How is compound interest calculated?
Compound interest is the addition of interest to the principal balance of a loan — in plain terms, you pay interest on interest. On a home loan this is simply how the maths works, and once you understand the mechanism you can structure your repayments to use it in your favour rather than against you. The difference between a passive borrower and a deliberate one, over the life of a loan, is frequently measured in hundreds of thousands of dollars.
Interest on a home loan is generally calculated daily on the outstanding balance, then charged monthly. What you actually pay depends on a handful of variables: the loan amount, the interest rate, the loan term, the repayment frequency, and whether you make extra repayments or hold funds in an offset account where your loan offers one.
The daily mechanism
The calculation itself is straightforward. A lender multiplies your outstanding balance by your interest rate, then divides by 365 days — some lenders divide by 366 in a leap year. That gives the daily interest charge. At the end of the month, the daily charges are added together, and that total is the monthly interest you see on your statement.
Consider a loan of $400,000 at an indicative annual rate of 3.5% — your own rate may sit higher or lower. Multiply $400,000 by 0.035, divide by 365, and the loan accrues roughly $38.36 in interest every day.
Because the charge is struck daily on the outstanding balance, the lever is the balance itself. The smaller the balance on any given day — or the lower the rate — the less interest accrues, and the faster you begin retiring principal rather than servicing interest. That is the whole game, and it is why every dollar you keep off the balance works for you from the day it lands.
Structuring repayments to use the mechanism
A few structural choices change the trajectory of the loan, and they compound on each other.
- Extra repayments. On the $400,000 loan above, an additional $300 per month can cut more than a decade off the term. Lift that to an extra $1,000 per month and the effect is larger again. The principle is constant: every additional dollar reduces the daily balance interest is calculated on, for every day that follows.
- Offset accounts. An offset account makes balance reduction effortless, because your savings are counted against the loan balance without being paid into the loan. Hold $100,000 in offset against the $400,000 loan and interest is calculated on $300,000 — yet the $100,000 remains yours to access.
- Fortnightly repayments. Because interest is calculated daily, paying fortnightly keeps the balance lower across the month. It also slips in an extra month's worth of repayments each year, since fortnightly instalments add up to thirteen monthly equivalents rather than twelve.
None of these requires a windfall. They are decisions about repayment frequency and where your savings sit — structure, not luck.
Modelling it before you commit
The figures above are illustrative, and the right combination depends on your loan, your cash flow and your lender's policy on offset, redraw and extra repayments. The point of understanding the daily mechanism is that it lets you model the trade-offs deliberately: how much an offset balance is worth to you, whether fortnightly repayments suit your income cycle, and how extra repayments reshape the term. That modelling is where a loan stops being a default arrangement and becomes a structured one.
Book a strategy session and we will work through how your loan is structured and where the daily mechanism can be put to work.
General information only — not personal financial product or credit advice. Loan features, offset availability and interest calculation methods vary by lender and are subject to each lender's policy and your full circumstances. AeFin is an Australian Credit Representative (CR 464548) of Finsure (ACL 384704).