Mortgage Offset Calculator: See How Much Interest You Could Save
An offset calculator shows how much interest and time you can cut off a home loan by keeping money in a linked offset account instead of paying it straight into the loan. Enter your loan and offset details into the offset calculator below and it will estimate your interest savings and how much sooner you could be debt-free.
What Is an Offset Account and How Does It Work?
A mortgage offset account is a transaction account linked to your home loan. Instead of earning interest itself, the balance in that account is subtracted from your loan balance before your lender calculates daily interest. If you owe $500,000 and keep $40,000 in a linked 100% offset account, your lender only charges interest on $460,000.
The money in the offset account isn’t paid into the loan, so you can still withdraw it at any time — unlike making an extra repayment, which reduces your loan balance directly but is often harder or slower to get back out. For an independent, non-bank explanation of how offset accounts work, ASIC’s MoneySmart offset accounts guide is a useful starting point before you rely on this offset calculator for your own numbers.
100% Offset vs Partial Offset
Most Australian home loans with an offset feature are “100% offset,” meaning every dollar in the account offsets a dollar of the loan balance. Some older loans or specific products offer a partial offset — commonly 40% or 50% — where only part of your balance reduces the interest-bearing loan amount. The offset percentage field in the calculator above lets you model either scenario; if you’re not sure which you have, check your loan contract or ask your lender, since it changes your savings significantly.
How Offset Interest Is Calculated
Lenders typically calculate offset mortgage interest daily, using this formula:
Daily interest = (Loan balance − Offset balance) × (Annual interest rate ÷ 365)
That daily interest is then totalled and usually charged to the loan monthly. Because the calculation happens daily, any change in your offset balance — a pay cheque landing, a bill being paid — immediately affects that day’s interest, even before you make a scheduled repayment. This is exactly how the offset account calculator above works out your savings: it recalculates the effective loan balance every month rather than treating your offset balance as a one-off discount.
Worked Example: Static Offset vs Growing Offset
Say you have a $500,000 loan at 6.00% p.a. with 25 years remaining, and $40,000 sitting in your offset account.
- Without offset: interest is calculated on the full $500,000 balance.
- With a static $40,000 offset: interest is calculated on $460,000 for as long as that balance stays roughly the same.
- With the same $40,000 plus a $500 monthly top-up: the interest-bearing balance shrinks a little further every month, so the savings compound faster than a static offset balance alone.
Because your scheduled repayment stays the same in both offset scenarios, less of each repayment goes toward interest and more goes toward paying down the principal. Over the life of the loan, this compounding effect can mean tens of thousands of dollars in interest savings and paying off the loan years earlier — the exact figures depend on your loan size, rate, term, and offset balance, which is what this offset calculator works out for your own numbers.
Is an Offset Account Right for You?
An offset account tends to suit people who already keep a meaningful cash buffer, rather than borrowers who spend down to zero every pay cycle. It’s worth considering if you fit one or more of these situations:
- You keep savings for emergencies or upcoming expenses. Rather than that cash sitting idle, it reduces your loan interest while remaining fully accessible.
- Your income is irregular (self-employed, commission-based, or contract work), so you want a buffer that still works for you between pay cycles.
- You hold an investment property loan and want to keep your personal savings separate from the loan balance for interest-deductibility reasons.
- You expect to need the funds again — for a renovation, a car, or a deposit on the next property — so locking the money into the loan via extra repayments isn’t ideal.
If you tend to carry little or no spare cash, or your loan doesn’t offer an offset feature without a costly package fee, a low-rate loan with no offset (or making extra repayments instead) may work out better for you.
Offset vs Redraw vs Extra Repayments
These three features are often confused, but they behave quite differently:
| Feature | Offset Account | Redraw Facility | Extra Repayments |
|---|---|---|---|
| Where funds sit | Separate linked transaction account | Paid directly into the loan, withdrawn on request | Paid directly into the loan, permanently reduces balance |
| Access to funds | Instant, like a normal bank account | Often has minimum amounts, fees, or processing delays | Only accessible again via redraw, if available |
| Effect on interest | Reduces interest-bearing balance daily | Reduces loan balance directly once paid in | Reduces loan balance directly and permanently |
| Tax treatment (investment loans) | Generally doesn’t affect deductibility of loan interest | Can complicate deductibility if redrawn for private use | Can complicate deductibility if later redrawn for private use |
| Typical fees | Sometimes a package or annual fee | Sometimes a per-withdrawal fee | Usually free, but some fixed-rate loans cap extra repayments |
If you’re using the loan for investment purposes, many accountants recommend an offset account over redraw or extra repayments specifically because it keeps the loan balance — and its interest deductibility — separate from your personal savings. This is general information, not tax advice; the ATO’s guidance on rental property interest expenses covers the deductibility rules in detail, and a registered tax agent can confirm how they apply to your situation.
