Prediction markets reached record trading volume in July, with Kalshi, Polymarket, and Polymarket US posting combined volume of $50.59 billion, with Kalshi accounting for roughly 74.5% of the total. Yet identical questions often trade at substantially different prices across venues, creating challenges for traders and those citing market-implied probabilities.
Predictions.io, a new data aggregation platform, now tracks more than 9,700 markets across the three largest prediction market venues in a single view. The service revealed significant pricing gaps on identically worded questions: "Blue tsunami in 2026?" was priced at 44.5% on Polymarket and 36.0% on Kalshi, while "Blue wave in 2026?" showed 82.5% against 74.0%. Across a sample of directly comparable binary markets, the median gap between venues exceeded four percentage points, with nearly half of market pairs differing by five points or more.
To help traders navigate these differences, Predictions.io offers two free tools. A fee calculator allows users to enter any trade and see the fee, total outlay, and effective all-in price on each venue, accounting for Kalshi's 0.07 × P × (1−P) taker formula and maker discounts against Polymarket's zero-fee standard markets. An odds converter translates American, decimal, and fractional odds into implied probability and prediction-market prices while showing the vig-free line.
The platform covers 23 categories including US politics, economics, crypto, sports, and geopolitics. Predictions.io operates as a data and comparison service only, taking no position in any contract and serving as neither an exchange, broker, nor investment adviser.
