Where the Money Waits: Polymarket's Open Interest Is 55% Politics, Kalshi's 44% Sports

Kalshi and Polymarket list many of the same questions and are different venues underneath. Kalshi is a CFTC-regulated exchange that trades in dollars. Polymarket's main venue is an on-chain order book settled in USDC on Polygon and closed to U.S. residents, with a separate, sports-first U.S. exchange that opened to the public in May 2026. Both charge takers a rate times p × (1 − p): at 50 cents, 100 contracts cost $1.75 to take on Kalshi and between nothing and $1.75 on Polymarket, depending on the category, and Polymarket makers pay no fee.

What they hold differs as much as how they work. Vultax read every open event on both venues late on 13 September 2026. On Polymarket, $341.3 million was open, and 55.3% of it sat on politics and elections; the economy and the Fed held 16.0%, crypto 12.3% and sports only 7.8%. On Kalshi, 1.04 billion contracts were open, and sports held the largest share, 43.6%, with elections close behind at 34.0%. Holding is not trading: esports was the largest sport by money traded on Polymarket in Vultax's record of 21 August to 12 September, yet it held 0.6% of the venue's open interest, because a match settles within hours.

Their prices sit close but seldom together. How far apart matched contracts trade, and whether a gap survives both venues' fees, is measured in the Kalshi–Polymarket arbitrage study.

The comparison a trader makes before choosing where to trade. Both venues' taker fees follow the same shape: they peak at 50 cents and shrink toward the ends of the price range.

Open interest is the value of positions still waiting to settle, and it shows where money stays. Vultax paged through every open event on Polymarket's public events endpoint between 23:22 and 23:24 UTC and on Kalshi's public events endpoint between 23:21 and 23:22 UTC on 13 September 2026. Polymarket reports each event's open interest in dollars; Kalshi reports open contracts for each market, which Vultax summed by event. A Kalshi contract pays $1 if it wins and is bought for less, so its contract count is an upper bound on the dollars at stake and not directly comparable with Polymarket's figure. Shares within each venue are comparable.

Kalshi assigns each event a category. Polymarket tags events with many labels, so Vultax assigned each event to the first matching group in this order: esports, sports, crypto, economy and Fed, politics and elections, geopolitics, culture, tech; an event tagged both sports and esports counts as esports.

More than half of Polymarket's open money waits on political outcomes, from the 2028 presidential election to Brazil's. The September Fed decision was the second-largest single event by open interest three days before the meeting.

Kalshi's open contracts divide between sports and elections, and its largest sports positions are not tonight's games but season-long questions such as the next NFL, MLB and NBA champions. Game contracts settle within hours and so hold little open interest even though they dominate Kalshi's daily trading; in a reading of Kalshi on 12 September, sports were 86% of contracts traded in 24 hours.

Each venue's headline trading figures describe fast markets; its open interest describes patient ones. Three examples from Vultax's September measurements show the gap.

• Esports on Polymarket - 21.4% of observed money traded between 21 August and 12 September, 0.6% of open interest on 13 September.
• Crypto up-or-down markets on Polymarket - 59.6% of observed trades in the same window, settling every five minutes to a day, so they barely register in open interest; crypto's 12.3% share is mostly long-dated price questions such as where Bitcoin will trade in 2026.
• Sports on Kalshi - 86% of contracts traded in a day, 43.6% of open contracts, most of them in championships decided months from now.

Open interest is the better guide to what a venue is exposed to and what its users are waiting on. Polymarket's depends heavily on U.S. politics through 2028 and on a handful of events; Kalshi's on the U.S. sports calendar and the same elections. Both would look very different after the midterms settle. For where daily trading goes, see the up-or-down study and Polymarket's sports and esports markets.

The two venues are less alike than a matched contract suggests. Kalshi is a CFTC-designated exchange whose contract volume reached about 90% of the pair's combined total in late August 2026, driven by sports and multi-leg combos; Polymarket's main venue is offshore, while its U.S. exchange, launched in December 2025 on the QCEX designation and opened to the public in May 2026, is sports-first and iOS-only. Since mid-2024 sports has been about 80% of Kalshi's volume and 39% of Polymarket's. Fed contracts are a small corner of both.

Regulation is now the largest source of structural difference. The Ninth Circuit ruled that federal law does not pre-empt state regulation of prediction markets, contradicting other rulings and pointing at the Supreme Court; Kalshi is blocked in Washington, Michigan and Nevada, Minnesota banned prediction markets from 1 August 2026, and the CFTC has proposed rules barring contracts on injuries and officiating. Each of those changes who can trade a given contract on which venue, which is what a gap ultimately measures.

Both venues publish calibration evidence of their own. Kalshi's August 2026 study over more than 2.2 million data points found forecasts improving with time, participation and volume; a separate 2026 academic study found its federal-funds-rate contracts near-perfectly calibrated by decision day.

Open interest is one reading at one moment. It moves with prices as well as with new positions, and the September Fed event was due to settle on 16 September. Polymarket figures cover its international exchange, not its U.S. exchange. Category groups for Polymarket are Vultax's assignment from Polymarket's tags and would shift slightly under a different order. Kalshi's contract counts overstate dollars at stake by an amount that depends on each contract's price.

Polymarket's fee rates were read from its documentation on 5 September 2026; Kalshi's fee schedule was cross-checked from a secondary source on that date because kalshi.com rate-limited automated reading. Nothing here is a recommendation to trade either venue, and nothing here is financial advice. Challenge a figure at [email protected]; material changes are logged in the changelog.