Rate Change Calculator
See how a rate rise or cut changes your mortgage repayment and total interest, under either a keep-repayment or recalculate-to-term policy. Working shown.
Repay interest only for the first years, then principal and interest.
Enter your loan and the new rate to see the repayment change and lifetime effect.
Summary
Two ledgers run from the same loan: one where the rate changes on the effective date, one where it never moves. The repayment shown is the first scheduled repayment on or after that date.
Working
- §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.
- §13.7 recurrence: each period: interest = accruing balance × i, then closing = opening + interest + fees − payment − extra repayment.
- §13.9 offset: interest accrues on max(0, balance − offset × effectiveness), floored at zero; offset cash is never a principal repayment.
- §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
How much will my mortgage repayment go up if rates rise?
Enter your loan, the new rate and the date it takes effect, and the calculator reruns the whole schedule on a ledger with the old rate applying up to that date and the new rate after it. It reports the repayment before the change, the repayment after it and the difference per week, fortnight or month. The lifetime interest is also recomputed against an otherwise identical schedule at the unchanged rate.
What happens to my loan term when interest rates change?
It depends on which policy your lender applies, and the calculator makes that an explicit choice rather than an assumption. Under keep-repayment the amount you pay stays the same and the payoff date moves; under recalculate-to-term the repayment is re-solved so the loan still closes at the original term. Switching the policy field reruns the schedule and shows the payoff date and lifetime interest under each.
Why does a small rate change cost so much over the life of a loan?
Because the extra interest is charged on the outstanding balance in every remaining period, and a higher interest charge leaves less of each repayment to reduce principal, which raises the balance the next period is charged on. The calculator captures that compounding by rerunning the full schedule rather than multiplying the rate difference by the balance. The interest difference shown is measured against the unchanged-rate schedule over the whole term.
Does the calculator predict future interest rate movements?
No. Every rate and every effective date is a figure you enter, and the calculator holds no forecast, no lender pricing and no view on where rates go. It reports what the schedule does under the change you specify, with the inputs listed in the assumptions panel.
Can I model more than one rate change?
This page models a single change on a single date so the before-and-after comparison stays clean. A sequence of dated rate changes, together with extra repayments, offsets and fees, runs on the full scheduled ledger in the mortgage simulator. Both pages use the same period-by-period engine and reconcile the balance on every period.
Related calculators
- Refinance Break-Even CalculatorCompare your current home loan against a refinance offer on cumulative cash flows including switching costs, cashback timing and the residual balance.
- Home Deposit CalculatorWork out the house deposit needed at your chosen loan-to-value ratio, plus the upfront costs you enter, with the savings gap and every step of the arithmetic.
- LVR CalculatorCalculate your loan-to-value ratio from a property value and loan amount, alongside the standard LVR bands Australian lenders reference. Full working shown.
- Affordability EstimateAn indicative borrowing range from your income, expenses and commitments under generic assumptions with an editable rate buffer. Not an offer or pre-approval.
- Stamp Duty CalculatorEstimate general transfer duty on a property purchase from official state revenue-office rate tables. Unsupported states are clearly blocked, never guessed.
- Property Buying Costs CalculatorUpfront costs of buying a house: transfer duty plus conveyancing, inspection, loan and registration fees for the total cash needed. Duty from official state tables.
All Property and mortgage calculatorsHow the Rate Change Calculator worksAll calculators