On September 13, 2026, VolRoc recorded 366,131 active prediction markets across Kalshi and Polymarket, holding $2.06B in combined open interest. Day over day, active markets rose 5.6% and open interest fell 3.3% (vs September 11, 2026). Politics & Elections was the largest domain by open interest (28%), ahead of Sports (20%); together they accounted for 48% of all open interest. Kalshi carried 85% of active markets and Polymarket 15%.
Deterministic · template v1 · 2026-09-13
As of 2026-09-13 · 00:00 UTC
Active Markets
366,131
00:00 UTC freeze
Open Interest
$2.06B
Kalshi OI + Polymarket liquidity
Markets Traded · 24h
30,326
with trading activity
Kalshi / Polymarket
85% / 15%
by active-market count
Domain Composition — 12 domains
Mixed / Multi-category
268,150
Sports
42,354
Other
35,218
Politics & Elections
10,625
Entertainment & Culture
3,215
Crypto & Digital Assets
1,652
Technology & AI
1,413
World & Geopolitics
876
Economics & Inflation
859
Companies & Business
784
Monetary Policy
553
Climate & Weather
317
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 = 366,131.
Largest Kalshi Series
Cross-category multi-markets
233,255
KXMVECROSSCATEGORY0
34,895
Midterm control
1,330
Ncaafspread
1,104
Votegeneral
936
Ncaaftotal
846
Music — artist streams
714
U.S. House races
647
Nfltd
510
Nflrecyds
455
Nextteamnba
454
Ncaafwins
424
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 13, 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 13, 2026.” https://volroc.ai/prediction-markets/2026-09-13/
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