How to Use a Mortgage Calculator to Compare Home Loan Options in Australia

Buying a home is likely the biggest financial commitment you’ll ever make, and choosing the right home loan can save you tens of thousands of dollars over the life of the loan. With hundreds of loan products on the market from dozens of lenders, comparing them manually is nearly impossible. That’s where a mortgage calculator comes in. In this guide, we’ll walk you through exactly how to use a mortgage calculator to compare home loan options in Australia, understand the true cost of borrowing, and make an informed decision before you apply.


Why Use a Mortgage Calculator?

A mortgage calculator is an essential tool for any home buyer or refinancer. It allows you to model different loan scenarios by adjusting variables like loan amount, interest rate, loan term, and repayment frequency. According to the Reserve Bank of Australia (RBA), the average variable interest rate for new owner-occupier home loans was 6.25% p.a. in January 2025, but rates can vary significantly between lenders and loan types. A mortgage calculator helps you:

• Estimate your monthly, fortnightly, or weekly repayments
• Compare the total interest payable over the life of the loan
• See the impact of extra repayments or offset accounts
• Understand how changes in interest rates affect your budget
• Determine your borrowing power based on your income and expenses

By using a calculator before you apply, you can narrow down your options and avoid applying for loans you can’t afford or that don’t suit your needs.

Step 1: Gather Your Financial Information

Before you start using a mortgage calculator, you’ll need to have some key figures ready:

• Loan amount: The amount you plan to borrow. This is typically the property price minus your deposit. For example, if you’re buying a $700,000 home with a 20% deposit ($140,000), your loan amount would be $560,000.
• Interest rate: You can use current market rates from comparison sites or lender websites. As of early 2025, variable rates for owner-occupiers range from about 5.75% to 6.50% p.a., while fixed rates for 1–3 years are slightly higher.
• Loan term: Most Australian home loans have a term of 25 or 30 years.
• Repayment frequency: Monthly is standard, but many calculators let you compare fortnightly or weekly repayments, which can reduce total interest.
• Fees: Some calculators allow you to include upfront and ongoing fees, such as application fees ($200–$600), annual fees ($0–$400), or monthly service fees.

Having accurate figures ensures your comparison is realistic. Don’t forget to factor in lenders mortgage insurance (LMI) if your deposit is less than 20%, which can add thousands to your loan.

Step 2: Compare Repayment Scenarios

Once you have your numbers, you can start plugging them into a mortgage calculator. Let’s look at a practical example comparing two different loan options for a $560,000 loan over 30 years.

Loan Feature	Option A	Option B
Interest rate	6.00% p.a.	6.25% p.a.
Monthly repayment	$3,357	$3,448
Total interest payable	$648,520	$681,280
Total cost of loan	$1,208,520	$1,241,280

As you can see, a difference of just 0.25% in the interest rate adds $91 more to the monthly repayment and over $32,000 in extra interest over 30 years. This illustrates why even small rate differences matter.

Many calculators also let you switch between principal-and-interest and interest-only repayments. Interest-only loans can have lower initial repayments but cost more in the long run because you’re not reducing the principal. For an investment property, interest-only might be tax-effective, but for owner-occupiers, principal-and-interest is usually the better choice.

Fortnightly vs Monthly Repayments

Another powerful feature is comparing repayment frequencies. If you opt for fortnightly repayments (half the monthly amount every two weeks), you’ll make the equivalent of 13 monthly payments per year instead of 12, because there are 26 fortnights in a year. This can shave years off your loan and save a significant amount in interest. Using the same $560,000 loan at 6.00% p.a.:

• Monthly repayments: $3,357/month, total interest $648,520
• Fortnightly repayments: $1,678/fortnight, total interest $538,200, loan paid off in 25 years and 9 months

That’s a saving of over $110,000 in interest and nearly 4.5 years off the loan term.

Step 3: Factor in Fees and Features

Interest rates aren’t the only cost. A good mortgage calculator will let you include upfront and ongoing fees. Some loans offer a lower rate but charge high annual fees, which can negate the savings. For example, a loan with a 5.90% rate and a $400 annual fee might end up costing more than a 6.10% loan with no annual fee, depending on the loan size.

Also consider features like:

• Offset account: A 100% offset account reduces the interest you pay by offsetting your savings against your loan balance. If you have $20,000 in an offset account on a $560,000 loan at 6.00%, you only pay interest on $540,000. Some calculators let you model this.
• Redraw facility: Allows you to withdraw extra repayments if needed, but may have fees.
• Split loans: You can fix a portion of your loan and keep the rest variable. Advanced calculators let you compare split scenarios.

When comparing loans, use the calculator to see the total cost over the loan term, including all fees. This is often called the comparison rate, which lenders are required to display alongside the advertised rate. The comparison rate combines the interest rate with most fees into a single percentage, making it easier to compare apples to apples.

