Guide

Kalshi vs Polymarket arbitrage: how it works

Kalshi and Polymarket often list the same event at different prices. When buying YES on one and NO on the other costs less than the $1 a winning contract pays, the difference is yours whoever wins. Here is how it works, with real fee math.

The idea in one example

Every contract on both platforms pays $1 if it wins and $0 if it loses. Suppose the same game is listed on both:

That is the whole trade. The hard parts are finding these gaps before they close, getting the fees right, and making sure both sides really are the same bet.

Why the prices differ

• Different traders. Kalshi and Polymarket have different users, so news and opinions hit their prices at different speeds.
• Different money. Moving money between platforms takes time, so traders can't instantly close every gap.
• Thin order books. On smaller markets one trade can move the price several cents.

What can go wrong

• The rules don't match. The single biggest risk. If Kalshi settles "by the official result at midnight" and Polymarket settles on a different source or time, both sides can lose. Read both rule pages before trading.
• Only one side fills. If you buy one side and the other price moves before you buy it, you're holding an ordinary bet. Most gaps last seconds to minutes.
• Fees eat the gap. Kalshi charges up to 1.75¢ per contract near 50¢ (see Kalshi fees). A 2¢ gap can be zero after fees.
• Not much is for sale. The best price may only have a few dollars behind it. The real profit is the gap times the size you can actually buy.
• Your money is tied up until the event settles, which can be days or weeks.
• Access. Polymarket's international site isn't available to US users. US traders can use Polymarket US instead, and should check what's allowed in their state.

How often these spreads appear

Real, fee-clearing gaps are rare and small. Most are a few percent and close within minutes, which is why checking by hand rarely works. ArbTrader's weekly reports show every spread the scanner found, how long it lasted and when it appeared.

Common questions

Is Kalshi Polymarket arbitrage risk-free?

No. The prices lock in a profit only if both sides fill and both platforms settle the event the same way. Mismatched settlement rules, one side not filling, and fees are the main risks.

How big are the profits?

Usually small: a few percent per trade before your money is tied up until the event settles. The size you can trade is limited by how much is for sale at the best price.

Do I need accounts on both platforms?

Yes. You need funded accounts on both, since each side of the trade is bought on its own platform.