What it measures
Velocity is nominal GDP divided by the money supply: how many times, on average, each dollar of money turns over in a year's transactions. With M2 at roughly $23 trillion and nominal GDP somewhat above that, M2 velocity today sits in the neighborhood of 1.3–1.4 turns per year. Velocity isn't measured directly — no one tracks individual dollars — it is computed as a residual, which is both its convenience and its weakness.
The equation it comes from
Velocity is one leg of the equation of exchange, MV = PQ: money times velocity equals the price level times real output. The equation is an identity — true by definition — and all the economics lives in what you assume about its parts. The strict quantity theory assumes V is stable and Q grows on its own trend, so money growth passes through to prices. Keynes's liquidity preference is, at bottom, a theory of why V moves. The equation settles nothing by itself, but it forces every argument about money and inflation to be explicit about velocity — which is exactly what most casual arguments skip.
Why velocity has fallen
M2 velocity climbed through the postwar decades to a peak around 2.2 in the late 1990s, then fell for twenty-five years, hitting record lows near 1.1 in 2020. Candidate explanations stack up: falling interest rates lowered the cost of holding idle money; households and firms built precautionary balances after 2008; an aging population holds more cash-like savings; and much of the money created by QE-era policy landed with holders who had no intention of spending it. The 2020 stimulus made the point dramatically — M2 jumped 25% while velocity collapsed, because the new money initially sat in accounts. When spending resumed in 2021–22, velocity turned back up, and inflation arrived with it.
Why it matters for reading money data
Velocity is the honest answer to "if the money supply grew 40%, why didn't prices rise 40%?" Money growth translates to inflation only to the extent velocity holds up. That makes velocity the crucial caveat for tools like this site's money supply calculator: dilution of the money stock is a fact, but its effect on prices is mediated by how eagerly the new dollars circulate. Watching M2 growth and velocity together tells you far more than either alone — one measures the fuel, the other the burn rate.
Related reading
Does Printing Money Cause Inflation? · What Is the M2 Money Supply? · Current M2 data