About ArbMonster
ArbMonster is a measurement service. It records the prices of the same outcome across US sportsbooks, Kalshi and Polymarket, subtracts every fee, and publishes where they disagree.
Why it exists
Prediction markets and sportsbooks price the same events independently, and for short windows they disagree by more than the fees. Spotting those windows by hand across twenty-plus venues is impossible; most public “free money” screenshots are gross numbers at sizes nobody could fill. We wanted a tool that told the truth about size and fees, and that measured its own reliability — so we built one and put the data online.
What it does
- Streams live order books from Kalshi and Polymarket and ingests pre-game lines from 20+ US sportsbooks every 15 minutes.
- Pairs markets that ask the identical question — with an automated reviewer and a visible confidence score — and prices both legs at fillable depth, net of fees.
- Publishes the measurements: every book's real vig, every Kalshi↔Polymarket pair, arbitrage by sport, monthly data reports and a Monday newsletter.
- Runs a paper-trading simulator around the clock to test whether the opportunities survive latency, depth and legging — results are simulated and disclosed as such.
What it will not do
It never places trades, never touches a venue account, never promises a return, and never shows a number it hasn't subtracted fees from. Nothing on the site is financial or betting advice; venue availability varies by state and you must be of legal age where you live.
Who
ArbMonster is built and operated by a small independent team based in North America. Data and methods questions, corrections, press and partnership enquiries: contact@arbmonster.com. We reply to everything.
Read the methodology for every rule behind every number, and the terms and privacy policy for the legal detail.