On September 15, 2026, VolRoc recorded 323,500 active prediction markets across Kalshi and Polymarket, holding $2.04B in combined open interest. Day over day, active markets rose 12.8% and open interest rose 16.7% (vs September 14, 2026). Politics & Elections was the largest domain by open interest (28%), ahead of Sports (19%); together they accounted for 47% of all open interest. Kalshi carried 85% of active markets and Polymarket 15%.
Deterministic · template v1 · 2026-09-15
As of 2026-09-15 · 00:00 UTC
Active Markets
323,500
00:00 UTC freeze
Open Interest
$2.04B
Kalshi OI + Polymarket liquidity
Markets Traded · 24h
24,997
with trading activity
Kalshi / Polymarket
85% / 15%
by active-market count
Domain Composition — 12 domains
Mixed / Multi-category
235,277
Sports
34,899
Other
33,193
Politics & Elections
10,531
Entertainment & Culture
3,095
Crypto & Digital Assets
1,656
Technology & AI
1,381
Economics & Inflation
895
World & Geopolitics
835
Companies & Business
789
Monetary Policy
553
Climate & Weather
282
Science & Health
114
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 = 323,500.
Largest Kalshi Series
Cross-category multi-markets
221,707
KXMVECROSSCATEGORY0
13,570
Midterm control
1,353
Votegeneral
1,150
Music — artist streams
701
U.S. House races
649
Nextteamnba
454
Ncaafwins
416
Nflwins
400
Ncaafspread
345
Midtermvoteturn
278
Mlbhrr
272
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 15, 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 15, 2026.” https://volroc.ai/prediction-markets/2026-09-15/
Get the daily statistics
A short daily read on the prediction-market universe across Kalshi and Polymarket.
Join the list →