On September 19, 2026, VolRoc recorded 513,795 active prediction markets across Kalshi and Polymarket, holding $2.51B in combined open interest. Day over day, active markets rose 13.1% and open interest rose 9.2% (vs September 18, 2026). Politics & Elections was the largest domain by open interest (23%), ahead of Sports (16%); together they accounted for 40% of all open interest. Kalshi carried 92% of active markets and Polymarket 8%.
Deterministic · template v1 · 2026-09-19
As of 2026-09-19 · 00:00 UTC
Active Markets
513,795
00:00 UTC freeze
Open Interest
$2.51B
Kalshi OI + Polymarket liquidity
Markets Traded · 24h
27,327
with trading activity
Kalshi / Polymarket
92% / 8%
by active-market count
Domain Composition — 12 domains
Mixed / Multi-category
429,358
Sports
33,872
Other
30,610
Politics & Elections
10,439
Entertainment & Culture
3,083
Crypto & Digital Assets
1,520
Technology & AI
1,389
Economics & Inflation
969
Companies & Business
846
World & Geopolitics
810
Monetary Policy
480
Climate & Weather
303
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 = 513,795.
Largest Kalshi Series
Cross-category multi-markets
351,352
KXMVECROSSCATEGORY0
78,006
Midterm control
1,396
Votegeneral
1,187
Ncaafspread
882
Music — artist streams
682
U.S. House races
649
Ncaaftotal
609
Ncaafwins
430
Nextteamnba
425
Nfltd
418
Nflwins
404
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 19, 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 19, 2026.” https://volroc.ai/prediction-markets/2026-09-19/
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