Kalshi and Polymarket price gaps after fees: 32,638 order-book readings

A price gap between Kalshi and Polymarket is easy to find and hard to be paid for. Vultax matched the same outcomes on both venues, NFL, college football and Premier League games, the 2026 midterms and the September Fed decision, and walked both order books to 20 levels every five minutes for 28 hours on 12 and 13 September 2026. That gave 32,638 checks of whether buying Yes on one venue and No on the other would pay after both venues' fees at the size the books held. The slow markets were where gaps paid. Midterm contracts cleared fees at depth in 10.1% of readings, and two pairs did so in every reading. The steadiest edge was a tick-size gap on the Fed decision worth 0.18%. Pre-game sports gaps almost never paid: none of 7,518 pre-game NFL quotes crossed between venues. In-play readings crossed 16.9% of the time and were discarded, because the two books were read seconds apart while the game moved. The study also records the two checks any cross-venue comparison needs: how far apart in time the venues were read (295 paired reads, all inside two seconds) and whether the contracts settle the same way. Matching was checked against both rule texts, and outcomes that could not be matched without doubt were left out. Deposited: the page at https://vultax.com/research/kalshi-polymarket-arbitrage-gaps-after-fees as a research note (PDF) and page (HTML, PDF) from 27 October 2026, the proof-scope record and a CITATION.cff. Kalshi-derived downloads are withheld while redistribution permissions are reviewed.