How Lenders Actually Calculate Your Borrowing Power (And Why the Number Feels So Low)

September 15, 2026

Punch your income into an online "how much can I borrow" calculator and you'll often get a number that feels wildly optimistic compared to what an actual lender later offers you. That gap isn't a bug in one calculator or the other — it's because most borrowing-power estimates skip steps that real lenders don't get to skip.

It starts with take-home pay, not gross income

Lenders work from what actually lands in your account: income after tax and the Medicare levy, not your advertised salary. A $120,000 gross salary and a $120,000 salary with a large salary-sacrificed novated lease don't produce the same take-home figure, and lenders care about the number you'd actually have available to service a loan.

Then every existing debt gets counted — at its limit, not its balance

This is the part that surprises people most. A credit card you pay off in full every month and rarely use still counts against your borrowing power at its full credit limit, not your current balance or your typical spend. A $10,000 limit card you've never carried a balance on is treated by most lenders as if you could draw the full $10,000 down tomorrow — because you could. The same logic applies to car loans, personal loans, buy-now-pay-later accounts, and other credit cards. Cancelling or reducing limits on cards you don't need before applying is one of the few borrowing-power levers actually within your control.

Living expenses are estimated whether or not you think you spend that much

Lenders use a benchmark measure of living expenses (based on your household size and income level) and compare it against what you declare on your own application — then use whichever figure is higher. Someone who genuinely lives frugally doesn't get credit for it if the benchmark for their situation is higher than their self-reported number; the lender is required to use the more conservative figure.

And then a safety buffer gets applied on top of the interest rate

This is the step most casual calculators skip entirely, and it's the single biggest reason a quick online estimate overshoots a real offer. Australian lenders are required to assess your ability to repay not at today's advertised interest rate, but at that rate plus a regulatory serviceability buffer — an extra few percentage points added on top, specifically so borrowers aren't approved right up to the edge of what they can afford at today's rate with nothing left if rates rise. A loan that looks comfortable at the advertised rate can look considerably tighter once the buffer is factored in, and that's deliberate: it's a genuine affordability stress-test, not red tape for its own sake.

Putting it together

Roughly, real borrowing-power assessment looks like:

Take-home income (after tax and Medicare levy)
  − Living expenses (the higher of your declared figure or the benchmark)
  − Existing debt repayments (based on full credit limits, not balances)
  = Income available to service a new loan

Applied against the interest rate PLUS the serviceability buffer
  = Maximum loan a lender will actually offer

That's why the number from a five-second online calculator that only asks for your income can be meaningfully higher than what a real lender comes back with — it's usually skipping the buffer, using your full credit limits' worth of headroom instead of accounting for them as debt, or both.

Our First Home Buyer Calculator applies the same logic real lenders use — take-home income, declared expenses and debts, and the standard regulatory buffer on top of your entered interest rate — so the estimate you get is closer to what you'll actually be offered, not a best-case number designed to look encouraging.

This article is general educational information, not personal financial or lending advice. Actual serviceability assessment varies by lender — confirm your borrowing power with a licensed mortgage broker or lender before making decisions.