Mortgage Serviceability Buffer Calculator (Australia)

What the serviceability buffer calculator does

The serviceability buffer calculator shows the gap between the loan your income could actually service and the loan a lender is allowed to approve. It converts your gross income to after-tax income using the current ATO brackets, the low income tax offset and the Medicare levy, subtracts your living expenses and existing commitments to find a monthly surplus, then turns that surplus into a loan amount twice: once at the rate you would really pay, and once at the rate the lender must assess you on.

The difference is the single most misunderstood number in Australian home lending. Borrowers routinely arrive at a broker with a repayment they can plainly afford and leave with an approval a hundred and fifty thousand dollars below it. This tool makes that arithmetic visible before the application rather than after.

How the buffer works

APRA requires authorised deposit-taking institutions to assess new home loan applications using an interest rate at least a set margin above the rate on the loan. The margin is a prudential expectation rather than a legislated rate, and APRA reviews it against household debt levels, rate expectations and lending standards. It is held here as a parameter so the calculator follows the current setting rather than a snapshot.

The buffer is applied to the loan being applied for and, importantly, to your other debts. A borrower with an existing mortgage at a low fixed rate is assessed on that mortgage as though it were priced at the buffered rate, which is why the arithmetic of refinancing is often worse than it looks. Lenders may also apply their own assessment floor: a minimum rate below which they will not assess, regardless of how cheap the product is. Where a floor is higher than the buffered rate, the floor governs. That is what the lender floor rate input models.

Two more layers usually sit alongside the buffer. Lenders benchmark declared living expenses against the Household Expenditure Measure and use the higher of the two, and most apply a net income surplus test on top. Neither is a published figure, so the expenses you enter here are used as given.

Why the effect is so large

Borrowing power is the present value of a stream of repayments. Present value is highly sensitive to the discount rate, and three percentage points on a rate near six is not a small adjustment: it is a fifty per cent increase in the assessment rate.

The consequence, on a thirty-year term at rates in the range Australia has seen recently, is that around a quarter of borrowing capacity disappears. The proportion grows as the term lengthens and shrinks as rates rise, because the buffer becomes a smaller fraction of a bigger number. That is one reason the practical bite of the buffer was greatest during the low-rate years and has eased somewhat since.

The result also explains a pattern many borrowers notice: a rate cut increases how much you can borrow by more than it reduces your repayment. The repayment falls in proportion to the rate; the capacity rises in proportion to the change in the discount factor across the whole term.

The inputs that move the answer most

Living expenses do more damage than almost anything else, because every dollar of monthly expense removes roughly a hundred and thirty dollars of borrowing power at typical assessment rates over thirty years. Cleaning up subscriptions and discretionary spending in the months before an application is not cosmetic; lenders read statements.

Credit card limits are the classic own goal. Assessment is on the limit, at a monthly percentage of it, whether or not you have ever used the card. A twenty thousand dollar limit sitting unused can cost tens of thousands of dollars of capacity. Reducing or closing limits before applying is usually the fastest single improvement available.

Loan term matters because a longer term spreads the same surplus over more payments, raising the loan amount. It also raises total interest, so the extra capacity is not free. Shading on rental and variable income is a lender policy, commonly a substantial haircut on gross rent to allow for vacancy, agent fees and rates, and is entered here rather than assumed.

What to do with the result

The borrowing power after the buffer is the number to plan around, because it is the number the credit team will produce. The borrowing power at the product rate is useful context: it tells you how much of your capacity is being withheld as a safety margin rather than because you cannot afford the loan.

The monthly headroom output is the practical one. It is the difference between your assessed surplus and the repayment you would actually make on the approved amount. That headroom is the buffer doing its job: it is roughly the room you would have if rates rose to the assessment level. If the headroom looks comfortable and the approval still falls short of what you need, the constraint is usually expenses or existing limits rather than income.

The debt-to-income multiple is worth watching separately. Lenders monitor high-DTI lending as a portfolio measure and many apply extra scrutiny above six times income, so a file that services on paper can still be declined on that basis.

Limits of this estimate

This calculator models the buffer, the tax treatment of your income and the annuity arithmetic. It does not model a specific lender's credit policy, the Household Expenditure Measure floor, lenders mortgage insurance, negative gearing add-backs, HELP debt repayments, or the treatment of variable income such as overtime, commission and bonuses. Those can move an assessment by a large margin in either direction.

It is an estimate based on the figures you enter and the current APRA and ATO parameters, not a credit assessment, a pre-approval, or personal financial advice. Speak to a lender or a licensed mortgage broker before relying on the number for a purchase decision.

About This Tool: Serviceability Buffer

Serviceability buffer: use this tool to get an instant, accurate estimate. The figures follow official ATO guidance and every rate and threshold is kept up to date for 2026. Adjust the inputs above to match your situation, or browse all our calculators to plan your next step.