How to Use the Mortgage Offset Calculator

Enter your current home-loan balance, annual interest rate and remaining loan term. Then enter the average balance you expect to maintain in your offset account.

You can also model:

• A monthly increase or decrease in the offset balance;
• Extra monthly mortgage repayments; and
• Different offset balances using the comparison table.

The calculator estimates:

• Standard monthly mortgage repayment;
• Effective loan balance used for interest;
• Total interest without an offset;
• Total interest with an offset;
• Estimated interest savings;
• Potential reduction in loan term;
• First-year interest savings; and
• A year-by-year mortgage balance schedule.

What Is a Mortgage Offset Account?

A mortgage offset account is a transaction or savings account linked to a home loan. The balance in the offset account can reduce the portion of the mortgage balance on which interest is calculated.

For example, if your mortgage balance is $500,000 and you have $50,000 in a 100% offset account, interest may effectively be calculated on:

The mortgage balance itself remains $500,000, but the interest calculation can be based on the lower net amount while the offset funds remain available.

Mortgage Offset Example

How Does an Offset Account Save Interest?

The basic concept is:

The lower interest-bearing balance means less interest is charged. If your mortgage repayment stays the same, more of the repayment may then go toward reducing principal.

Over time, this can potentially:

• Reduce total mortgage interest;
• Pay the mortgage off sooner; or
• Provide both benefits.

What Is a 100% Offset Account?

A 100% offset account generally means the full eligible account balance is used to offset the linked loan balance for interest-calculation purposes.

For example:

Loan products and offset conditions vary, so check your lender’s terms.

Offset Account vs Extra Repayments

Both strategies can reduce mortgage interest, but they work differently.

Offset account

Money remains in a separate linked account and may remain accessible for transactions or withdrawals, subject to the account terms.

Extra mortgage repayments

Money is paid directly onto the mortgage principal. Access to those funds later may depend on redraw availability and lender rules.

This calculator lets you combine an offset balance with extra monthly repayments.

Offset Account vs Redraw

An offset account and redraw facility are not the same thing.

An offset account is generally a separate bank account linked to the loan. Redraw generally refers to accessing eligible extra repayments previously made directly to the loan.

The legal, tax and access implications can differ, particularly for loans that may later become investment-property loans.

Does an Offset Reduce the Loan Balance?

No, not directly.

If you owe $500,000 and have $50,000 in your offset account, your loan balance is still generally $500,000.

The offset changes the amount used for the interest calculation.

How Much Interest Does $10,000 in Offset Save?

A useful rough first-year estimate is:

For $10,000 at a 6% mortgage rate:

$10,000 × 6% = approximately $600 per year

The actual long-term saving depends on repayments, changing mortgage balances, the offset balance and interest-rate changes.

How Much Does $50,000 in Offset Save?

At a mortgage rate of 6%, a simple static-balance estimate would be:

Because mortgage balances decline over time, the exact lifetime benefit requires an amortisation calculation such as the calculator above.

Can an Offset Account Pay My Mortgage Off Faster?

Potentially, yes.

If the offset reduces interest but you continue making the same scheduled loan repayment, a larger share of that payment can go toward principal.

This can reduce the mortgage balance more quickly and may shorten the effective loan term.

Does the Monthly Mortgage Repayment Change With an Offset?

Often the contractual repayment itself does not automatically fall simply because money is held in an offset account.

Instead, the reduced interest charge can mean a larger portion of each payment reduces the loan principal.

Individual lenders and loan products can operate differently, so check your actual repayment arrangement.

What If My Offset Balance Changes Every Month?

Real offset balances often change as salary is deposited and living expenses are paid.

The calculator therefore includes a Monthly Change in Offset Balance field.

For example:

• Enter $500 if you expect the offset balance to grow by about $500 each month;
• Enter -$500 if you expect the balance to decline by about $500 each month; or
• Enter $0 to model a constant offset balance.

Can My Offset Balance Be Higher Than My Mortgage?

You can hold more cash than the remaining mortgage balance, but the amount used to reduce mortgage interest generally cannot reduce the interest-bearing loan balance below zero.

For example, if the mortgage is $20,000 and the linked eligible offset balance is $30,000, the calculator limits the effective interest balance to $0.

Offset Accounts and Investment Property Loans

Offset accounts can be particularly important when a property may later become an investment property because keeping savings separate from the loan can have different practical and tax consequences from directly paying down and later redrawing the loan.

Does an Offset Account Earn Interest?

Many mortgage offset accounts do not pay ordinary deposit interest in the same way as a separate high-interest savings account.

Instead, their economic benefit may come from reducing mortgage interest.

For example, if your mortgage rate is 6%, each eligible dollar in the offset can effectively avoid mortgage interest at around that rate while it remains offsetting the loan, subject to the loan’s terms.

Offset Account vs Savings Account

The better mathematical option depends on the after-tax savings return compared with the mortgage interest avoided.

For owner-occupied debt, mortgage interest is generally a personal expense, while interest earned in a savings account may be assessable income.

This means a mortgage offset can sometimes provide an attractive effective after-tax benefit, but the correct comparison depends on your circumstances.

Does an Offset Save More When Interest Rates Are Higher?

Yes, all else equal.

The interest avoided by an offset balance is directly related to the mortgage interest rate.

For a $50,000 offset:

• At 4%: rough annual saving ≈ $2,000;
• At 5%: rough annual saving ≈ $2,500;
• At 6%: rough annual saving ≈ $3,000;
• At 7%: rough annual saving ≈ $3,500.

What Is the Effective Loan Balance?

For the purpose of this calculator, the effective balance is:

This is the balance used in the simplified interest calculation, capped at zero.

How the Calculator Estimates Time Saved

The calculator first determines the standard monthly repayment required to amortise the mortgage over the entered remaining term.

It then simulates two scenarios:

• The mortgage with no offset; and
• The mortgage with the entered offset balance and offset changes.

Both scenarios use the same regular mortgage repayment plus any extra repayment entered.

The difference between the simulated payoff dates is displayed as the estimated time saved.

Why the Actual Bank Result May Differ

Actual mortgage calculations can differ because lenders may:

• Calculate interest daily;
• Charge interest monthly;
• Use actual calendar-day counts;
• Change interest rates during the loan;
• Change minimum repayments;
• Apply fees;
• Have partial rather than full offset arrangements; or
• Apply product-specific conditions.

The calculator therefore provides an estimate rather than a lender repayment schedule.

Mortgage Offset Calculator Assumptions

This calculator assumes:

• The mortgage is principal and interest;
• The entered interest rate remains constant;
• The standard repayment remains constant;
• The offset account provides a 100% offset effect;
• The starting offset balance changes monthly by the entered amount;
• The offset balance cannot reduce the interest-bearing loan below zero;
• Extra repayments are made monthly;
• No redraws are modelled;
• No loan fees are included;
• No offset-account fees are included;
• No tax consequences are calculated;
• No lender-specific daily interest method is reproduced exactly; and
• Results are estimates rather than lender quotes.

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