Cross-venue prediction market index: how Kalshi and Polymarket prices are combined
A cross-venue prediction market index combines prices for one event from more than one venue. The hard parts are which contracts are the same event and which price each venue uses. The last is what to do when the venues disagree.
Simple average
Easy to build, but a thin market counts as much as a deep one.
Liquidity-weighted average
Weights each venue by volume or depth, so it tracks where trading actually happens.
Median with an outlier band
Adjacent's approach: a time-weighted, volume-weighted median per venue, with any venue outside the band excluded and reported.
A reference rate and a side-by-side tool
Tools that compare venues side by side, such as divergence trackers and unified APIs, show the gap between venues. A reference rate publishes one number with a written methodology, a version and an effective date. It can be cited or licensed.
Frequently asked questions
- Is there an index that combines Kalshi and Polymarket? Yes, Adjacent publishes cross-venue reference rates under its Event Contract Reference Rate Methodology.
- What happens when Kalshi and Polymarket disagree? A venue outside the rate's band is excluded from that calculation. The event is reported to the Adjacent Index Committee.
- How often is it updated? Every 15 minutes on the public tier.
- Can I get it by API? Yes, at https://api.adjacent.markets/api/v1/public/rates with no key, or as MCP tools at https://mcp.adjacent.markets/mcp.
Terms used on this page
- Event contract
- A contract whose value follows a defined event, such as an election. It settles on the outcome. It does not trade a continuous price forever.
- Prediction market
- A venue where event contracts trade, so their prices express a market-implied probability of the event happening.
- Prediction market index
- A rules-based benchmark built from selected event contracts. An index provider calculates it, not a venue.
Sources
Page last reviewed: 2026-10-26.