Net Interest Margin Explained

Key takeaways

• NIM = (interest income - interest expense) / average interest-earning assets.
• It is a margin, not a profit - operating costs and losses come out of it afterwards.
• Deposit competition compresses NIM; lending competition compresses it too.
• Rising rates can expand NIM initially, then compress it as deposits reprice.

What does NIM measure?

Take everything the bank earns in interest on loans and securities, subtract everything it pays in interest on deposits and wholesale funding, and divide by the average interest-earning assets. The result is usually expressed in basis points and reported every half.

Australian bank NIMs typically sit in a narrow band measured in a small number of basis points of movement between reporting periods, which is why a shift of even a few basis points is treated as significant news. On a balance sheet of hundreds of billions, small percentages are large dollars.

NIM is not profit. Out of the margin come operating costs, credit losses, tax and the cost of capital. A bank can expand NIM and still report a worse result.

What actually moves it?

The last one is worth understanding. A bank funded by transaction accounts paying near zero has a structural advantage when rates rise. As customers notice and shift money into term deposits and savers, that advantage erodes - which is why NIM often peaks partway through a tightening cycle rather than at the top of it.

Why does this explain bank behaviour?

Because it reframes the question "why did the bank not pass on the full cut?" Banks manage a margin, not a rate. When the cash rate moves, they choose how much to pass through to which loans and which deposits, and the answer that protects margin is rarely symmetrical.

It also explains the loyalty tax. Winning new lending requires discounting, which compresses margin on the front book. The back book - existing borrowers who do not ask - carries a higher rate and supports the average. The gap is a deliberate consequence of managing NIM, not an oversight.

For a borrower the operative conclusion is simple: your rate is a commercial variable the bank actively manages, so asking for a reprice is not cheeky. It is participating in the mechanism.

Frequently asked questions

What is a good net interest margin for a bank?

There is no universal figure - it depends on the mix of lending, funding and market. What analysts watch is the direction and the drivers rather than the absolute level, and Australian bank NIMs move in small increments between reporting periods.

Does a higher NIM mean the bank is overcharging?

Not necessarily. NIM must cover operating costs, credit losses, tax and the cost of capital before any profit. A bank with high operating costs can have a wide margin and poor returns.

Why does NIM expand when rates rise?

Because loans generally reprice faster than deposits, and a portion of funding sits in accounts paying little or no interest. That advantage erodes as depositors move money into higher-paying products.

How does NIM affect my mortgage rate?

It is the constraint the bank prices against. Decisions about how much of a cash rate move to pass through, and whether to discount for new borrowers, are made to manage margin - which is why new and existing customers often pay different rates.

Related reading

• Why Banks Do Not Always Pass On the Full Rate Move
• Where Banks Actually Get the Money They Lend You
• Digital-Only Lenders: Speed vs Flexibility

Sources

• Financial Stability Review — Reserve Bank of Australia
• Quarterly authorised deposit-taking institution performance statistics — APRA