Pros and Cons of an Offset Account
Advantages
- Full and instant access to your money, unlike funds paid into the loan.
- The “return” you get is effectively tax-free, since you’re avoiding interest rather than earning taxable interest income.
- Works automatically — you don’t need to manually make extra repayments to benefit.
- Useful for keeping investment loan interest cleanly deductible.
Disadvantages
- Loans with an offset feature sometimes carry a slightly higher interest rate or an annual package fee compared with a basic loan.
- The benefit is proportional to your offset balance — a small balance delivers a small saving, and fees can outweigh that saving.
- Most lenders allow only one (or a limited number of) offset accounts per loan, so funds need to be consolidated to get full value.
- It’s easy to overestimate savings if your offset balance regularly gets drawn down for spending.
How to Open or Set Up an Offset Account
- Check if your current loan already includes one. Many packaged home loans include an offset account as a standard feature — log into your banking app or check your loan documents first.
- Compare the package cost against the benefit. If there’s an annual fee, use the calculator above to estimate whether your typical offset balance saves more in interest than the fee costs.
- Ask your lender to link the account, or open a new one as part of a loan package if you’re refinancing.
- Redirect your salary and savings into it so the balance builds automatically rather than sitting in a separate, non-offsetting account.
- Review it periodically, especially after rate changes, since a higher rate increases the dollar value of every dollar you keep offset.
How Much Can You Actually Save With an Offset Account?
Your savings depend on three things working together:
- How much you keep in the offset account — a bigger balance offsets more of the loan.
- How long you keep it there — a balance you draw down every few months saves far less than one you build steadily.
- Your loan’s interest rate and remaining term — higher rates and longer terms mean a bigger dollar impact from the same offset balance.
This is why this offset calculator includes a monthly top-up field: many borrowers don’t hold a static balance in offset, they gradually build one from salary, then draw it down for something like a renovation or holiday. Modelling that top-up gives a far more realistic picture than assuming the balance never changes.
Common Mistakes When Using an Offset Account
- Treating it like a savings account that pays interest. It doesn’t pay interest directly — the “return” is the loan interest you avoid.
- Spreading savings across multiple accounts. Some banks only offer one offset account per loan; consolidating idle cash into it maximises the offset benefit.
- Confusing offset with extra repayments. Extra repayments reduce the loan balance directly and permanently; offset funds remain accessible and can be withdrawn at any time.
- Forgetting fees can outweigh the benefit on small balances. Check that your typical offset balance is large enough to make the feature worthwhile compared with a lower-rate loan without one.
- Assuming the balance never moves. If you regularly draw the account down to near zero, your real-world savings will be much lower than a calculator assuming a constant balance.
Limitations of This Calculator
Like any offset calculator, this tool works from assumptions that simplify a real loan:
- It assumes your interest rate stays constant for the remaining term — in reality, variable rates change over time.
- It assumes standard principal-and-interest repayments, not interest-only periods.
- It assumes your offset balance grows only by the monthly top-up you enter, with no unplanned withdrawals.
- It doesn’t include loan fees, offset account fees, lenders mortgage insurance, or other loan features such as rate discounts for higher offset balances.
For a decision as significant as restructuring your mortgage, use this offset calculator to understand the general mechanics and scale of potential savings, then confirm exact figures with your lender or a mortgage broker.
Frequently Asked Questions
Interest is calculated daily on your loan balance minus your offset account balance, then usually totalled and charged monthly. If your loan balance is $460,000 after offset and your rate is 6% p.a., that day’s interest is roughly $460,000 × (6% ÷ 365).
With a partial offset (for example 50%), only that percentage of your offset balance is deducted from the loan balance before interest is calculated. A $40,000 balance at 50% offset reduces the interest-bearing loan amount by $20,000, not the full $40,000.
Enter your remaining loan balance, interest rate, remaining term, and current offset balance into the offset calculator above. It compares your total interest with and without the offset account to show your estimated savings and time saved.
It depends on your offset balance relative to your loan size. A larger, steadily growing offset balance shortens your loan more than a small or shrinking one. Use the “monthly offset top-up” field in the calculator to model how a growing balance affects your payoff timeline.
Related Calculators
If you’re working through your overall home loan strategy, you may also find these useful:
- Australian Mortgage Calculator — work out your standard repayments before comparing offset scenarios.
- Australia Stamp Duty Calculator — estimate upfront costs when buying.
- Australia Salary After Tax Calculator — check take-home pay to plan realistic offset top-ups.
- Superannuation Calculator — see how extra super contributions compare with offset savings.
- All Australia Financial Calculators — the full toolkit.