Home loan interest in Australia is almost always calculated daily and compounded — but most borrowers think it works monthly. That misunderstanding costs people money, because the way interest is calculated determines how every extra dollar in your offset or extra repayment actually works.
Here's exactly how the math works on a standard Australian home loan.
The Basic Daily Interest Formula
Daily interest = (Loan balance × Annual interest rate) ÷ 365
Each day, the bank calculates interest on your current loan balance, divides that by 365, and adds it to a running tally.
At the end of the month, the accumulated daily interest is charged to your loan account — that's the figure you see on your monthly statement.
Example: $600,000 Loan at 6.5%
| Calculation | Amount |
|---|---|
| Loan balance | $600,000 |
| Annual rate | 6.5% |
| Daily interest = $600,000 × 6.5% ÷ 365 | $106.85/day |
| Monthly interest (30 days) | $3,205.48 |
Why Daily Calculation Matters
Because interest is calculated on your current balance every day, anything that reduces that balance — even temporarily — saves interest.
This is how offset accounts work. If you have $30,000 in your offset, the bank treats your loan balance as $570,000 (not $600,000) when calculating interest each day. That saves you $5.34/day or about $1,950/year — even though you haven't paid a cent off the loan.
Principal & Interest Repayments Explained
Each monthly P&I repayment is split into two parts:
- Interest portion — covers the month's accumulated interest
- Principal portion — reduces the loan balance
Early in the loan, the interest portion is much bigger than the principal portion. Over time, as the balance falls, the split shifts.
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The Power of Extra Repayments
Because interest is daily, extra repayments aren't just "future debt reduction" — they immediately reduce tomorrow's interest.
An extra $100/week on a $600,000 loan at 6.5% saves approximately $80,000 in total interest and shortens the loan by 5+ years. The earlier you start, the bigger the impact, because that money compounds against itself.
Fixed vs Variable Rates
Fixed-rate loans calculate interest the same way — daily on the balance — but the rate is locked. With a fixed loan, RBA rate changes don't affect your repayments during the fixed period.
Variable rate loans are subject to changes whenever the lender adjusts. When the RBA changes the cash rate, lenders typically pass on changes within 2–4 weeks.
How Banks Display This on Statements
Your monthly statement usually shows:
- Opening balance
- Repayments received
- Interest charged for the month
- Closing balance
The interest figure is the sum of every day's interest in that statement period — calculated on the running daily balance.
Bottom Line
Daily compounding is what makes offsets, extra repayments, and rate reductions so powerful. Even small balance reductions, sustained over time, save real money.
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