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Prediction markets have never been bigger — or more contested. As volume hits record highs and regulators circle, identically worded midterm questions are trading several points apart by venue. Predictions.io now tracks 9,700+ markets across Kalshi, Polymarket, and Manifold in one place, with free fee and odds calculators so traders can see what a price actually costs them.
A Market at Record Highs, Under Record Scrutiny
Kalshi, Polymarket, and Polymarket US together posted a record $50.59 billion in combined volume in July, with Kalshi accounting for roughly 74.5% of the total. In the same month, New York City opened a probe into both leading venues, a Washington judge ordered Kalshi to halt most wagers in the state, and the CFTC began an internal review of so-called “mention markets.”
Amid that scrutiny, a simpler question has gone largely unexamined: when two venues list the same question, do they agree on the answer?
Often, They Don’t
On identically worded midterm markets tracked by Predictions.io, “Blue tsunami in 2026?” was priced at 44.5% on Polymarket and 36.0% on Kalshi. “Blue wave in 2026?” showed 82.5% against 74.0%. Both gaps are 8.5 percentage points — on questions whose wording is identical across the two venues.
 
Across a sample of directly comparable binary markets live on more than one venue, the median gap was more than four points, and nearly half of the pairs differed by five points or more. (Prices as of 05:08 UTC on August 28, 2026; both venues’ live prices are shown side by side on Predictions.io.)
Those gaps matter to anyone quoting a single number. A market priced at 44.5% on one venue and 36.0% on another doesn’t have one “market-implied probability” — it has two, and which one gets cited is arbitrary unless the reader is told both.
“A single venue’s price is a data point. The spread between venues is the information,” said a spokesperson for Predictions.io. “When the two biggest markets in the world disagree by seven points on the same sentence, that disagreement is the story — and nobody who runs one of those markets is in a position to report it.”
Comparing Markets in One View
Predictions.io aggregates markets from Kalshi, Polymarket, and Manifold, matching equivalent questions across venues so the same event can be compared directly. The platform currently tracks more than 9,700 event pages across 23 categories, including US politics, economics, crypto, sport, and geopolitics.
Alongside the comparison pages, Predictions.io publishes two free tools:
Fee Calculator — Enter any trade to see the fee, total outlay, and effective all-in price on each venue, including Kalshi’s 0.07 × P × (1−P) taker formula and maker discount against Polymarket’s zero-fee standard markets.
https://predictions.io/tools/fee-calculator
Odds Converter — Convert American, decimal, and fractional odds into implied probability and prediction-market prices, and see the vig-free line.
https://predictions.io/tools/odds-converter
A direct venue comparison is available at predictions.io/compare/polymarket-vs-kalshi, with live midterm markets at predictions.io/lobby/us-politics.
Predictions.io operates no market and takes no position in any contract. It is a data and comparison service — not an exchange, broker, or investment adviser.
About Predictions.io
Predictions.io is an independent aggregator of prediction markets, bringing prices from Kalshi, Polymarket, and Manifold into a single view so you can compare the same question across venues. It publishes free tools for traders and journalists, including a cross-venue fee calculator and odds converter.
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