Repayment & rate stress calculator
What would your repayment actually be — and what if the rate rose?
Set your loan amount, rate and term. Choose weekly, fortnightly or monthly. Then move the stress buffer and see what a rate rise would do to the number you have to find every month. No sign-in, nothing to download.
How a loan repayment is worked out
A standard principal-and-interest repayment is a level payment: the same amount every period, split between interest and principal, with the interest share shrinking as the balance falls. It comes from the annuity formula, where the repayment equals the loan amount multiplied by the periodic interest rate, divided by one minus one-plus-the-periodic-rate to the power of minus the number of periods.
The periodic rate is simply the annual rate divided by how many repayments you make each year — 12 monthly, 26 fortnightly, 52 weekly. That single detail is why a fortnightly repayment is not half a monthly one. This calculator computes the real figure for the frequency you pick rather than halving or quartering a monthly number, because the shortcut overstates what you would actually pay.
A worked example
A $600,000 loan at 6.00% per annum over 30 years, repaid monthly, comes to $3,597.30 a month. Over the full term that is about $1,295,028 repaid, of which roughly $695,028 is interest — more than the amount borrowed. Seeing the total interest next to the monthly figure is usually the more useful of the two.
| Loan amount | Rate | Term | Monthly repayment |
|---|---|---|---|
| $500,000 | 6.50% | 30 years | $3,160.34 |
| $600,000 | 6.00% | 30 years | $3,597.30 |
| $750,000 | 5.75% | 30 years | $4,376.80 |
| $1,000,000 | 6.14% | 30 years | $6,085.81 |
These are arithmetic, not offers. They exclude fees and assume the rate never moves.
Why stress-test the repayment
The repayment you can afford today is not the repayment you might have to find in three years. A variable rate moves, and a fixed rate ends. The buffer on this page answers one narrow, useful question: if my rate rose by this much, what would the payment become?
A 3% default is used because it is a common way to sanity-check a repayment against a rate rise, and because it is large enough to be informative rather than reassuring. You can set it anywhere from 0% to 10%.
What it is not: a lender serviceability test. Lenders apply their own buffer, their own living-expense benchmarks, their own treatment of your income and existing commitments, and their own credit policy — and those differ between lenders and change over time. A buffer on a calculator tells you about your own budget. It tells you nothing about what a lender will do.
What these figures exclude
- Establishment, ongoing, annual and discharge fees
- Valuation, settlement and government charges
- Lenders mortgage insurance, where it applies
- Offset and redraw behaviour
- Rate changes, and the timing of them
- Your full circumstances, and any lender's credit policy
Extra repayments, in real numbers
Extra repayments come straight off the principal, so they cut both the interest charged from that day forward and the number of years the loan runs. The saving shown here is measured against the identical loan with the extra removed — the only honest comparison.
If your extra is at a different frequency to your loan, it is converted by preserving the annual total. One hundred dollars a week is $5,200 a year, which is about $433 a month. Treating it as $100 a month would understate the benefit more than fourfold.
Interest only, honestly
An interest-only period lowers the repayment because you are not repaying principal. It does not make the loan cheaper. The whole principal still has to be repaid across the remaining term once the period ends, so the repayment steps up — and interest has been charged on the full balance the entire time. This calculator shows both figures, because showing only the low one would be misleading.
What this calculator will not do
It will not tell you how much you can borrow. It will not name a lender, rank a product or produce a "best refinance" result. It will not suggest you are likely to be approved. Those are credit decisions and credit advice, and they belong with a licensed broker who has seen your actual situation — not with a page of arithmetic.
What it is good for is the conversation before that: getting a realistic feel for the numbers, testing whether a rate rise would hurt, and arriving at your appointment already knowing which questions matter to you.
Questions people actually ask
How is a loan repayment calculated?
A standard principal-and-interest repayment uses the annuity formula: the repayment is the loan amount multiplied by the periodic interest rate, divided by one minus (one plus the periodic rate) to the power of minus the number of periods. The periodic rate is the annual rate divided by the number of repayments per year — 12 for monthly, 26 for fortnightly, 52 for weekly. This calculator uses exactly that formula and rounds only the figures it displays, so a 30-year loan carries no accumulated rounding drift.
What does the 3% stress buffer mean?
It shows what your repayment would be if your interest rate rose by 3 percentage points. It is a repayment stress scenario only. It is not a lender serviceability test, not a borrowing-capacity figure and not an indication of approval. Lenders apply their own buffers and their own policies, which differ between lenders and change over time. You can set the buffer to any value between 0% and 10%.
Does paying weekly or fortnightly actually save money?
At the same interest rate, repaying more often reduces total interest, because interest is charged on a balance that falls sooner. The effect is real but modest on its own. The larger effect people notice usually comes from paying more in total across a year — for example paying half the monthly amount every fortnight is 26 payments a year, which is 13 monthly equivalents rather than 12. This calculator computes the true repayment for the frequency you choose rather than halving a monthly figure.
How much difference do extra repayments make?
Extra repayments come off your principal, so they reduce both the interest charged from that point on and the time the loan runs. The calculator shows the interest saved and the time saved against the identical loan with no extra repayment. If you enter an extra amount at a different frequency to your loan, it is converted by preserving the annual total — $100 a week is $5,200 a year, which is about $433 a month, not $100 a month.
Is interest only cheaper?
The repayment during an interest-only period is lower, because you are not repaying any principal. The loan is not cheaper overall — it is usually more expensive, because the full principal still has to be repaid over the remaining term after the interest-only period ends, and interest has been charged on the whole balance in the meantime. This calculator shows both the interest-only repayment and the higher repayment it reverts to.
Does this tell me how much I can borrow?
No, and it deliberately does not try to. Borrowing capacity depends on each lender's credit policy, how they assess your income and commitments, their own buffer, living-expense benchmarks and your full circumstances. This calculator only answers the question you asked it: what a repayment would be at the numbers you entered. A senior broker can tell you what is realistic across a lender panel.
Do the results include fees?
No. The figures exclude establishment fees, ongoing or annual fees, valuation and settlement costs, lenders mortgage insurance, government charges and any offset or redraw behaviour. They also assume the interest rate you entered stays the same for the whole term, which is not how a variable rate behaves. Treat every figure as indicative only.
Will you recommend a lender or a product?
Not from a calculator. This page names no lender, ranks no product and produces no best-rate result. If you want a recommendation, that is a conversation with a credit-licensed broker who has looked at your actual situation and documented why a particular option suits it.