M0 and the monetary base
The narrowest layer is physical currency — the notes and coins in circulation. Add the reserve balances that commercial banks hold in accounts at the Federal Reserve and you get the monetary base (sometimes written MB, and often loosely called M0). This is the only layer the Fed creates directly. It is also the layer the public can least observe in daily life: reserves never leave the banking system, and you cannot spend them at a store. When the Fed "prints money" through asset purchases, the monetary base is what expands first — whether broader money follows is a separate question, taken up in How Money Is Created.
M1: money you can spend today
M1 is currency held by the public plus deposits that can be spent immediately — checking accounts and, since May 2020, savings accounts as well. That date matters for anyone reading charts: when the Fed suspended Regulation D's six-withdrawal limit on savings accounts, savings deposits became fully liquid by definition and were reclassified from M2-only into M1. M1 jumped roughly fourfold overnight for definitional reasons, not because money was created. Any chart of M1 that spans 2020 has that cliff in it.
M2: money plus near-money
M2 is M1 plus small time deposits (CDs under $100,000) and retail money market fund balances — assets that are one short step from spendable. It is the headline U.S. money supply figure, currently about $23 trillion, and the series this site tracks on its dashboard. A fuller treatment is in What Is the M2 Money Supply?
M3 and broader measures
M3 added large time deposits, institutional money market funds, repurchase agreements, and eurodollars — money as held by institutions rather than households. The Fed stopped publishing it in March 2006, judging that it added little information beyond M2 and was costly to compile; the story is told in Why the Fed Stopped Publishing M3. Other countries' central banks (the ECB among them) still publish M3, and private reconstructions of U.S. M3 exist, with varying methodology.
Which one should you watch?
For most questions about the U.S. money supply, M2 is the practical choice: it is broad enough to capture household money, narrow enough to exclude wealth, and published weekly. The monetary base is the right lens for analyzing Fed operations specifically. M1 has been hard to interpret since the 2020 reclassification. And comparisons across layers are themselves informative — the gap between base money growth and M2 growth after 2008 is one of the clearest illustrations that central bank money creation and bank money creation are different processes.
Related reading
What Is the M2 Money Supply? · The Fed's Balance Sheet, Explained · Money supply calculator