Calculator · Central Lending Solutions
Compound Interest Calculator
Compound interest is interest earned on your original balance plus all the interest already added to it, so the balance grows faster every period. It works for you in savings and offset accounts and against you on a Perth home loan, where a typical $500,000 mortgage at 6.45% over 30 years costs around $635,000 in repayments and $135,000 in compounded interest.
Why this matters
What this calculator is actually telling you.
This calculator shows the future value of a balance when interest is added to the principal each period and then earns interest itself in the next period. The same maths drives every savings account, term deposit, offset account and home loan in Australia. Change the rate, the contribution and the compounding frequency and you can see exactly how the curve bends, because compounding is not linear, it accelerates over time.
For a Perth household the most useful framing is the comparison between three places to put a spare dollar. A high-interest savings account at around 4.50% pays interest you then pay tax on. A term deposit locks in a rate but is also taxed. An offset account linked to your home loan effectively earns the loan rate, currently around 6.45% on a Perth variable mortgage in 2026, and it is tax-free because you are avoiding interest rather than earning it. For someone on the 37% marginal rate, a 6.45% tax-free offset is the same as roughly 10.24% in a savings account before tax. At the top 45% bracket plus Medicare it is closer to 12% gross. There is no investment product on the market that beats that risk-free return.
The other lesson the calculator makes obvious is how heavily front-loaded a Perth mortgage is. On a $500,000 loan at 6.45% over 30 years, more than two-thirds of your repayments in the first five years go to interest, not principal. That is why every extra dollar you put against the loan in years 1 to 7 has roughly three times the lifetime impact of the same dollar in year 25. An extra $200 a month into the offset from day one of a $500,000 Perth loan can clear the mortgage around five years early and save in the order of $90,000 in compounded interest.
Use the calculator to test scenarios, then bring the numbers to a broker who can show you what your actual offset, redraw or repayment structure looks like across more than 30 lenders. The structure of the loan often matters more than the headline rate.
Questions
Frequently asked.
How does compound interest actually work?
Compound interest is calculated on the original balance plus any interest already added. Each period the interest is added to the balance, then the next period's interest is calculated on the new, larger balance. The more often interest compounds, the faster the balance grows. Most Australian home loans compound interest monthly, most savings accounts compound monthly or daily.
What is the difference between simple and compound interest?
Simple interest is calculated only on the original principal, so it grows in a straight line. Compound interest is calculated on the principal plus all previously added interest, so it grows on a curve that gets steeper over time. Over 30 years on a $500,000 balance at 6.45%, the difference between the two methods is hundreds of thousands of dollars.
How does compound interest work on my Perth home loan?
On a typical Perth home loan, interest is calculated daily on the outstanding balance and charged to the loan monthly. Because the unpaid interest is added to the balance you owe, you then pay interest on that interest in the following month. On a $500,000 loan at 6.45% over 30 years, total repayments come to around $635,000, with roughly $135,000 of that being compounded interest. Anything that lowers the daily balance, such as an offset account or extra repayments, reduces the interest charge that compounds the following month.
Is an offset account better than a high-interest savings account?
For a Perth homeowner with a mortgage, almost always yes. Money sitting in an offset linked to a 6.45% home loan saves you 6.45% in interest, tax-free. A high-interest savings account in 2026 pays around 4.50% before tax. At a 37% marginal rate, that 4.50% savings rate is worth about 2.84% after tax, while the 6.45% offset return is the equivalent of earning roughly 10.24% in a taxable account. The offset wins on rate, on tax treatment and on risk.
How much do extra repayments actually save on a Perth mortgage?
On a $500,000 Perth loan at 6.45% over 30 years, adding an extra $200 a month from day one clears the loan around five years early and saves roughly $90,000 in interest over the life of the loan. Adding $500 a month clears it almost nine years early and saves close to $170,000. The same extra repayments made in year 20 instead of year 1 have less than a third of that impact, because there is less remaining balance left to compound against.
Where do I go for help applying this to my own situation?
Call Central Lending Solutions on 0489 082 257 or book a free chat. A Perth broker will model the compound interest impact of an offset account, extra repayments or a refinance against your actual loan balance and rate, comparing structures across more than 30 lenders. There is no fee for most clients and no obligation to switch.
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