Step 4: Stress Test Your Budget

Interest rates can change, so it’s wise to stress test your repayments. Use the calculator to see what your repayments would be if rates rose by 1%, 2%, or even 3%. For example, on a $560,000 loan at 6.00%, monthly repayments are $3,357. If rates rose to 8.00%, repayments would jump to $4,109 – an increase of $752 per month. The Australian Prudential Regulation Authority (APRA) requires lenders to assess borrowers at an interest rate buffer of at least 3 percentage points above the loan’s rate, so it’s smart to do the same for yourself.

You can also use the calculator to see how much you could save by making extra repayments. Even an extra $100 per month can cut years off your loan. On the same $560,000 loan at 6.00%, adding $100 extra per month reduces the total interest by about $47,000 and pays off the loan 4 years and 3 months earlier.

Step 5: Use Multiple Calculators for Different Purposes

Not all mortgage calculators are the same. Different types serve different needs:

• Repayment calculator: Shows your regular repayment amount based on loan amount, rate, and term.
• Borrowing power calculator: Estimates how much you can borrow based on your income, expenses, and debts. Lenders use their own criteria, but this gives a rough idea.
• Comparison rate calculator: Calculates the true cost of a loan including fees.
• Split loan calculator: Compares fixed and variable portions.
• Stamp duty calculator: Estimates the upfront stamp duty cost, which varies by state and property value.

For a comprehensive comparison, you might start with a borrowing power calculator to determine your budget, then use a repayment calculator to compare loan options, and finally a stamp duty calculator to factor in all upfront costs.

Common Mistakes to Avoid

Even with a calculator, it’s easy to misinterpret the results. Here are some pitfalls:

• Ignoring fees: Always check the comparison rate, not just the headline rate.
• Assuming rates will stay the same: For variable loans, model different rate scenarios.
• Forgetting about LMI: If your deposit is less than 20%, LMI can cost several thousand dollars and is usually added to the loan.
• Not accounting for life changes: Consider if you might have children, change jobs, or need to renovate – all affecting your ability to repay.
• Using unrealistic expenses: Be honest about your living costs. Lenders will scrutinize your expenses, so your estimate should be accurate.

How to Find a Reliable Mortgage Calculator

You can find mortgage calculators on most bank and lender websites, as well as on independent comparison sites like Canstar, RateCity, and Mozo. Government sites like ASIC’s MoneySmart also offer unbiased calculators. When using a calculator, ensure it’s from a reputable source and updated with current interest rates. Some calculators allow you to input custom rates, which is useful for comparing specific loan products.

Next Steps After Using a Mortgage Calculator

Once you’ve narrowed down your options using a calculator, you can:

1. Get pre-approval: This gives you a clearer idea of your borrowing limit and shows sellers you’re serious.
2. Consult a mortgage broker: A broker can access a wide range of loans and may find deals not available directly to consumers. They can also help you understand the fine print.
3. Read the product disclosure statement (PDS): Before committing, understand all fees, features, and conditions.
4. Apply formally: Once you’ve chosen a loan and have a property in mind, submit your application with all required documents.

Remember, a mortgage calculator is a starting point. It provides estimates, not guarantees. Your actual repayments may vary based on the lender’s assessment, your credit history, and market conditions.

FAQ

What is the difference between a comparison rate and an advertised rate?

The advertised rate is the interest rate charged on the loan. The comparison rate includes both the interest rate and most fees (like application and ongoing fees) expressed as a single percentage. It helps you see the true cost of the loan. For example, a loan with a 5.90% advertised rate but high fees might have a comparison rate of 6.20%, while a no-fee loan at 6.00% might have a comparison rate of 6.00%.

Can a mortgage calculator tell me exactly how much I can borrow?

No, a calculator gives an estimate based on the information you provide. Lenders assess your borrowing capacity using their own criteria, including your credit score, employment stability, and detailed living expenses. Use a borrowing power calculator as a guide, but expect the actual amount to be confirmed during pre-approval.

How often should I use a mortgage calculator?

You should use a mortgage calculator whenever you’re considering a new loan, refinancing, or if interest rates change significantly. Regularly reviewing your loan can help you spot opportunities to save by switching to a better rate or making extra repayments.

Do mortgage calculators work for investment properties?

Yes, but you may need to adjust settings. Investment loans often have higher interest rates and you might choose interest-only repayments. Some calculators have a specific mode for investors. Also, remember that interest on investment loans may be tax-deductible, which isn’t reflected in a standard calculator.

Are online mortgage calculators accurate?

They are accurate for the inputs you provide, but they assume a constant interest rate and don’t account for future rate changes, fees not included, or changes in your financial situation. Always treat the results as estimates and confirm with a lender or broker before making decisions.

References

1. Reserve Bank of Australia – Statistical Tables: Housing Lending Rates (2025). https://www.rba.gov.au/statistics/tables/
2. Australian Securities and Investments Commission – MoneySmart Mortgage Calculator. https://moneysmart.gov.au/home-loans/mortgage-calculator
3. Canstar – Home Loan Comparison (2025). https://www.canstar.com.au/home-loans/
4. RateCity – Home Loan Calculators and Comparison. https://www.ratecity.com.au/home-loans/calculators
5. Australian Prudential Regulation Authority – Prudential Practice Guide APG 223: Residential Mortgage Lending (2024). https://www.apra.gov.au/