On September 18, 2026, VolRoc recorded 454,156 active prediction markets across Kalshi and Polymarket, holding $2.29B in combined open interest. Day over day, active markets rose 41.0% and open interest rose 12.5% (vs September 17, 2026). Politics & Elections was the largest domain by open interest (25%), ahead of Sports (18%); together they accounted for 44% of all open interest. Kalshi carried 90% of active markets and Polymarket 10%.
Deterministic · template v1 · 2026-09-18
As of 2026-09-18 · 00:00 UTC
Active Markets
454,156
00:00 UTC freeze
Open Interest
$2.29B
Kalshi OI + Polymarket liquidity
Markets Traded · 24h
25,331
with trading activity
Kalshi / Polymarket
90% / 10%
by active-market count
Domain Composition — 12 domains
Mixed / Multi-category
369,536
Sports
33,510
Other
31,102
Politics & Elections
10,450
Entertainment & Culture
3,101
Crypto & Digital Assets
1,556
Technology & AI
1,387
Economics & Inflation
975
Companies & Business
839
World & Geopolitics
817
Monetary Policy
476
Climate & Weather
290
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 = 454,156.
Largest Kalshi Series
Cross-category multi-markets
311,814
KXMVECROSSCATEGORY0
57,722
Midterm control
1,382
Votegeneral
1,181
Ncaafspread
737
Music — artist streams
689
U.S. House races
649
Ncaaftotal
519
Ncaafwins
438
Nextteamnba
425
Nflwins
403
Nflspread
322
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 18, 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 18, 2026.” https://volroc.ai/prediction-markets/2026-09-18/
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