Prediction markets reached record volume levels in July, with Kalshi, Polymarket and Polymarket US recording combined volume of $50.59 billion, according to data tracked by Predictions.io. Kalshi accounted for approximately 74.5% of that total.
Despite identical question wording across venues, the same events are frequently priced differently. On comparable midterm election markets, "Blue tsunami in 2026?" was priced at 44.5% on Polymarket versus 36.0% on Kalshi, a gap of 8.5 percentage points. "Blue wave in 2026?" showed a similar spread, trading at 82.5% against 74.0%. Across a sample of directly comparable binary markets, the median price gap exceeded four percentage points, with nearly half of comparable market pairs differing by five points or more.
Predictions.io, an independent data aggregator, has launched tools to help traders navigate these differences. The platform currently tracks more than 9,700 markets across Kalshi, Polymarket and Manifold, matching equivalent questions to enable direct comparison.
Two free tools are now available:
- Fee Calculator — displays the total cost of trades on each venue, accounting for Kalshi's 0.07 × P × (1−P) taker formula and Polymarket's zero-fee standard markets
- Odds Converter — converts between American, decimal and fractional odds formats and displays implied probabilities
Predictions.io operates as a data and comparison service only, taking no positions in any contracts and providing no investment advice.
