Guide

How to find prediction market arbitrage

A step-by-step way to spot real arbitrage between Kalshi, Polymarket and Polymarket US, check it's not a trap, and place it before it closes.

Step by step

1. Find the same event on two platforms. Search the event name on each. Sports games, Fed decisions and elections are listed most widely.
2. Check the rules match exactly. Same outcome, same deadline, same source of truth (for example "official league result" vs "AP call"). If anything differs, skip it.
3. Use the ask, not the last price. The price you can buy at is the best ask. The big number on the market page is often the last trade or a midpoint.
4. Add YES on one side to NO on the other. For events with several outcomes on one platform, add YES on every outcome instead.
5. Add the fees. Kalshi and Polymarket US charge fees; most Polymarket markets don't. Use the calculator.
6. Check how much is for sale at that price on each side. The profit is the gap times the smaller of the two sizes.
7. Place the side with less for sale first. If it doesn't fill, you've lost nothing. Then place the other side right away.
8. Plan for a miss. If the second side moves before you get it, decide in advance whether you'll pay up to finish or sell back the first side.

Why doing it by hand rarely works

There are tens of thousands of open events across the three platforms, and the gaps that clear fees are rare and usually gone within minutes. By the time you've searched two sites, compared rules and added fees, most have closed. That's the problem scanners solve: they read every order book continuously and only show gaps that still clear fees at a size you can actually buy.

Checklist before you trade

Common questions

Can you still find arbitrage between Kalshi and Polymarket?

Yes, but fee-clearing gaps are uncommon and short-lived. They tend to appear around news and in less-watched markets, and most close within minutes.

What's the biggest mistake beginners make?

Assuming two markets are the same bet when their settlement rules differ. Always compare the rules, not just the titles.