Kalshi, Polymarket open interest falls 20% as World Cup frenzy winds down

A few days before Sunday's World Cup Final, combined Kalshi and Polymarket open interest (OI) is down 20% from its peak of ~$2 billion at the beginning of July.

On a smoothed weekly basis, total OI ran from $1.2 billion in late May to nearly $1.8 billion in the week ending July 5, then fell to around $1.5 billion by the week ending July 19.

Kalshi’s sports volume peaked at ~$9 billion in the week ending July 5 and fell to around $4 billion by the week ending July 19, down about 55%. While Polymarket’s sports volume peaked near $2.3 billion and dropped to $740 million over the same span, down nearly 70%.

Sports accounted for roughly 80% of both platforms’ total volumes throughout the tournament, as all eyes were on the World Cup. On Sunday, Spain defeated Argentina 1-0 to win the country's second-ever World Cup title.

Polymarket profit and loss data compiled by DeFiOasis following the tournament's conclusion found that 194,000 addresses traded Polymarket’s World Cup winner market, of which roughly two-thirds lost money. Most gains and losses were below $100, but 54 addresses made more than $100,000 each — including five that cleared $1 million — while 43 lost more than $100,000 each.

The decline is unsurprising, as the World Cup group stage that ran from mid to late June packed the schedule with matches across 48 teams, flooding both platforms with many markets to bet on. As the tournament bracket narrowed through the knockout rounds and the number of matches per day thinned out, the amount of markets people could bet on dried up, and with the final played on July 19, the catalyst is now gone entirely.

For context, OI has fallen more gently than volume. Volume is daily turnover, which collapses the moment there are fewer matches, while OI is the outstanding stake that unwinds only as markets resolve.

Following the world’s biggest sporting event, it is likely that prediction markets activity will remain subdued compared to its June-July levels, at least until the next major event catalyst, that being the U.S. midterm cycle into the fall.

This is an excerpt from The Block's Data & Insights newsletter. Dig into the numbers making up the industry's most thought-provoking trends.