How Do Lenders Calculate How Much You Can Borrow for a Home Loan?

Lenders don’t pull your maximum loan amount out of thin air. They run two core checks — a serviceability assessment and a deposit‑and‑costs test — and the lower number usually sets your borrowing ceiling. Getting your head around the process early helps you test your own capacity before you speak to a lender, and it makes rate and policy comparisons far less confusing.

What serviceability really measures

Serviceability is where the heavy arithmetic lives. A lender works out whether you could keep making repayments if interest rates rise — not just at today’s rate, but at a materially higher rate. The number they land on is your net borrowing capacity, and it’s built from four main inputs.

1. Assessable income

Not every dollar you earn counts fully. Lenders include your base salary, regular overtime (often shaded to 80% or less), rental income (typically shaded to 75–80%) and consistent bonuses or commissions over a two‑year track record. Investment income, family tax benefits and some Centrelink payments may be accepted, but the treatment varies by lender.

2. Existing debts and living costs

Outgoing money trims your borrowing power fast. Credit‑card limits are assessed on the full drawn limit, not the balance owing, so even a zero‑balance card can bite. Other loans (car, personal, investment‑property debt), HECS‑HELP repayments and any buy‑now‑pay‑later facilities all reduce what’s left for a home‑loan repayment. On top of that, lenders use a benchmark for everyday living costs — often the Household Expenditure Measure — and check it against your declared expenses.

3. The assessment rate (the serviceability buffer)

The key variable is not the advertised headline rate. Under current Australian Prudential Regulation Authority (APRA) guidance, lenders must assess your ability to repay at either 3 percentage points above the loan’s real rate or a predetermined floor rate — whichever is higher. If a lender is offering you a variable loan at 5.89% p.a., the assessment is likely run at 8.89% p.a., which significantly reduces the loan size you can service.

This buffer is in place to protect borrowers from being stretched if rates rise further. It means your borrowing power is deliberately lower than a simple “repayments‑based‑on‑current‑rates” calculation would suggest.

4. Principal‑and‑interest repayment calculation

For most owner‑occupier home‑loans, lenders require principal‑and‑interest repayments over the remaining term, typically 25 to 30 years. The repayment is calculated using the assessment rate, not the actual rate, and is then tested against your after‑tax income surplus.

The deposit, costs and LVR side of the equation

Even if your serviceability allows a high figure, the Loan‑to‑Value Ratio (LVR) can cap the amount. Most lenders will not go above 80% LVR without lenders mortgage insurance (where available). For a property worth $800,000, an 80% LVR gives a maximum loan of $640,000 — regardless of what your income supports. You will also need to cover stamp duty, legal fees and other purchase costs from your savings; those outlays don’t form part of the loan.

How other expenses and buffers affect your result

Beyond the headline numbers, a few moving parts regularly surprise borrowers:

• Strata and council rates on the intended property are factored in as ongoing costs.
• Dependants increase the minimum living‑cost benchmark, lowering the surplus available for repayments.
• Interest‑only periods can temporarily boost short‑term cash flow but don’t escape the buffer — lenders still test long‑term principal‑and‑interest repayments from day one.

Why a borrowing‑capacity calculator helps — and what it can’t do

Tools such as the Moneysmart mortgage calculator can model different interest rates, incomes, expenses and loan terms. They’re useful for getting a ballpark figure and for understanding the gap between headline rates and the assessed rate. The results are not a guarantee; you still need to satisfy a lender’s full lending criteria.

On ozLoan you can use a free borrowing‑capacity estimator that follows the APRA 3% buffer framework. It’s a quick way to test different scenarios — salary change, extra debts, smaller deposit — before you approach a lender or mortgage broker.

Where ozLoan fits in

OzLoan provides independent, general research on the Australian home‑loan market — rate comparisons, calculator tools and lender‑policy explainers. It may accept service enquiries for Australian loan assistance within the scope of an authorised credit representative. OzLoan is not a lender, does not promise approval, rates or savings, and does not give personal financial advice. Use the calculators and research to build your knowledge, then speak with a licensed professional before making any decision.