On September 21, 2026, VolRoc recorded 212,165 active prediction markets across Kalshi and Polymarket, holding $681.4M in combined open interest. Day over day, active markets fell 55.8% and open interest fell 49.6% (vs September 20, 2026). Politics & Elections was the largest domain by open interest (45%), ahead of Sports (13%); together they accounted for 58% of all open interest. Kalshi carried 83% of active markets and Polymarket 17%.
Deterministic · template v1 · 2026-09-21
As of 2026-09-21 · 00:00 UTC
Active Markets
212,165
00:00 UTC freeze
Open Interest
$681.4M
Kalshi OI + Polymarket liquidity
Markets Traded · 24h
10,902
with trading activity
Kalshi / Polymarket
83% / 17%
by active-market count
Domain Composition — 12 domains
Mixed / Multi-category
172,787
Sports
15,939
Other
11,450
Politics & Elections
7,102
Crypto & Digital Assets
1,373
Technology & AI
1,288
World & Geopolitics
763
Companies & Business
483
Entertainment & Culture
352
Economics & Inflation
242
Climate & Weather
152
Monetary Policy
151
Science & Health
83
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 = 212,165.
Largest Kalshi Series
Cross-category multi-markets
137,321
KXMVECROSSCATEGORY0
35,466
Ncaafspread
167
Ncaaftotal
161
Btcd
132
Mlbhit
107
Mlbhr
98
Nflrecyds
64
Mlbhrr
64
Ethd
63
Mlbtb
57
Rain
54
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 21, 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 21, 2026.” https://volroc.ai/prediction-markets/2026-09-21/
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