4.4 Banking and the Expansion of the Money Supply

Q: What is a bank balance sheet (T-account) and what is its fundamental accounting equation?

A bank balance sheet is a financial statement where Assets must equal Liabilities plus Equity ($Assets = Liabilities + Equity$).

Q: What is the difference between Required Reserves and Excess Reserves?

Required Reserves are the minimum funds a bank must legally hold, while Excess Reserves are any funds held beyond that requirement that are available for lending.

Q: How do you calculate the total change in the money supply from a central bank bond purchase?

$ ext{Total Change} = ext{Initial Excess Reserves} imes ext{Money Multiplier}$.

Q: Why is the actual expansion of the money supply often less than the "Maximum Money Multiplier" suggests?

Expansion is limited by leakages , specifically banks holding excess reserves and the public holding cash (currency drain).

Q: What is the difference between the Monetary Base and the Money Supply?

The Monetary Base is currency plus bank reserves, while the Money Supply is currency plus checkable deposits.

AP Macroeconomics · AP (Advanced Placement) · 5 min read · Updated: 7 Sept 2026

Welcome to the Money Factory: How Banks Create Money

Have you ever wondered how a $\$100$ bill in your pocket can turn into $\$500$ or even $\$1,000$ in the total economy? It sounds like magic, but it’s actually the result of the banking system! In this chapter, we are going to look under the hood of a bank to see how they use your deposits to expand the money supply. This is a core part of Unit 4: The Financial Sector and is a favorite topic for AP Exam questions involving calculations.

Don’t worry if the math seems intimidating at first. Once you see the pattern, it’s as simple as basic division!

1. The Bank Balance Sheet (The T-Account)

To understand how money expands, we first need to look at a bank's Balance Sheet, often called a T-Account. In accounting, a balance sheet must always balance: Assets = Liabilities + Equity. For AP Macroeconomics, we focus on two main sides:

Liabilities: Money the bank owes to others.
• Demand Deposits: This is the money you put into your checking account. You can "demand" it back at any time, so the bank owes it to you.

Assets: Money the bank owns or is owed by others.
• Required Reserves: The percentage of deposits the law says the bank must keep in the vault (or at the Federal Reserve).
• Excess Reserves: Any reserves held above the required amount. This is the money the bank is free to lend out.
• Loans: Money lent to borrowers. This is an asset because the borrower must pay it back with interest.

Quick Review: If you deposit $\$1,000$ into a bank, the bank’s Liabilities go up by $\$1,000$ (Demand Deposits). Its Assets also go up by $\$1,000$ (Reserves).

2. Fractional Reserve Banking and the Reserve Ratio

In our economy, we use Fractional Reserve Banking. This means banks only keep a fraction of your money in the vault and lend the rest out. This is how the money supply grows!

The Required Reserve Ratio ($rr$): This is the fraction of deposits that the central bank (the Fed) requires banks to hold.
Example: If the reserve ratio is $10\%$ (or $0.10$), and you deposit $\$100$:
• Required Reserves = $\$10$
• Excess Reserves = $\$90$ (The bank can now lend this out!)

Key Formula to Remember:

$\text{Total Reserves} = \text{Required Reserves} + \text{Excess Reserves}$

3. The Money Multiplier

When a bank lends out its Excess Reserves, that money gets spent and eventually deposited into another bank. That second bank then lends out its excess reserves, and the cycle continues. This "ripple effect" is measured by the Money Multiplier.

The Formula:

$\text{Maximum Money Multiplier} = \frac{1}{\text{Required Reserve Ratio (rr)}}$

Example: If the reserve ratio is $20\%$ ($0.2$):
$\text{Multiplier} = \frac{1}{0.2} = 5$

This means for every $\$1$ of new excess reserves created in the banking system, the total money supply can potentially increase by $\$5$.

4. Calculating the Expansion of the Money Supply

There are two main scenarios the AP exam will ask you to calculate. Pay close attention to the wording!

