Polymarket vs Kalshi: World Cup bets and fee race

Prediction markets recorded $31.2 billion in notional turnover in May, with open interest near $1.3 billion. In that month Kalshi accounted for roughly 58% of flow, about $18 billion, while Polymarket handled about 28%, or $8.8 billion. Sports trading was the primary driver of activity in May.

Polymarket’s World Cup winner market has about $2 billion in lifetime volume, roughly $436 million in reported liquidity and single-day spikes that reached $137 million. The platform lists more than 330 active World Cup-related markets. Kalshi’s World Cup exposure is divided into 48 markets that together accumulated about $182.3 million in volume.

Kalshi’s sports category produced about $10.44 billion in volume in May. In the same month Kalshi’s elections category totaled about $173.66 million, crypto markets did about $2.02 billion and a sports-adjacent exotics bucket added roughly $4.88 billion.

Polymarket’s sports trading was a large share of its annual flow. In January sports on Polymarket accounted for about $6.2 billion of a $14.34 billion monthly total and peaked in March with $8.77 billion of a $19.58 billion month. By June total monthly activity on Polymarket had cooled to about $5.91 billion while the sports share rose to roughly 56.5%. In June Polymarket’s crypto volume was about $1.73 billion and politics was about $831.25 million.

Fee revenue in May was concentrated at Kalshi, which collected about $137.86 million in trading fees. Polymarket reported about $28.07 million in fees for May. A smaller platform, Opinion, recorded about $159,330 in fees that month. One smaller venue reported a week in January with $729.52 million in crypto volume and by the week of June 1 had sports accounting for roughly 99.4% of its activity.

The two venues use different market structures. Kalshi offers multiple match-level books across fixtures, while Polymarket pools liquidity in a single tournament-scale market. Both platforms priced Spain near a 17% win probability in their World Cup markets, with Kalshi quoting roughly 5.56x on Spain and France.