Short answer: Net Interest Margin (NIM) is a bank's net interest income divided by its average earning assets, usually expressed as an annualized percentage. It measures how profitably the bank lends. Most US community banks run between 3.0% and 4.0%.
The formula
Component breakdown:
- Interest income is what the bank earns on loans, investment securities, and interest-bearing balances held at other banks.
- Interest expense is what the bank pays on deposits, FHLB borrowings, and other interest-bearing funding.
- Earning assets are loans plus investments plus interest-bearing balances. (Total assets minus things like premises, equipment, and non-interest-bearing cash.)
NIM is reported on every UBPR and on virtually every bank's earnings release. It's one of the four ratios you should know cold if you're looking at any bank.
What's a "good" NIM?
| Range | What it usually means |
|---|---|
| Below 2.5% | Compressed. Often a very large bank, a bank heavy in low-yield investments, or one with expensive funding. |
| 2.5% to 3.0% | Below average for a community bank. Typical for mid-size and large banks. |
| 3.0% to 3.5% | Average for community banks. |
| 3.5% to 4.0% | Strong. Usually a bank with healthy small business or consumer loan books. |
| Above 4.0% | Very strong. Often consumer-heavy, ag-heavy, or high-yield commercial portfolios. |
As always, the peer-group percentile beats the absolute number. NIM varies systematically by business model and region. A 3.2% NIM is fine for a $5B regional bank and weak for a $200M rural ag bank.
How NIM behaves through a rate cycle
NIM is one of the most rate-sensitive metrics on a bank's income statement. When the Fed raises rates:
- Asset yields tend to rise first. Floating-rate commercial loans reprice quickly. Treasury yields adjust. New fixed-rate loans get written at higher rates.
- Funding costs follow, with a lag. Demand deposits cost zero and stay that way for a while. CDs reprice as they mature. The lag between asset repricing and funding repricing widens NIM in early rate hikes.
- Eventually, funding catches up. Customers chase yield, money market funds compete, brokered CDs come into the mix. NIM tends to flatten or compress later in the cycle.
When the Fed cuts:
- Asset yields fall first, especially on floating-rate loans.
- Funding costs follow, but stickier deposits resist coming down quickly.
- NIM tends to compress in the first 2-4 quarters of a cutting cycle.
Banks with heavy floating-rate commercial books outperform during hikes. Banks with sticky low-cost deposit franchises (lots of consumer demand deposits) outperform during cuts. The interesting question with NIM is almost never "what is it." It's "what direction is it going, and why."
NIM vs. net interest spread
Net interest spread is the gap between the average yield on earning assets and the average cost of interest-bearing liabilities. NIM is similar but also reflects the benefit of non-interest-bearing funding (like demand deposits, which cost the bank zero in interest).
A bank with lots of free checking will have a NIM noticeably higher than its spread. That gap is part of why deposit-rich community banks can look "more profitable" than larger competitors with the same yield-cost spread.
What to watch for
- Trend, not level. Eight quarters of NIM history tell a more useful story than one quarter's number.
- Securities vs. loans mix. A bank shifting from securities into loans usually sees NIM rise. The reverse compresses it.
- Loan growth quality. Rising NIM driven by aggressive, high-yield lending might also show up as rising non-performing loans 12-18 months later.
- Deposit beta. When rates change, what percent of the change shows up in the bank's funding cost? Low-beta deposit franchises are gold during hikes.
Related reading
- What is a UBPR? - NIM is one of the four ratios on the front page.
- What is a bank's efficiency ratio? - efficiency and NIM together drive most of a bank's profitability.
- How do FFIEC peer groups work? - peer-group context is essential for reading NIM.