The Overround

The Overround compares the same event across Kalshi, Polymarket and a panel of nine US sportsbooks, normalises the prices so they can be read against each other, and shows which venue is cheapest to back a given side. It also publishes the overround on each market, the sum of what it costs to back every outcome at each venue, so you can see what the house is charging before you trade. Every market gets its own page carrying the full field, the settlement source and the contract terms. Free, no account, and the underlying data is available as unauthenticated JSON.