On September 25, 2026, VolRoc recorded 382,971 active prediction markets across Kalshi and Polymarket, holding $2.18B in combined open interest. Day over day, active markets rose 35.9% and open interest rose 8.3% (vs September 24, 2026). Politics & Elections was the largest domain by open interest (27%), ahead of Sports (18%); together they accounted for 45% of all open interest. Kalshi carried 92% of active markets and Polymarket 8%.
Deterministic · template v1 · 2026-09-25
As of 2026-09-25 · 00:00 UTC
Active Markets
382,971
00:00 UTC freeze
Open Interest
$2.18B
Kalshi OI + Polymarket liquidity
Markets Traded · 24h
24,863
with trading activity
Kalshi / Polymarket
92% / 8%
by active-market count
Domain Composition — 12 domains
Mixed / Multi-category
308,094
Other
28,408
Sports
26,639
Politics & Elections
10,109
Entertainment & Culture
3,124
Crypto & Digital Assets
1,619
Technology & AI
1,344
Economics & Inflation
1,046
Companies & Business
881
World & Geopolitics
775
Monetary Policy
491
Climate & Weather
325
Science & Health
116
Venue-agnostic subject taxonomy from Kalshi series prefixes and
Polymarket title keywords. A residual Other bucket is reported honestly.
Yes-price across both venues, 10 buckets from 0 to 100. n = 382,971.
Largest Kalshi Series
Cross-category multi-markets
258,674
KXMVECROSSCATEGORY0
49,420
Midterm control
1,543
Votegeneral
1,256
Ncaafspread
779
U.S. House races
656
Music — artist streams
654
Ncaaftotal
495
Ncaafwins
437
Nextteamnba
425
Nflwins
407
Midtermvoteturn
317
Kalshi market families (Polymarket is not series-structured).
About & Citation
This is the fixed-hour (00:00 UTC) state of every active market on
Kalshi and Polymarket for September 25, 2026. The live
Observatory
shows the current diurnal value; this record is immutable. Open interest combines Kalshi
open interest with Polymarket USD liquidity as comparable USD magnitudes. Sources: Kalshi, Polymarket.
How to cite
“VolRoc Prediction Market Archive — September 25, 2026.” https://volroc.ai/prediction-markets/2026-09-25/
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