Guide
What Is Net Interest Margin (NIM), and Why Banks Live or Die by It
Published Oct 12, 2026
Net interest margin is what a bank earns on loans and securities minus what it pays for deposits and borrowing, as a share of those assets — the single number that drives most of a bank's profit.
Net interest margin (NIM) is the gap between what a bank earns on the money it lends out or invests and what it pays for the money it borrowed, expressed as a percentage of its interest-earning assets (loans, securities, and similar holdings). It is the bank equivalent of a retailer's gross margin: the markup on the bank's core product, which is money itself.
The formula
Net interest margin = net interest income / average interest-earning assets
Net interest income is interest earned on loans and securities minus interest paid on deposits and borrowed funds. A bank with $10 billion in interest-earning assets and $350 million of net interest income has a NIM of 3.5%.
Why it moves
NIM is not a fixed number — it shifts with interest rates and with the bank's own choices:
- The rate environment. When central banks raise rates, a bank's loans usually reprice upward faster than the deposits funding them, which can widen NIM for a while — until deposit rates catch up. When rates fall, the reverse can happen.
- Funding mix. A bank funded mostly by low-cost checking and savings deposits has a lower cost of funds than one that relies on more expensive borrowed money or large time deposits (CDs), so its NIM tends to run higher for the same loan book.
- Loan mix. Credit cards and consumer loans typically carry much higher rates (and higher NIM) than mortgages or loans to large, low-risk corporate borrowers.
- Competition for deposits. When banks compete hard for deposits, they pay more for them, which compresses NIM even if loan rates stay flat.
Why investors watch it
Net interest income is usually the largest single piece of a bank's revenue, so a small change in NIM — even a few tenths of a percentage point — can move profit by a lot more than the same percentage change would at a typical company. A bank that says revenue grew mainly "due to margin expansion" is describing a rate or funding-mix tailwind, not necessarily more lending activity; one that grew loan volume while NIM shrank is trading margin for growth.
What to check alongside it
- Is it reported GAAP or on a fully taxable-equivalent (FTE) basis? Some banks report NIM adjusted to make tax-exempt municipal bond income comparable to taxable income, which nudges the number up slightly. The filing will say which.
- Total loans and deposits. A widening NIM on a shrinking loan book is a different story than a widening NIM on a growing one — see the bank's own net interest income and margin figures alongside loan and deposit growth.
- Efficiency ratio and charge-offs. A high NIM funded by riskier, higher-rate loans can come with higher loan losses later; the efficiency ratio and net charge-off ratio on the same report show whether that trade-off is showing up yet.
NIM is specific to lenders — insurers, brokers, and regular companies don't report it, because they don't fund assets with deposits and borrowed money the way a bank does.
For information only; not investment advice.