Prediction market arbitrage: Kalshi vs Polymarket vs sportsbooks

ThinBlimp guide · facts checked October 18, 2026 · fees from each venue's published schedule · nothing here is advice

An arbitrage is buying both sides of the same outcome on two venues for less than the $1 that one of them is sure to pay. If Kalshi sells Yes for 46¢ and Polymarket US sells No on the same question for 50¢, the pair costs 96¢ and pays $1 whichever way it goes.

The catch is in the four words "the same outcome", and in the fees.

Fees decide most gaps

Both Kalshi and Polymarket US charge a taker fee that peaks at 50¢ and shrinks toward either end. Buying 100 of each side:

So a 2¢ gap between two coin-flip prices is not an arbitrage; it loses money once both fees are paid. Near 50¢ the gap has to be about 4¢. The same 2¢ gap between a favorite and a longshot does clear, because the fees there are small.

Resting a limit order instead of buying the ask avoids Kalshi's taker fee on most markets, but then one side may fill and the other may not. See limit orders on Kalshi.

What we found when we looked

On October 18, 2026 we checked 102 fields that Kalshi and Polymarket US both list as one complete set of outcomes, such as every candidate in a Senate race. Buying every outcome across the two venues at the cheapest price, after fees, the median field lost 0.94¢ a set.

The best one was real and small. Montana's Senate race paid 1.79¢ a set, checked live against both order books, but only for 1,070 contracts before the price ran out: about $19 of profit on roughly $2,100 of capital, held until Kalshi's January 3 close. That is about 4.3% a year.

Across 470 pregame games, a venue's moneyline sat more than 2.83¢ away from the other venues' median only 1% of the time. The exchanges and the sportsbooks we track mostly agree.

Four ways an arbitrage turns out not to be one

• The rules differ. Two markets with the same title can settle on different facts. A 16¢ gap we saw on Venezuela's leader came from a rules difference: Kalshi's clarification counts who officially holds the office, not an acting leader. Read both venues' rules before treating a gap as free money.
• One price is stale. On October 1 our sportsbook feed still held FanDuel's opening spread on Texas State at San Diego State (−4.5) while FanDuel itself had moved to −7.5. Seven "locked" spread hedges appeared against Kalshi; none existed. A gap that only shows up against one feed is usually that feed.
• The book is thin. The best price may cover a few contracts. The rest of your size fills at worse prices, often at a loss.
• Your money is tied up. Profit arrives at settlement, which can be months away. Judge the edge per year, not per trade.

Polymarket International is not open to US persons, so for a US trader the pairs are Kalshi, Polymarket US and the sportsbooks.

Sportsbooks

A Kalshi or Polymarket US price can be paired with a DraftKings or FanDuel line once the line is turned into a price without the vig. That is what the no-vig calculator does. Sportsbooks can limit accounts that only bet into lines they have not moved yet, so a cross-venue pair that leans on a book may not be repeatable.

Checking a pair

The arbitrage calculator takes two prices on two venues and shows the result after each venue's fee. The Screener reads every venue's order book and shows where a given size fills cheapest, which is how a gap that looks real at one contract is checked at the size you would actually trade.

Questions