A worked example
Megan from Perth borrows $650,000 to buy a house in Bentley. Her lender offers a fixed rate of 6.19% p.a. for three years, and she chooses a 30-year loan term with monthly repayments.
Using the calculator, she enters:
Loan amount: $650,000
Interest rate: 6.19% p.a.
Loan term: 30 years
Repayment frequency: Monthly
The calculator shows her monthly repayment is $3,962. Over 30 years, she'll repay a total of $1,426,320, which means $776,320 in interest alone.
Megan then switches the repayment frequency to fortnightly. Her fortnightly repayment becomes $1,828 (roughly half the monthly amount). Because there are 26 fortnights in a year, she actually pays slightly more each year ($47,528 vs $47,544 monthly). This small difference shaves about seven months off her loan term and saves around $18,000 in interest. The calculator's amortisation schedule shows her exactly when she'll be mortgage-free under each scenario.
State-by-state differences
Mortgage repayment calculations work the same across all Australian states, but your borrowing capacity and upfront costs vary significantly by location:
- NSW: Stamp duty is among the highest in Australia. First home buyers can access exemptions or concessions on properties up to $800,000 (full exemption up to $650,000 as at 2025-26). This affects how much deposit you need upfront.
- VIC: Similar stamp duty settings to NSW, with first home buyer exemptions to $600,000 and concessions to $750,000. Regional Victoria often has lower median prices, affecting typical loan amounts.
- QLD: Lower stamp duty than southern states. First home buyers pay no transfer duty on homes up to $500,000. Brisbane and Gold Coast prices have climbed substantially in recent years.
- WA: Stamp duty concessions for first home buyers on properties up to $530,000. Perth's median dwelling price sits lower than Sydney or Melbourne, so typical loan amounts are often smaller.
- SA: First home buyers can access stamp duty concessions on homes up to $650,000. Adelaide's lower median prices mean many buyers borrow $400,000-$550,000.
- TAS: Duty concessions available. Hobart prices rose sharply post-COVID but remain below mainland capitals.
- ACT: No stamp duty; instead, higher annual rates. This changes the upfront vs ongoing cost balance.
- NT: Lower property prices, smaller typical loans, and different First Home Owner Grant settings.
Common mistakes people make
- Forgetting to include all borrowing costs in the loan amount. Many people calculate repayments on the property price alone, but your actual loan includes stamp duty (if you're capitalising it), lender's mortgage insurance if your deposit is under 20%, and other settlement costs. Always use your total loan amount, not just the purchase price.
- Comparing rates without checking the comparison rate. A headline rate of 5.99% looks better than 6.19%, but the comparison rate (which includes most fees) might reveal the first loan is actually more expensive. The comparison rate is required disclosure in Australia and gives a clearer picture of true cost.
- Ignoring the impact of offset accounts when choosing repayment frequency. If you have a 100% offset account, parking your salary there reduces interest daily. Switching from monthly to weekly repayments has less benefit because your money isn't sitting in offset as long. Run the numbers both ways before committing.
- Assuming advertised rates apply to them. The rate you actually get depends on your deposit size (LVR), whether you're owner-occupier or investor, and your credit history. A 5.89% advertised rate might only apply to loans with 30%+ deposit. Use your actual approved rate in the calculator.
- Not factoring in future rate rises on variable loans. If you're on a variable rate, model what happens if rates increase by 0.5% or 1%. The RBA cash rate has moved substantially in recent years, and your repayments will too.
What this calculator doesn't account for
This calculator shows scheduled principal and interest repayments based on the inputs you provide. It does not account for:
- Interest-only periods: Many investors or borrowers choose interest-only for the first few years, then switch to principal and interest. This calculator assumes P&I from day one.
- Redraw facilities or offset accounts: These reduce the interest charged on your loan balance, but the calculator assumes a standard amortisation schedule without extra payments sitting in offset.
- Variable rate changes: If you have a variable loan, your rate will change over the loan term. The calculator uses a fixed rate assumption throughout.
- Fees and charges: Monthly account-keeping fees, annual package fees, and discharge fees are not included in the repayment figures.
- Lender-specific features: Some loans have unique structures (split loans, multiple offset accounts, or loyalty discounts) that alter the effective cost.
- Early repayment benefits: The calculator can show you the effect of a fixed extra payment each period, but not irregular lump sums or varying extra amounts.
Edge cases and nuances
Some scenarios produce misleading results if you rely solely on the standard calculator output:
Construction loans: If you're building, you draw down the loan in stages as construction progresses. You pay interest only on the drawn amount during construction, then switch to principal and interest once complete. A standard mortgage calculator will overstate your repayments during the construction phase.
Split loans: Many Australians split their loan into fixed and variable portions (say, 50/50). Each portion has a different rate and may have different repayment terms. You need to calculate each split separately, then add the repayments together.
Low-doc or alt-doc loans: Self-employed borrowers or those with non-standard income often pay a rate premium of 0.5% to 1.5% above standard rates. This significantly increases repayments and total interest compared to standard full-doc loans.
Bridging loans: If you're buying before you sell, bridging finance usually involves interest-only repayments on both loans until your sale settles. The calculator won't model the short-term cashflow crunch accurately.
Non-resident borrowers: Australian lenders typically charge non-residents 0.5% to 2% higher rates and require larger deposits (often 20-30% minimum). If you're a temporary resident or foreign buyer, don't rely on standard advertised rates.
