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Kalshi Takes Share as August Volume Falls 15%

The Signal

Kalshi, Polymarket and Polymarket US generated $45.33 billion of trading volume in August, down 14.5% from July and the first monthly decline since August 2025, according to The Block. Kalshi contributed $37.17 billion, a 7.3% drop, while Polymarket’s global and U.S. venues combined for $8.16 billion, down 36.7%.

The pullback erased $7.66 billion of monthly turnover after the World Cup lifted activity in June and July. Kalshi still gained 6.3 percentage points of combined market share, cushioning the industry’s first clear test of post-tournament demand.

The Mechanism

• Kalshi captured 82% of tracked volume. Its share rose from roughly 75.7% in July as Polymarket contracted almost five times faster. Kalshi processed about $4.55 for every dollar traded across Polymarket’s two venues in August.

• Polymarket’s decline complicates its valuation story. The platform’s reported $1 billion raise at a $21 billion valuation would fund expansion from a stronger capital base, but August turnover shows that recent volume has been more seasonal than the yearlong growth streak suggested.

• The comparison spans different market structures. Kalshi’s figure comes from its U.S. CFTC-regulated exchange. Polymarket’s $8.16 billion combines its crypto-based global platform with Polymarket US, so the headline market-share split is not a clean onshore-to-onshore comparison.

• Sports distribution remains the immediate growth lever. Kalshi followed the summer tournament surge with an exclusive US Open partnership, including digital promotion and on-court signage. September will show whether exclusive inventory and advertising can convert event-driven users into recurring traders.

• Market integrity is becoming a platform cost center. Kalshi issued its first lifetime ban to former congressman George Santos and imposed penalties tied to alleged self-referential trading, according to The Wall Street Journal. Additional investigations may follow, Semafor reported, raising the compliance burden as political contracts attract more scrutiny.

The Landscape

Market Position. Kalshi widened its lead even as its own volume declined. Polymarket lost $4.73 billion from July, versus a $2.93 billion reduction at Kalshi, accounting for nearly 62% of the combined contraction. Still, August volume remained 76.7% above May’s $25.66 billion, leaving the industry on a much larger base than it had before the summer surge.

Regulatory Environment. Sports contracts remain the pressure point for both platforms. More than a dozen states have pursued enforcement or litigation against Kalshi and Polymarket, and Connecticut recently sued to block Kalshi’s sports markets, according to The Block. Those cases will determine how far federal derivatives regulation can shield CFTC-regulated event exchanges from state gambling laws—especially as sports supply drives customer acquisition and trading frequency.

Key Data

• $45.33 billion: Combined August volume, down 14.5% month over month.
• $37.17 billion: Kalshi volume, down from $40.1 billion in July.
• $8.16 billion: Combined Polymarket and Polymarket US volume, down from $12.89 billion.
• 82.0%: Kalshi’s August share of the three-platform total, up approximately 6.3 percentage points.
• $25.66 billion: Combined May volume, 43% below August’s level.

What’s Next

September’s platform data will separate a one-month sports-calendar reset from a broader slowdown. Watch Kalshi’s conversion of US Open exposure into repeat volume, Polymarket’s global-versus-U.S. mix, the closing status of Polymarket’s reported financing, and the next state-court decisions over sports event contracts. A second consecutive decline would put more pressure on both platforms to prove that distribution deals and institutional APIs can produce activity outside marquee tournaments.

Predict This covers the evolution of prediction markets — platforms, regulation, volume, and methodology. For questions or tips: reply to this email.

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This is an independent project by Michael McDonough, built with the assistance of AI. Content is aggregated and summarized automatically—errors, omissions, or inaccuracies may occur. This newsletter is for informational purposes only and does not constitute professional advice.

Oracle is our AI event-derivatives analyst. Obsessed with market structure and liquidity — where the money actually is, and where the odds diverge from the headlines.