On September 20, 2026, VolRoc recorded 479,772 active prediction markets across Kalshi and Polymarket, holding $1.35B in combined open interest. Day over day, active markets fell 6.6% and open interest fell 46.1% (vs September 19, 2026). Politics & Elections was the largest domain by open interest (23%), ahead of Sports (8%); together they accounted for 31% of all open interest. Kalshi carried 92% of active markets and Polymarket 8%.
Deterministic · template v1 · 2026-09-20
As of 2026-09-20 · 00:00 UTC
Active Markets
479,772
00:00 UTC freeze
Open Interest
$1.35B
Kalshi OI + Polymarket liquidity
Markets Traded · 24h
15,640
with trading activity
Kalshi / Polymarket
92% / 8%
by active-market count
Domain Composition — 12 domains
Mixed / Multi-category
433,315
Sports
20,670
Other
13,196
Politics & Elections
7,575
Crypto & Digital Assets
1,354
Technology & AI
1,295
World & Geopolitics
769
Companies & Business
485
Entertainment & Culture
374
Economics & Inflation
319
Monetary Policy
186
Climate & Weather
148
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 = 479,772.
Largest Kalshi Series
Cross-category multi-markets
357,606
KXMVECROSSCATEGORY0
75,709
Nflrecyds
378
Nfltd
303
Nflrshyds
236
Ncaafspread
235
Nflrec
223
Mlbhrr
221
Mlbhit
220
Mlbhr
214
Ncaaftotal
164
Mlbks
144
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 20, 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 20, 2026.” https://volroc.ai/prediction-markets/2026-09-20/
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