Arbitrage Between Kalshi and Polymarket: A Realistic Guide

The same contract trades on both Kalshi (opens in a new tab) and Polymarket (opens in a new tab). Most of the time, the prices are within a cent or two. Occasionally. After a major news event, on thin contracts, or during weekend hours, they diverge. That gap is arbitrage. Here's how it actually works, why it persists, and what you need to know before trying to capture it from California.

What is prediction market arbitrage?

Prediction market arbitrage involves simultaneously purchasing opposing contracts for the same event on different platforms to try to profit from price differences. Fees, different settlement rules and slippage can erase the gap, so it is never guaranteed.

Arbitrage is buying a contract on one platform and selling the same contract on another platform simultaneously, aiming to profit from the price difference. If the 2026 California governor market trades YES at 38¢ on Kalshi and NO at 60¢ on Polymarket (so YES = 40¢), there's a 2-cent gap.

Buy YES on Kalshi, buy NO on Polymarket, and you've spent 98¢ on a position that pays $1 at resolution: a 2-cent margin before fees, if both contracts settle the same way. Run the numbers on any pair with this outcome calculator (opens in a new tab) before sizing in.

Why do gaps exist?

Five reasons:

What costs eat into arbitrage profits?

Round-trip per-contract:

Realistic per-contract cost: 3-5 cents round trip. Below that gap, arbitrage isn't worth executing.

If the gap is under 5 cents, fees eat the profit. Above 5 cents, you have to move fast.

What's the actual workflow?

Traders pre-fund both platforms, monitor live price discrepancies, and simultaneously execute opposing positions when a profitable gap exceeds transaction costs and slippage.

1. Pre-fund both Kalshi and Polymarket. You can't arbitrage with one-side capital.
2. Monitor pricing across both platforms. ElectionBettingOdds (opens in a new tab) shows live cross-platform odds for elections; for sports and other contracts, you'll need to manually check.
3. When you spot a gap above your cost threshold, execute both sides simultaneously. Speed matters. The gap may close within seconds.
4. Hold both positions until resolution. Don't try to leg out of one side; you'll usually give back the profit.

What contracts are most arb-friendly?

Three categories:

• Major elections: same contract terms on both platforms, deep liquidity.
• Major sports: Super Bowl, NBA Finals, World Series outright winner.
• Macro events: Fed FOMC (opens in a new tab), jobs reports, CPI prints.

Avoid arbitraging niche or scripted-TV contracts. Resolution rules often differ between platforms and you can lose on a technicality.

What's the catch?

Three traps:

1. Resolution rule mismatch. Kalshi and Polymarket sometimes word contracts differently. "Trump wins 2024 election" on one may resolve based on AP call; on the other, based on Electoral College certification. Read both contract specifications before assuming they're identical.
2. Liquidity disappears. A 5-cent gap on a low-volume contract may have only $200 of size at the displayed price. Your fill may be much worse.
3. Capital lockup. A 4-cent edge sounds great until you realize your capital is locked for 14 months until resolution.

Is arbitrage legal?

Arbitrage is legal in the United States, as no federal laws prohibit investors from simultaneously buying and selling the same asset across different platforms to profit from price gaps.

Yes. There's nothing illegal about trading the same contract on two platforms. Both Kalshi and Polymarket are CFTC (opens in a new tab)-regulated and available to eligible California residents (dispute pending). See our legality guide for the broader regulatory picture.

Is it worth doing?

Retail arbitrage is generally not worthwhile for individual investors due to high competition from automated systems and the significant capital required to capture narrow price gaps.

Honestly: at retail size, usually not. The cost structure means you need a meaningful price gap and meaningful capital to make the math work. Most professional arb desks are running automated systems that capture sub-second gaps you'll never see.

Where retail traders can win: spotting persistent gaps on contracts that the pros aren't watching. California-specific weather contracts, niche entertainment markets, or low-volume sports props. Even there, expect modest returns relative to the operational complexity.

What else should I read?

Relevant resources include a detailed fee breakdown comparing Kalshi to Polymarket and a technical guide explaining the underlying mechanics of how event contracts work.

For a fee-by-fee breakdown of the two platforms involved, see Kalshi vs Polymarket fees. For the underlying market mechanics, see how event contracts work.

About the author

Catie Di Stefano

Catie Di Stefano has spent 15 years in the regulated gambling industry, starting at Betsson Group in Malta and spending nearly five years at Gaming Innovation Group (GiG) before building an independent consultancy across European and US iGaming brands, including a licensed engagement with Hard Rock Hotel & Casino Atlantic City under a New Jersey DGE vendor license. She's been interviewed by the California Business Journal on the rise of online gambling in the US, and spoke on a 2022 Next.io panel specifically about California's gaming market. She went on to build the OnlineGamblers.com brand and hold marketing director roles at Smartico.ai and Vegas Kings, and has been named to multiple iGamingExpress industry rankings. She now covers regulated prediction markets full-time.

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Is prediction market arbitrage legal?

Available, dispute pending (as of October 13, 2026). Trading the same contract on two CFTC-regulated platforms is available to eligible California residents (dispute pending). Both Kalshi and Polymarket allow it.

What's the typical cost of an arbitrage trade?

Round-trip costs run 3-5 cents per contract including trading fees, spreads and the cost of keeping money on two platforms. Below that gap, arbitrage isn't profitable.

Why do price gaps exist?

Different user bases, capital inefficiency (most users only fund one platform), different fee structures, settlement timing differences, and occasional resolution rule differences.

What contracts are best for arbitrage?

Major elections, Super Bowl, NBA Finals, World Series, and Fed FOMC. These have identical contract terms on both platforms and deep liquidity.

Is Kalshi. Polymarket arbitrage legal in California?

Available, dispute pending (as of October 13, 2026). Both platforms are CFTC-regulated venues legal in California, so trading opposite sides of the same event across them is legal. What matters is fee-adjusted math, tax reporting, and settlement risk: not legality.

Do you pay taxes on arbitrage profits?

Yes. Every leg of an arb is a taxable event. Net profits from both platforms are taxable federally (the IRS hasn't said how event contracts are classified), and California residents owe state income tax on top. Kalshi provides a profit-and-loss statement for your trades; for Polymarket US, keep the app's trade history and any tax documents it provides.

How much capital do you need to arb Kalshi vs Polymarket?

Meaningful arbs typically need $2K-$10K per side to overcome fees and settlement drift. Below that, per-contract minimums, fees and price movement between the two fills usually eat the spread.

Is it worth doing at retail size?

Usually not. Cost structure means you need meaningful price gaps and capital. Where retail can win: persistent gaps on contracts the pros aren't watching.

Do transaction fees eat the profit of a Kalshi-Polymarket arb?

Transaction costs include Kalshi's per-contract trading fees and the fee the Polymarket US app shows on each order. Traders must make sure the price spread covers these costs and the slippage from order book depth.

Which platform offers faster settlement for arbitrageurs?

Polymarket often settles immediately following a verified UMA protocol resolution. Kalshi settlements occur after the official source data is verified by their clearinghouse. California traders should account for the capital lock-up period on both platforms when calculating the annual percentage yield of an arbitrage trade.