Scenario A: A Central Bank Bond Purchase

When the Federal Reserve buys bonds from a bank, they are putting "fresh" money into the system that wasn't there before.
Total Change in Money Supply = $\text{Initial Excess Reserves} \times \text{Money Multiplier}$

Scenario B: A Cash Deposit by a Customer

If you take $\$100$ from under your mattress and put it in a bank, the immediate money supply (M1) doesn't change—you just traded cash for a digital deposit. However, the bank can now lend out a portion of that $\$100$.
Maximum Change in Checkable Deposits = $\text{Initial Excess Reserves} \times \text{Money Multiplier}$

Memory Trick: If the question asks for the "Maximum Change," they want the total amount created. If they ask for the "Change in Loans," they want the same thing!

5. Real-World Limitations: Why the Multiplier is "Maximum"

In the "perfect" world of textbook math, the money supply expands by the full multiplier. In the real world, the expansion is usually smaller. Why?

Banks Holding Excess Reserves: If banks are nervous about the economy, they might hold onto more money than required rather than lending it all out.
Currency Drain (Leakage): If a borrower gets a loan and keeps the cash in their wallet instead of depositing it back into a bank, the "multiplication" stops.

Key Takeaway: The Simple Money Multiplier overstates the actual expansion of the money supply because it assumes banks hold zero excess reserves and the public holds no cash.

6. Monetary Base vs. Money Supply

It is important to distinguish between these two concepts, which we first encountered in Chapter 4.3:

• Monetary Base: Includes only currency in circulation and bank reserves (required and excess). Think of this as the "raw material."
• Money Supply: Includes currency in circulation and checkable deposits.
The Money Multiplier is the ratio that links these two. It shows how the Money Supply relates to the Monetary Base.

Did you know? The Federal Reserve can influence the money supply by changing the reserve requirement, though in the modern U.S. system (as noted in Chapter 4.6), they often use other tools like interest on reserves!

Summary and Quick Review

1. Required Reserves: What the bank must keep ($\text{Deposits} \times \text{rr}$).
2. Excess Reserves: What the bank can lend ($\text{Total Reserves} - \text{Required Reserves}$).
3. Multiplier: $\frac{1}{rr}$.
4. Maximum Expansion: $\text{Excess Reserves} \times \text{Multiplier}$.
5. Leakages: Holding excess reserves or cash reduces the multiplier's effect.

Common Mistake to Avoid: When calculating Required Reserves, only apply the percentage to the Demand Deposit. Do not apply it to the bank's other assets like buildings or existing loans!

Quick check

Can you answer these now?

Open each question to check the key ideas from this chapter.

What is a bank balance sheet (T-account) and what is its fundamental accounting equation?

A bank balance sheet is a financial statement where Assets must equal Liabilities plus Equity ($Assets = Liabilities + Equity$).

What is fractional reserve banking?

Fractional reserve banking is a system where banks keep only a fraction of their deposits as reserves and lend out the remainder.

What is the difference between Required Reserves and Excess Reserves?

Required Reserves are the minimum funds a bank must legally hold, while Excess Reserves are any funds held beyond that requirement that are available for lending.

What is the Money Multiplier and how is it calculated?

The Money Multiplier measures the maximum amount of money the banking system generates with each dollar of excess reserves; it is calculated as $\frac{1}{rr}$.

How do you calculate the total change in the money supply from a central bank bond purchase?

$\text{Total Change} = \text{Initial Excess Reserves} \times \text{Money Multiplier}$.

How does a cash deposit by a customer affect the money supply (M1) immediately versus over time?

Initially, M1 remains unchanged; over time, the money supply expands by $\text{Initial Excess Reserves} \times \text{Money Multiplier}$.

Why is the actual expansion of the money supply often less than the "Maximum Money Multiplier" suggests?

Expansion is limited by leakages, specifically banks holding excess reserves and the public holding cash (currency drain).

What is the difference between the Monetary Base and the Money Supply?

The Monetary Base is currency plus bank reserves, while the Money Supply is currency plus checkable deposits.

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