On September 28, 2026, VolRoc recorded 258,081 active prediction markets across Kalshi and Polymarket, holding $823.5M in combined open interest. Day over day, active markets fell 43.3% and open interest fell 65.2% (vs September 27, 2026). Politics & Elections was the largest domain by open interest (37%), ahead of Sports (15%); together they accounted for 53% of all open interest. Kalshi carried 88% of active markets and Polymarket 12%.
Deterministic · template v1 · 2026-09-28
As of 2026-09-28 · 00:00 UTC
Active Markets
258,081
00:00 UTC freeze
Open Interest
$823.5M
Kalshi OI + Polymarket liquidity
Markets Traded · 24h
14,659
with trading activity
Kalshi / Polymarket
88% / 12%
by active-market count
Domain Composition — 12 domains
Mixed / Multi-category
218,379
Sports
14,580
Other
12,877
Politics & Elections
7,028
Crypto & Digital Assets
1,339
Technology & AI
1,269
World & Geopolitics
747
Companies & Business
522
Entertainment & Culture
514
Economics & Inflation
343
Climate & Weather
199
Monetary Policy
198
Science & Health
86
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 = 258,081.
Largest Kalshi Series
Cross-category multi-markets
174,175
KXMVECROSSCATEGORY0
44,204
Nflrecyds
439
Nflrec
275
Nflrshyds
261
Nfltd
247
Dpworldtour
151
Votecounty
144
Nflspread
143
Ncaafgame
141
Ggnom
110
Ncaafspread
109
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 28, 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 28, 2026.” https://volroc.ai/prediction-markets/2026-09-28/
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