AP Syllabus focus: ‘The money multiplier is the ratio of the money supply to the monetary base.’
Understanding the money multiplier helps you link central bank actions to broader measures of money. This topic explains the monetary base, how bank balance sheets amplify it, and what the multiplier means conceptually.
Core idea: base money vs. broader money
The financial system contains a small set of “high-powered” funds that support a larger quantity of spendable bank money. The connection between them is summarised by the money multiplier, which compares a broad money measure to the monetary base.
Monetary base (high-powered money)
Monetary base (MB): The sum of currency held by the public plus bank reserves held at the central bank.
The monetary base matters because it is the foundation on which depository institutions can create additional checkable deposit money through lending and deposit creation.
Money supply (what the public uses for payments)
Money supply (M): A measured stock of money used for transactions, typically captured by a monetary aggregate such as M1 or M2.
In this subtopic, the key is not which aggregate is chosen, but that the same aggregate must be used consistently when interpreting the multiplier.
The money multiplier: what the ratio means
The money multiplier is a number that indicates how many dollars of measured money supply exist for each dollar of monetary base. It captures the banking system’s ability to transform base money into deposits that households and firms can spend.
= money multiplier (unitless ratio)
= money supply (dollars, as measured by a chosen aggregate)
= monetary base (dollars)
= implied money supply supported by a given monetary base (dollars)
= underlying monetary base (dollars)
A higher multiplier means the same monetary base supports a larger measured money supply; a lower multiplier means less deposit expansion relative to the base.
How the monetary base supports a larger money supply
The multiplier is fundamentally about balance sheets and the distinction between reserves and deposits.
Mechanism in words (no arithmetic)
Reserves are the banking system’s most liquid funds for meeting payments and withdrawals.
When a bank makes a loan, it typically creates a deposit for the borrower.
That new deposit can be spent; when it is received and redeposited elsewhere, it becomes part of the measured money supply.
Because only part of deposits must be held as reserves at any moment, the system can maintain deposits larger than reserves, so can exceed .
This is why the money multiplier is usually greater than 1 in a fractional reserve banking environment, even though the monetary base is the original support.
This graph illustrates how deposit creation accumulates through repeated rounds of lending and redepositing in a fractional-reserve system. Each curve corresponds to a different reserve requirement, showing that a lower reserve ratio implies a larger eventual expansion of deposits (i.e., a larger effective multiplier). Source
Interpreting changes in the multiplier
Because , the multiplier can change even if the central bank does not change the monetary base.
This article’s figures compare the monetary base (cash plus bank reserves) with a broader money measure and discuss how their ratio—the money multiplier—can shift over time. The visual separation of the monetary base into currency and reserves supports the idea that changes in banks’ desired reserve holdings can weaken the link from to even when the formula stays the same. Source
What can move the ratio (conceptually)
Banks’ desired reserve holdings: if banks choose to hold a larger fraction of their funds as reserves, deposit expansion tends to be smaller relative to , reducing .
Public preference for currency vs. deposits: if households hold more currency rather than leaving funds as deposits, a larger share of sits as currency and a smaller share supports deposit creation, reducing .
Financial conditions and risk perceptions: when lending is less attractive, banks may expand credit less aggressively, weakening the link from to and lowering .
Why AP Macroeconomics emphasises the multiplier-to-base link
The syllabus focus—“the money multiplier is the ratio of the money supply to the monetary base”—provides a compact way to connect:
central bank influence over MB
the banking system’s role in shaping M
the idea that broader money can expand by a multiple of base money under typical banking practices
FAQ
No. It can vary with banks’ willingness to lend, regulatory pressures, and shifts in the public’s preference for currency versus deposits.
Even if $MB$ is unchanged, $M$ can change if the ratio $m=\dfrac{M}{MB}$ changes.
It depends on the question’s definition of “money supply.” Use the aggregate specified (e.g. M1 or M2) and keep it consistent.
Different aggregates can produce different multipliers because they include different kinds of deposits.
It is possible in principle if measured $M$ is small relative to $MB$, for example when banks hold very large reserves and deposit creation is weak.
Whether that occurs depends on institutions and what is included in $M$.
If the central bank pays interest on reserves, banks may be more willing to hold reserves rather than expand lending.
That behavioural shift can reduce deposit expansion and lower $m$ for a given $MB$.
Because it is the portion of money most directly linked to the central bank and it underpins the banking system’s ability to support deposits.
Small changes in $MB$ can correspond to larger changes in $M$ when $m$ is sizeable.
Practice Questions
(2 marks) Define the money multiplier and state its relationship to the monetary base.
1 mark: Defines money multiplier as a ratio of money supply to monetary base.
1 mark: States (or equivalently ).
(5 marks) Using the concept of , explain how the money supply can be larger than the monetary base and describe two reasons the multiplier might fall.
1 mark: Explains that the monetary base consists of currency plus reserves.
1 mark: Explains that bank lending can create deposits, increasing measured money supply.
1 mark: Links this to exceeding (multiplier greater than 1).
1 mark: One valid reason multiplier falls (e.g. banks hold more reserves relative to deposits).
1 mark: Second valid reason multiplier falls (e.g. public holds more currency relative to deposits).
