Banks make money by charging borrowers more interest than they pay for funding.
The size of that difference is closely related to net interest margin, or NIM.
For investors, net interest margin is one of the simplest ways to understand whether a bank’s core lending business is becoming more—or less—profitable.
The basic idea is:
interest earned on loans and securities − funding costs = net interest income
But interest rates and the yield curve can change that equation quickly.
What Is Net Interest Margin?
A bank receives interest from assets such as:
- mortgages
- business loans
- credit cards
- government bonds
It also pays interest on funding such as:
- customer deposits
- savings accounts
- certificates of deposit
- wholesale borrowing
Net interest margin compares the bank’s net interest income with its interest-earning assets.
Higher NIM generally means the bank is earning a wider spread.
A Simple Example
Imagine a bank has $100 billion of interest-earning assets.
Those assets earn an average:
5% = $5 billion of interest
The bank’s funding costs:
2% = $2 billion
Simplified net interest income:
$5 billion − $2 billion = $3 billion
That represents roughly a:
3% interest spread
If lending yields increase while funding costs remain stable, profitability can rise.
But if deposit costs increase faster, margins can shrink.
Why the Yield Curve Matters
The yield curve compares interest rates across different maturities.
Normally:
short-term rates < long-term rates
That can help banks because some funding is short-term while many loans have longer maturities.
For example:
Deposits cost 2%
Long-term loans earn 5%
The spread is attractive.
But if the yield curve becomes flat or inverted:
short-term funding costs can approach—or exceed—long-term yields
That can put pressure on net interest margin.
Banks Do Not Simply “Borrow Short and Lend Long”
That phrase is useful, but reality is more complicated.
Banks hold many different assets and funding sources.
Some loans have floating rates.
Some deposits pay almost no interest.
Other deposits reprice quickly.
A bank’s profitability therefore depends on how fast its assets and liabilities respond to changing interest rates.
This is called interest-rate sensitivity.
Why Rising Rates Can Help—or Hurt
Imagine lending rates rise:
4% → 6%
But deposit costs rise only:
1% → 2%
The bank’s spread expands.
That can be positive for earnings.
Now imagine deposits become highly competitive:
Loan yield: 6%
Deposit cost: 4.5%
The spread becomes much smaller.
So rising rates alone do not tell investors whether banks are becoming more profitable.
The key is:
Which side reprices faster?
Why an Inverted Yield Curve Matters
An inverted yield curve occurs when short-term rates are above longer-term rates.
That can create problems because:
funding becomes expensive while long-term lending yields may not rise enough
It can also signal expectations of slower economic growth.
For banks, that creates two potential risks:
lower margins + weaker loan demand
If the economy deteriorates, credit losses may also rise.
Expected Return vs Risk
Strong net interest margins can support higher bank earnings.
But investors should never examine NIM alone.
| Positive Signal | Risk |
|---|---|
| Rising loan yields | Higher deposit costs |
| Cheap deposits | Deposit outflows |
| Steeper yield curve | Economic slowdown |
| Loan growth | Higher defaults |
| Expanding NIM | Credit deterioration |
A bank can have excellent margins today but still face problems if borrowers begin defaulting.
That is why expected return must be balanced against funding, credit and interest-rate risk.
What Investors Should Watch
When analysing bank stocks, look at:
Net interest margin — Is it expanding or shrinking?
Deposit costs — Are customers demanding higher rates?
Loan growth — Is the bank finding profitable lending opportunities?
Yield curve — Are long-term yields sufficiently above short-term rates?
Credit losses — Are borrowers still performing?
Together, these numbers reveal far more than interest rates alone.
The Bottom Line
Net interest margin explains the economics at the heart of banking.
The simplest framework is:
asset yield − funding cost = banking spread
A favorable yield curve and cheap deposits can strengthen profitability.
But rising funding costs, an inverted yield curve or worsening credit quality can quickly reduce the benefit.
For investors, the question is therefore not simply:
“Are interest rates high?”
It is:
“How much is the bank earning after paying for the money it uses?”
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