Kalshi vs Polymarket arbitrage: fees, rules and speed

Kalshi vs Polymarket arbitrage comes down to four differences: how each venue charges fees, how it settles a market, how fast its prices move, and what it takes to get an account. The price gap is the easy part to see. These four decide whether you keep any of it.

This page compares the two on those points only. It says nothing about which is the better place to trade, and it does not rank them.

Kalshi vs Polymarket at a glance

What matters for arbitrage	Kalshi	Polymarket
Taker fee	A fee on the expected earnings of the contract; some markets differ	shares x 0.05 x price x (1 - price) for sports
Maker fee	Charged on resting orders once they fill; canceling is free	Makers are never charged
Price format	Cents; Yes at 70 cents means a 70 percent chance	Share price in dollars; winners pay $1 per share
Settlement	Per-market rules, read each one	UMA optimistic oracle, 2-hour challenge period
Account	Account on Kalshi	Identity verification before trading or API access on Polymarket US
API	REST, WebSocket and FIX	REST and WebSockets, SDKs for TypeScript and Python

The same gap is worth different amounts after fees

Take a hypothetical pair: Team A at 0.46 on Kalshi, Team B at 0.50 on Polymarket. That is 0.96 for a $1 payout, a gross margin of 0.04.

Polymarket's sports taker fee on the 0.50 leg is 0.05 x 0.50 x 0.50 = 0.0125 per share, or 2.5 percent of that leg's cost. If you instead rest an order and get filled as a maker, you pay no fee there. On the Kalshi side the fee depends on the market and on whether you took or rested, and the full schedule is a separate document. Assume a hypothetical 0.01 for the example: the 0.04 gross becomes about 0.0175 net, 1.8 percent of cost.

The Polymarket fee is largest near 50 cents and shrinks toward the ends, which means a gap on a heavy favorite or a long shot costs less to take than the same gap on a coin flip. At 0.20 the fee is 0.05 x 0.20 x 0.80 = 0.008 per share, or 4 percent of that leg's cost, which is a bigger bite of a cheap contract. Work out the percentage of cost for the leg you are actually taking.

Two rulebooks for one game

Polymarket resolves through the UMA optimistic oracle: an outcome is proposed, there is a challenge period of 2 hours, and winning shares then pay $1. Each Kalshi market has its own rules page. Read both for the exact market, because a contract titled "Team A wins" can handle a postponement, a shortened game or a tie differently on each side.

In a hypothetical case, a game is suspended and finished the next day. One venue keeps the market open until the game ends. The other voids at a cutoff and refunds. You end up with one leg paid and one leg returned, which is no longer an arbitrage. Resolution mismatch is the main reason a pair that looks hedged on a screen is not.

Who leads on price, and for how long

A study of political contracts across Polymarket, Kalshi and PredictIt, as summarized by Quantpedia, reported that Polymarket generally led Kalshi because of its higher liquidity, and that cross-platform opportunities "typically exist only for a few seconds, at best a few minutes." That was political markets in October and November 2024. It is not a statement about sports contracts today, but it is a reasonable prior: a gap you notice by eye has probably been seen by faster traders.

In practice that means the second leg is the risk. If you buy the cheaper side on one venue and the other venue moves before you fill, you hold a directional position, not a hedge.

Access and legal status

Polymarket US asks you to verify your identity before you can trade or access the API. Whether either venue can list sports contracts in your state is not settled: the Ninth Circuit ruled 3-0 against Kalshi on August 28, 2026, after the Third Circuit ruled in its favor, and further cases are pending. Check where you live before opening an account on either.

Which is easier to work with

It depends on the job.

• Taking prices that appear: compare after-fee cost on each leg. Polymarket's formula is public and checkable; Kalshi's needs the schedule for your market.
• Resting orders to avoid fees: Polymarket makers pay nothing, so patient orders there cost less, but you may not fill.
• Automating your own research: both publish APIs. Kalshi lists REST, WebSocket and FIX, and Polymarket US offers REST and WebSockets with SDKs, though trading needs a verified account.

Where quant+ fits

The quant+ arbitrage scanner puts Kalshi and Polymarket (USA) prices on one board with the sportsbooks and flags pairs whose implied probabilities sum under 100 percent. It shows the stake split for a total you choose. It does not subtract each venue's fees or compare settlement rules for you, and it never places bets or connects to your accounts. For the method, see prediction market arbitrage, and for one venue at a time see Kalshi sports markets and Polymarket sports analytics. The arbitrage calculator has the stake math.

Quant+ is analytics for informational purposes. 21+. If you or someone you know has a gambling problem, call 1-800-GAMBLER. Check that sports betting and prediction markets are legal where you live, and read the risk disclosure.