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Mortgage repayment, extra repayment and offset calculator
Four paths side by side: your current repayment, paying extra, holding money in an offset, and doing both.
- No sign-up or email
- Runs in your browser
- Assumptions you can edit
- Print or save as PDF
What would paying extra — or keeping cash in my offset — actually save me?
A repayment figure on its own does not tell you much. What people want to know is what changes if they pay a bit more each month, or park their savings in an offset account. This calculator runs all four paths against the same loan and reports the difference in interest and in time.
$
%
years
$
On top of the scheduled repayment.
$
Assumed constant for the whole term.
$
Applied immediately, before the first repayment.
Scheduled repayment
$3,597.30
per month
Total interest, as it stands
$695,029
Paid off 30 years
Compare a change
—
Add an extra repayment or an offset balance to see what it saves.
| Scenario | Total interest | Interest saved | Time saved | Paid off |
|---|---|---|---|---|
| As it stands baseline | $695,029 | — | — | 30 years |
| With extra repayments | $695,029 | $0 | 0 months | 30 years |
| With offset | $695,029 | $0 | 0 months | 30 years |
| Extra + offset | $695,029 | $0 | 0 months | 30 years |
Payoff dates are counted from today. Excludes all fees, and assumes the rate and the offset balance stay where they are for the whole term.
Estimate only, based on the figures and assumptions you entered. It is general information, not financial advice, and it does not take your objectives, financial situation or needs into account. Model version 1.1.0, not independently reviewed.
Reading the result
What the numbers mean
Scheduled repayment is what the loan requires. It is the same in every path — extra repayments and offset balances change what the repayment achieves, not what is required.
Interest saved and time saved are measured against your current repayment with no extra and no offset.
An offset reduces the balance that accrues interest, not the balance you owe. Money in an offset is still yours and can be withdrawn; money paid into the loan generally cannot, without redraw.
Extra repayments and an offset are not additive in a simple way. Doing both saves less than the two savings added together, because each one shrinks the interest the other could have saved.
Worked example
An example, start to finish
A $600,000 loan at 6.00% with 30 years remaining, repaid monthly.
- Scheduled repayment
- $3,597.30 per month
- Total interest, no changes
- $695,029
- Paying $500 extra each month
- saves $212,713 and 7 years 11 months
- Holding $50,000 in offset instead
- saves $202,175 and 4 years 8 months
Both changes are worth roughly $200,000 over the life of this loan, which is a third of the total interest. The offset achieves it without the money leaving your control.
Methodology
What this calculator assumes
Principal and interest for the whole term. Interest-only periods are not modelled.
One constant interest rate for the whole term. Rate rises and falls are not modelled, and over 30 years they will dominate any of the savings shown here.
The offset balance is held constant for the whole term, and is capped at the amount owing — an offset larger than the loan takes interest to zero and no further.
Interest accrues on the outstanding balance each repayment period, and the final repayment is reduced to exactly clear the loan.
No fees of any kind: no application, ongoing, offset account, discharge or break costs.
Limitations
What it does not do
This is not a lender quote, a loan approval, a borrowing-capacity estimate or a refinance comparison.
Fees are excluded, and an offset account often carries one. A package fee can consume a meaningful part of the saving on a small offset balance.
Real offset balances move constantly as money is spent. A constant balance is optimistic in most households.
Whether to direct spare cash at a mortgage, super, or an investment is a personal decision involving tax, risk and access. This calculator answers only the arithmetic part.
About this model
Built by the Souffle team. Model version 1.1.0, in effect from 2026-09-10. It has not been independently reviewed, and no review badge is shown for that reason. The calculation runs entirely in your browser — nothing you type is sent to us or to anyone else, and nothing is stored.
Questions
Common questions
Is an offset better than paying extra off the loan?
For the same amount of money the interest effect is nearly identical — both reduce the balance interest is charged on. The practical difference is access: offset money stays available, extra repayments generally do not unless the loan has redraw. Which suits you depends on your circumstances.
Why does the total interest differ from my lender's figure?
Lenders typically accrue interest daily and charge it monthly, and they include fees. This calculator accrues on the repayment period and excludes fees. Expect a difference; treat this as an estimate.
What if my repayment does not cover the interest?
The calculator flags it rather than returning a very long term. A repayment below the interest charge means the balance grows every period and the loan never clears.
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