NIM: Net Interest Margin

Imagine a grocer who buys vegetables at a certain price and sells them at a markup, the gap between those two prices, relative to how much stock the grocer typically carries, tells you how healthy that core business actually is. Net Interest Margin does exactly this for a bank's core business of borrowing cheap and lending dearer.

NIM is calculated as the interest a bank earns on its loans and investments, minus the interest it pays out on deposits and borrowings, expressed as a percentage of its average interest-earning assets. It captures, in one number, how efficiently a bank is converting its balance sheet into actual lending profit, independent of fee income, trading gains or other non-core revenue.

NIM is shaped by several forces at once: a bank's CASA ratio, since cheap deposits widen the margin directly, the interest rate environment, since rate cuts and hikes affect loan and deposit repricing at different speeds, and the bank's loan mix, since unsecured retail and small business loans typically carry higher rates, and therefore higher margins, than large corporate or secured lending.

Indian banks typically report NIM in the range of roughly 3 to 4.5%, though this varies meaningfully by business mix, a bank leaning heavily into retail and small business lending will generally post a higher NIM than one dominated by large corporate loans, simply because the pricing power and risk premium differ across those segments.

Whenever a bank's quarterly results show NIM compressing or expanding even while overall profit looks stable, that movement is usually the first place analysts look, since NIM changes reveal whether the bank's core lending economics are genuinely improving or whether stable headline profit is being propped up by something else entirely, like lower provisioning or one-off gains.