A blended industry figure showed a 14.5% monthly decline. Splitting it by venue shows one platform roughly flat and the other down 45%.
The story the aggregate numbers have been telling about prediction markets is wrong, or at least badly incomplete.
The industry figure that has circulated since late August showed combined volume of $45.33 billion, down 14.5% month over month. Read as an industry statistic, it describes a category cooling off after an expansion.
Venue-level data on a rolling 30-day basis through tells a different story. Kalshi's 30-day volume was $14.4 billion, up 0.7%. Polymarket's was $2.8 billion, down 45.3%.
One platform is flat. The other lost nearly half its volume. The blended number averages those into a decline that describes neither.
The two windows are not the same
The figures are not measured on the same basis. The 14.5% decline is a calendar-month comparison. The venue split is a rolling 30-day window ending on a different date. Some of the difference between them is arithmetic rather than substantive, and no full reconciliation between the two windows has been established.
What survives that objection is the relative comparison, because both venue figures are measured on the same basis over the same window. Whatever the correct industry number is, Kalshi and Polymarket did not experience the same 30-day period.
Why the gap is plausible
The two platforms have diverged structurally. Kalshi operates as a CFTC-designated contract market with U.S. retail brokerage distribution, including order flow routed through mainstream trading apps. Polymarket's growth has historically been driven by crypto-native users and by a smaller number of very large event cycles.
A platform whose volume depends on a handful of headline markets is more exposed to the gap between them than one carrying continuous retail flow across many contracts. The relative size of the two figures, $14.4 billion against $2.8 billion, is itself part of the answer: Kalshi is now roughly five times Polymarket on this measure.
What it means for the fall
Both platforms are heading into the same catalysts, an NFL season and a midterm election cycle. If the volume decline were a category-wide demand problem, those catalysts would lift both. If it is specific to one venue's mix, the fall will widen the gap rather than close it.
That makes the next 30-day comparison the test. Anyone using blended industry volume as an indicator of prediction market health should stop, because the blend is currently hiding the only thing in the data worth knowing.