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Mortgage Simulator

Run a mortgage through dated rate changes, extra repayments, offset balances and fees on a scheduled ledger that reconciles on every single period.

AustraliaScheduled modelClass B · rule-based estimateNo statutory rules requiredMethodology
AUD

Repay interest only for the first years, then principal and interest.

AUD

Scheduled model: each event takes effect from the first repayment date on or after its date, in order: rate changes first, then offset movements, repayments and fees. Mid-period timing is not modelled.

    Enter the loan to simulate it through dated rate changes, extra repayments, offsets and fees on a scheduled ledger.

    Summary

    One scheduled ledger carries your dated events (rate changes, extra repayments, offset movements and fees) in date order, and each period is reconciled before it is reported.

    Working

    1. §13.5 payment: the scheduled repayment solves P = B·i ÷ (1 − (1 + i)^−n) from the balance B, the periodic rate i and the remaining periods n.
    2. §13.7 recurrence: each period: interest = accruing balance × i, then closing = opening + interest + fees − payment − extra repayment.
    3. §13.9 offset: interest accrues on max(0, balance − offset × effectiveness), floored at zero; offset cash is never a principal repayment.
    4. §12.5.8 reconciliation: the ledger identity is checked on every period. A reconciliation failure invalidates the result rather than warning about it.

    Assumptions

    • Interest accrues once per repayment period on the payment-period ledger. Daily accrual is not modelled at P0.
    • The first repayment falls on 1 October 2026; every date in the schedule follows from that and the repayment frequency.
    • The rate is held constant except where a dated rate-change event moves it.
    • Weekly, fortnightly and monthly frequencies use 52, 26 and 12 periods per year.

    Limitations

    • Lender daily accrual, transaction timing, fee timing and rounding can differ from this model; compare the settings with your loan contract and statements.
    • Redraw availability, offset eligibility conditions, break costs and any lender fee that was not entered are not modelled.
    • Amounts beyond the entered term are not projected; a balance left unpaid at term is reported rather than extended.

    Common questions

    What is a mortgage simulator?

    It runs your loan forward on a scheduled ledger, one repayment period at a time, with dated events applied on the periods they actually fall in. Rate changes, extra repayments, offset balances and fees each land on their own date instead of being averaged across the term. Every period is reconciled (opening balance plus interest less repayment must equal the closing balance), and a reconciliation failure stops the result rather than being shown as a warning; the methodology page lists the invariants.

    How do I compare two mortgage scenarios?

    Switch the simulator into compare mode and edit scenario A and scenario B separately; both run through the same ledger on the same loan so the difference is attributable to the events you changed. The results show each scenario's total interest and payoff date alongside the interest difference and the time difference between them. Nothing is inferred about which scenario suits you. The calculator reports the two schedules and the gap.

    Does the simulator model daily interest accrual?

    No. Interest is accrued per repayment period, and daily accrual is not modelled in this release, which the limitations panel states on every result. Contracts that accrue daily and debit monthly will differ from this schedule by small amounts that grow with the term. The period basis is shown with the working so the difference is visible rather than hidden.

    How does an offset balance change the simulation?

    An offset balance is netted against the loan balance before interest is charged for that period, so it lowers the interest component without reducing the amount you owe. The simulator keeps the offset money separate from principal in every row, which is why it still shows as cash available. For an offset-only view, the offset account calculator runs the same ledger with the offset inputs brought forward.

    Are the simulator's interest rates real lender rates?

    No. Every rate in the simulator is a figure you enter, including the dated changes. The calculator holds no lender pricing and makes no forecast of where rates go next. It reports what the schedule does at the rates and dates you supply, with those inputs listed in the assumptions.

    Why does the payoff date move when I add a fee?

    Fees are charged to the ledger on their dates, so they either increase the balance interest is charged on or consume part of a repayment that would otherwise have reduced principal. Either way the schedule takes longer to close and the payoff date shifts. The schedule rows show the period each fee lands in.

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