THE BANK THAT MADE A FORTUNE FROM THE DIFFERENCE BETWEEN TWO INTEREST RATES
A bank can borrow money at one rate, lend it at another, and make money from the gap.
This sounds almost too simple.
But it is one of the fundamental ideas behind banking.
Imagine a bank pays its depositors:
2%
and lends that money to customers at:
7%
There is a:
5 percentage-point difference.
That difference is part of the bank's net interest income.
Scale it across billions of euros or dollars of loans and deposits, and a small percentage difference can become an enormous amount of money.
THE BANK ISN'T JUST "KEEPING YOUR MONEY"
When you deposit €10,000 into a bank, the bank doesn't simply put your exact €10,000 into a box with your name on it.
Your deposit becomes part of the bank's funding.
The bank can use its balance sheet to finance assets such as:
The bank pays interest on some of its funding.
It earns interest on its assets.
That creates the basic banking spread.
HERE'S THE SIMPLE VERSION
Imagine a bank has:
€1 billion of deposits
It pays depositors an average:
2%
Annual interest expense:
€20 million
Now imagine the bank uses its balance sheet to hold:
€1 billion of interest-earning assets
earning an average:
6%
Annual interest income:
€60 million
The simplified difference is:
€60 million − €20 million = €40 million
That is the basic idea behind net interest income.
The Federal Reserve defines net interest margin as the difference between interest income and interest expense relative to interest-earning assets.
THE BANK MAKES MONEY FROM THE GAP
Think of the process like this:
Depositors provide funding
↓
Bank pays interest
↓
Bank uses funding to support interest-earning assets
↓
Loans and securities generate interest
↓
Interest earned − interest paid
↓
Net interest income
The bigger the spread and the larger the balance sheet, the greater the potential income.
But there is a catch.
The bank doesn't get to keep all of it as profit.
THE 5% GAP IS NOT PURE PROFIT
Suppose the bank earns:
€60 million
and pays:
€20 million
The €40 million difference isn't automatically €40 million of profit.
The bank still has expenses.
It may have to pay for:
So the interest-rate spread is an important source of revenue, not a magic profit number.
WHAT IF INTEREST RATES CHANGE?
This is where banking becomes much more complicated.
Suppose the bank has loans earning:
6%
But the interest it pays on deposits rises from:
2% → 5%
The gap shrinks from:
4 percentage points
to:
1 percentage point.
The bank can suddenly be earning much less from the same basic business.
The Federal Reserve has noted that bank net interest income and margins can change as market rates change and as banks' funding costs and asset composition change.
THIS IS WHY BANKS CARE ABOUT DEPOSIT RATES
Imagine you have €100,000 sitting in a bank account.
The bank might prefer paying you:
1%
rather than:
4%
because its funding cost is lower.
But if another bank offers you:
4%
you may move your money.
Now the first bank has a problem.
It may have to raise the rate it pays depositors to keep the money.
That increases its funding cost.
And that can squeeze its interest margin.
BANKS ARE CONSTANTLY BALANCING TWO SIDES
A bank's balance sheet has two sides.
ASSETS
Things that can generate income:
LIABILITIES
Sources of funding:
The bank wants its assets to generate enough income to cover:
Funding costs + operating costs + credit losses
with something left over.
THE STRANGE PART: THE BANK DOESN'T NECESSARILY NEED A HUGE SPREAD
A bank doesn't necessarily need a gigantic difference between lending and deposit rates.
Suppose a bank has:
€100 billion
of interest-earning assets.
A net interest margin of just:
2%
would correspond to roughly:
€2 billion
of net interest income before considering other expenses and adjustments.
That is why tiny percentage changes can matter enormously to large banks.
BUT THE BANK TAKES RISKS TO EARN THAT INTEREST
The bank isn't simply collecting free money.
If it lends €300,000 for a mortgage, the borrower might stop paying.
If it lends €10 million to a company, that company could fail.
If it buys a bond, its value can change when interest rates move.
So the bank is being compensated for taking financial risks.
That's why the interest rate charged to a borrower is not simply:
"Whatever the bank wants."
It reflects funding costs, credit risk, competition, market conditions and other factors.
THE MOST DANGEROUS MISMATCH
Here's where the simple model breaks down.
Imagine a bank funds itself largely with deposits whose rates can change quickly.
But it owns a large portfolio of loans that pay fixed interest for many years.
Suppose those loans earn:
3%
Then market rates rise dramatically.
New loans can earn:
6%
But the bank is still receiving only:
3%
from its old fixed-rate loans.
Meanwhile, depositors may start demanding higher rates.
The bank's margin gets squeezed from both directions.
Asset income stays low.
Funding costs rise.
THIS IS WHY BANKS WATCH THE YIELD CURVE
Banks don't all borrow and lend at the same maturities.
A bank might have:
Short-term funding
supporting
Longer-term assets.
That creates interest-rate risk.
If short-term rates rise faster than the returns on longer-term assets, the spread can narrow.
If the opposite happens, the bank's margin can improve.
The exact effect depends on the bank's balance sheet.
THE FEDERAL RESERVE WATCHES THIS TOO
The Federal Reserve regularly tracks bank assets, liabilities, loans, deposits and other balance-sheet data.
As of July 2026, U.S. commercial banks had roughly:
$6.87 trillion in total assets
and:
$5.64 trillion in deposits
according to the Fed's H.8 data.
At that scale, even a relatively small change in interest rates can have significant effects across the banking system.
THE BUSINESS MODEL IS OLDER THAN MODERN BANKING
The underlying idea isn't new.
A financial institution takes funds from one group of people or institutions and deploys those funds into assets that can generate returns.
The difference between what it earns and what it pays is one of the fundamental economics of banking.
Modern banks simply do this on an enormous scale, with complicated balance sheets and many different sources of income and funding.
WHY BANKS CAN MAKE MONEY WHEN RATES ARE HIGH
Higher interest rates don't automatically mean banks lose.
If a bank can reprice its loans faster than its deposits become more expensive, its interest income can rise faster than its funding costs.
That can increase net interest income.
But if deposit costs rise quickly while the bank's assets are locked into low rates, the opposite can happen.
So the important question isn't simply:
"Are interest rates high?"
It's:
"How quickly do the bank's assets and funding reprice?"
THE REAL BUSINESS IS THE SPREAD
You can simplify the entire concept to:
Money comes in
↓
Bank pays for that money
↓
Bank deploys the money
↓
Bank earns a return
↓
Difference becomes net interest income
↓
Expenses and losses are deducted
↓
Remaining earnings contribute to profit
That's the basic engine.
AND THE NUMBERS SCALE FAST
Imagine a bank has:
€500 billion
of interest-earning assets.
A hypothetical:
1 percentage-point
net interest spread across that amount would represent approximately:
€5 billion
of annual net interest income before other adjustments and expenses.
That's why banks spend so much time thinking about fractions of a percentage point.
To an individual customer, a 0.25 percentage-point difference might look tiny.
To a bank with hundreds of billions in assets, it can be enormous.
MAACAT PERSPECTIVE
One of the simplest ways to understand banking is to stop thinking about a bank as a company that merely "holds your money."
Think of it as a company managing a giant balance sheet.
It has:
funding costs
and
asset returns.
The difference between them is one of the central engines of bank earnings.
A bank doesn't need to make €1 million on every transaction.
It can make a relatively small amount on a huge amount of money.
And that is the fascinating part:
A fraction of a percentage point can become billions when the balance sheet is big enough.
