On September 17, 2026, VolRoc recorded 322,110 active prediction markets across Kalshi and Polymarket, holding $2.04B in combined open interest. Day over day, active markets rose 1.2% and open interest fell 3.5% (vs September 16, 2026). Politics & Elections was the largest domain by open interest (29%), ahead of Sports (19%); together they accounted for 47% of all open interest. Kalshi carried 86% of active markets and Polymarket 14%.
Deterministic · template v1 · 2026-09-17
As of 2026-09-17 · 00:00 UTC
Active Markets
322,110
00:00 UTC freeze
Open Interest
$2.04B
Kalshi OI + Polymarket liquidity
Markets Traded · 24h
25,470
with trading activity
Kalshi / Polymarket
86% / 14%
by active-market count
Domain Composition — 12 domains
Mixed / Multi-category
236,110
Sports
34,124
Other
31,844
Politics & Elections
10,506
Entertainment & Culture
3,082
Crypto & Digital Assets
1,639
Technology & AI
1,380
Economics & Inflation
939
World & Geopolitics
823
Companies & Business
803
Monetary Policy
460
Climate & Weather
285
Science & Health
115
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 = 322,110.
Largest Kalshi Series
Cross-category multi-markets
199,770
KXMVECROSSCATEGORY0
36,340
Midterm control
1,365
Votegeneral
1,170
Music — artist streams
696
U.S. House races
649
Ncaafspread
615
Ncaaftotal
449
Ncaafwins
433
Nextteamnba
424
Nflwins
399
Ncaafgame
314
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 17, 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 17, 2026.” https://volroc.ai/prediction-markets/2026-09-17/
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