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Net Interest Margin

The net interest margin (NIM) is how much profit a bank makes on the spread between what it earns on loans and investments and what it pays out on deposits and borrowings. It is calculated as (interest income minus interest expense) divided by average earning assets, and it is the single largest lever on bank profitability.

How Net Interest Margin Is Calculated

A bank earns interest on mortgages, commercial loans, treasury bills, and other assets. It pays interest on checking accounts, savings deposits, money market deposits, and borrowings from other banks or the Federal Reserve.

The difference is net interest income (NII). To convert that to a margin:

Net Interest Margin = (NII) ÷ (Average Earning Assets)

Example: A bank earns $8 billion in interest income, pays $3 billion in interest expense, and holds $400 billion in average earning assets.

  • Net interest income: $8B − $3B = $5B
  • Net interest margin: $5B ÷ $400B = 1.25%

That is a compressed margin by historical standards. The same bank earning 3.0% NIM would generate $12 billion in net interest income on the same asset base.

The Yield Curve and NIM

The shape of the yield curve—the spread between short-term and long-term interest rates—is the most powerful lever on NIM.

A steep yield curve (long-term rates much higher than short-term) favors banks. They borrow short (pay low rates on deposits) and lend long (earn high rates on mortgages and bonds). A steep curve in the early 2010s—with short-term fed funds rates near zero and 10-year Treasury yields at 2.5%+—created spectacular NIM expansion for banks. JPMorgan and other large commercial banks posted NIM above 3.5%.

A flat or inverted yield curve (short-term rates equal to or higher than long-term rates) crushes NIMs. When the curve inverted in 2022, with 3-month Treasury bills yielding 4.5% and 10-year Treasuries at 3.5%, banks’ borrowing costs rose toward their lending rates. Competition for deposits also intensified—banks had to offer 5%+ on savings accounts to retain depositors—further squeezing the spread.

A steep curve + rising rates is the ideal environment for NIM expansion. Rates move up across the board, but long-term rates rise faster than short-term, and deposit competition lags behind loan-rate increases. This was the environment of 2023–2024: the Fed raised the fed funds rate to 5.25–5.50%, long-term rates rose more, banks repriced loan portfolios upward, and deposit rates lagged. NIMs expanded broadly.

Components: What Moves the Margin

Interest income sources:

Interest expense sources:

As rates rise, newly originated loans reprice immediately, but the deposit side lags. Customers do not notice a 50 basis-point fed funds rate increase right away, and many banks can keep deposit rates flat for months. This creates a window of NIM expansion.

But as rates stay elevated, deposit competition intensifies. Competitors raise rates, and depositors move funds to whoever offers the highest yield. To retain deposits, a bank must raise its rates, and the margin benefit shrinks.

Real-World Example: 2022–2024

In 2021, with fed funds rate near zero and the yield curve steep but normalized, large banks’ NIMs were 1.5–2.0%. Not great, but steady.

In March 2022, the Fed began aggressive rate hikes. By June 2022, the fed funds rate was 1.50–1.75%. Banks immediately repriced floating-rate loans and benefited from higher yields on new securities. But deposits still paid near zero. NIM expanded rapidly.

By September 2022, the fed funds rate was 3.00–3.25%, and the NIM expansion was obvious: JPMorgan and Bank of America reported NIMs near 2.5%—a gain of ~0.5 percentage points in just six months.

By late 2023, the fed funds rate was stable at 5.25–5.50%, and deposit rates had caught up. Banks could no longer expand NIMs through repricing; new loans yielded more, but deposits demanded higher rates too. NIM expansion slowed and eventually stabilized or contracted.

A 0.1% decline in NIM on a $400 billion asset base is $40 million in lost annual earnings—material for a large bank but absorbed into overall profitability.

Non-Interest Income and the Full Picture

Net interest margin is not the whole story of bank profitability. Banks also earn non-interest income:

  • Fees on deposits and loans
  • Investment banking and advisory fees
  • Wealth management fees
  • Trading revenue
  • Insurance and card interchange fees

A bank with a 2.0% NIM but strong fee income (another 1.0%+ as a percentage of assets) can be quite profitable. Conversely, a bank with a bloated balance sheet and low margin might not be.

But NIM is the foundation. When NIMs compress due to a flat curve or deposit competition, banks struggle to offset losses in non-interest income.

Margin Compression vs. Expansion

Compression occurs when:

  • The yield curve flattens or inverts (long-term rates fall relative to short-term).
  • Deposit rates rise faster than loan rates can.
  • Credit quality deteriorates, forcing banks to book loan-loss provisions against future losses, which reduce net interest income on an accounting basis.
  • The Federal Reserve raises rates faster than banks can reprice mortgages and fixed-rate loans.

Expansion occurs when:

  • The curve steepens.
  • Rates rise, and deposit inflows exceed outflows, allowing banks to delay deposit-rate increases.
  • Loan demand is strong and credit quality is good, so banks can charge wider spreads to compensate for risk.
  • The Fed signals a long hiking cycle, and banks can lock in wider spreads on long-term mortgages before rates stabilize.

Why Banks and Analysts Care

For equity investors, NIM is often the single most important driver of bank stock valuations. A 0.25% expansion in NIM can translate to 5–10% earnings growth if the bank holds assets flat. Conversely, compression can erase profit growth overnight.

For credit ratings and bond holders, NIM affects the bank’s ability to absorb loan losses. A bank with a 2.0% NIM has less cushion to cover credit deterioration than one with a 3.0% NIM.

For regulators and capital adequacy rules, NIM trends inform stress tests. If rates fall sharply, what happens to the bank’s profitability? If NIM falls to 1.5%, can the bank still meet capital requirements?

See also

Wider context