Can you arbitrage between Kalshi and Polymarket?
Two exchanges, no sportsbook margin on either side, and two fee schedules that disagree about where trading is expensive. Kalshi charges a curve that peaks on near-even contracts; Polymarket charges a small flat fraction with a ceiling on it. A two-sided position between them is the one case on this site where the fee on BOTH legs has to be paid, and where ignoring either one changes the answer.
Two order books listing the same outcome will not hold the same price, because the people trading them are not the same people. Kalshi carries the wider menu of sports markets and the steadier participation; Polymarket concentrates its depth on whatever is being traded heavily that day. Where the two list the same bet, the gap that opens a position is a genuine difference of opinion between two crowds rather than a stale number.
What the fees do to a Kalshi and Polymarket position
One example, the same two posted prices on every pair page, put through these two venues' own published schedules.
| Leg | Price | Implied | Venue fee | Net cost |
|---|---|---|---|---|
| Kalshi | +112 | 47.17% | 2¢ per $1 | 49.17% |
| Polymarket | -104 | 50.98% | 1.25¢ per $1 | 52.23% |
| Both sides | — | 98.15% | — | 101.40% |
On the posted numbers, +112 at Kalshi against -104 at Polymarket is a position worth $1.85 per $100.00 staked. Kalshi charges 2¢ per $1 on its leg, and Polymarket charges 1.25¢ per $1 on its leg. Once Kalshi and Polymarket have both been paid there is nothing left: the two sides cost 101.40% of a certainty between them, so holding both would cost $1.40 rather than return anything. A calculator that ignores the Kalshi vs Polymarket fee schedules would have reported the $1.85 and stopped.
Fees are stated in dollars per $1 of potential payout, which is the one unit both models fit in — a per-contract trading fee and a commission charged on winnings are otherwise not comparable. Schedules: Kalshi — Kalshi taker fee schedule, Jul 2026; Polymarket — Polymarket sports theta, raised 0.03 -> 0.05 Jul 2026.
Live Kalshi and Polymarket positions
These decay in minutes. An arbitrage is a snapshot of two prices that were live moments ago, and it decays in minutes — often in seconds once either venue notices. Confirm BOTH prices at BOTH venues before staking either leg. A position with one leg filled and the other gone is not an arbitrage; it is a one-sided bet you did not intend to make. Venues also limit, void or refuse stakes at their discretion, so the profit is guaranteed by the arithmetic and by nothing else.
Kalshi and Polymarket are both quoting First Touchdown, Hits, Hits + Runs + RBIs, Passing Touchdowns, Passing Yards, Pitcher Outs, RBIs, Receiving Yards, Receptions, Rushing Yards, Strikeouts, Total Bases, Moneyline, Spread, and Total on the board right now, and a two-sided position can only exist inside that overlap.
No arbitrage between Kalshi and Polymarket on the board right now
Premium shows both sides of each position: the bet, the venue and price for each leg, how to split a $100.00 stake, and what it returns either way. The count above is live and is usually zero — arbitrage is rare, which is the point of watching for it.
Go PremiumTaking one: what goes wrong on this pair
Both legs are order books, so both legs can move under you — this is the only pair where neither side is a venue that must honour a posted price. Take the thinner book first. Beyond that, the two venues settle on different schedules and hold your capital for different lengths of time, and the two fees compound: a position whose posted prices imply a one percent margin can be flat once Kalshi's per-contract fee and Polymarket's taker fee are both paid.
Kalshi. Kalshi charges a per-contract trading fee instead of building a margin into the line. The fee is a curve, not a flat rake: it peaks at roughly 1.75¢ on a contract priced near 50¢ and shrinks toward zero at both extremes, and maker fees are a quarter of taker fees.
Polymarket. Polymarket's taker fee is a fraction of a percent, which makes its headline quote very close to the all-in cost and often the cheapest of the exchanges in the middle of the price range.
Kalshi vs Polymarket arbitrage FAQ
Can you arbitrage between Kalshi and Polymarket?
Yes. Both list binary contracts that settle at one dollar or nothing on the same real-world outcomes, so opposite sides at the two exchanges form a two-sided position whenever their prices add up to less than a certainty.
Is arbitrage betting allowed on Kalshi?
Yes. Kalshi is a CFTC-regulated exchange and taking both sides of related contracts is ordinary trading there, not a prohibited strategy. Kalshi earns a per-contract fee on volume rather than on your losses, so it has no reason to restrict an account for trading profitably.
Which is cheaper to trade, Kalshi or Polymarket?
It depends on the price. Kalshi's fee is a curve that is largest near 50 cents and shrinks toward both extremes, while Polymarket charges a small flat fraction with a cap, so Polymarket is usually cheaper in the middle of the range and the two converge at the tails.
Do I pay a fee on both legs of a Kalshi and Polymarket position?
Yes. Both venues charge the taker, so a position across the two pays two fees rather than one. That is why the margin on this pair has to be measured net of both before it means anything.
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