On September 9, 2026, VolRoc recorded 345,302 active prediction markets across Kalshi and Polymarket, holding $2.19B in combined open interest. Day over day, active markets rose 43.2% and open interest rose 16.6% (vs September 8, 2026). Politics & Elections was the largest domain by open interest (27%), ahead of Sports (19%); together they accounted for 46% of all open interest. Kalshi carried 82% of active markets and Polymarket 18%.
Deterministic · template v1 · 2026-09-09
As of 2026-09-09 · 00:00 UTC
Active Markets
345,302
00:00 UTC freeze
Open Interest
$2.19B
Kalshi OI + Polymarket liquidity
Markets Traded · 24h
27,590
with trading activity
Kalshi / Polymarket
82% / 18%
by active-market count
Domain Composition — 12 domains
Mixed / Multi-category
205,240
Other
75,628
Sports
42,353
Politics & Elections
10,839
Entertainment & Culture
3,069
Crypto & Digital Assets
1,898
Climate & Weather
1,429
Technology & AI
1,368
Economics & Inflation
1,024
Companies & Business
918
World & Geopolitics
876
Monetary Policy
543
Science & Health
117
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 = 345,302.
Largest Kalshi Series
Cross-category multi-markets
205,240
—
40,026
Midterm control
1,267
Music — artist streams
728
U.S. House races
645
Nextteamnba
485
Ncaafwins
413
Ncaafspread
413
Nflwins
386
Nflspread
367
Mlbhrr
363
Ncaafgame
307
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 9, 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 9, 2026.” https://volroc.ai/prediction-markets/2026-09-09/
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