan term is unchanged, what rate applies in each phase, whether the rate is fixed or variable, and how the principal-and-interest repayment will be calculated. Ask whether extra repayments, redraw and offset are available and whether fees or break costs apply.
Confirm the loan purpose and security, repayment frequency, rate-change notice, default rate and hardship contact process. Read the credit contract and key facts sheet where applicable. Do not rely on refinancing, future income, property growth or sale proceeds without a contingency. If repayments become difficult, contact the lender early and use reputable financial counselling or hardship support.
Limits of the calculation
Calculations are monthly and rates are nominal annual inputs divided by twelve. Real lenders may calculate interest daily, apply different day counts, change rates during either phase, round payments, debit fees at different times or set a final adjustment. The annual fee estimate is prorated for comparison even if a full fee is charged at an anniversary. Results exclude stamp duty, mortgage insurance, conveyancing, property costs and tax.
The simulation is most useful as a transparent sensitivity model. Reconcile it with the lender’s repayment schedule and actual statements. A small difference may reflect daily interest or rounding; a large difference can reveal a different term, rate, fee, repayment type or balance. Keep each scenario’s input date because product pricing changes.
Frequently asked questions
Does an interest-only payment reduce the loan balance?
Not in this model. The payment covers interest, leaving the starting principal unchanged until principal-and-interest or separate principal repayments begin.
Why is the later repayment higher?
The full balance must be amortised over the shorter remaining term, and the post-period rate may also be higher. Both can increase the monthly amount.
Is the immediate P&I benchmark a product quote?
No. It is a mathematical comparison that amortises the starting balance over the combined term at the initial rate. A real quote can use different rates, fees and terms.
Does the calculator include an offset account?
No. Offset benefits depend on daily balances and product rules. Use lender statements or a dedicated daily model and avoid counting the same saving twice.
Can investors claim all interest as a tax deduction?
Not automatically. Deductibility depends on use of borrowed funds and circumstances. This page provides no tax classification or estimate; obtain qualified advice.
What happens when the interest-only period finishes?
Under the entered structure, principal-and-interest repayments begin on the full balance for the remaining term. Check the contract, because refinance, extension or renewal is not guaranteed.