Prediction market arbitrage is the practice of buying and selling the same event across platforms like Kalshi and Polymarket to lock in a guaranteed profit. When the same contract is priced differently on two exchanges, the gap is free money—if you move fast enough.
What is prediction market arbitrage?
Arbitrage happens when two markets disagree on the probability of the same event. For example:
- Kalshi: "Will the Fed cut rates in June 2026?" priced at Yes 62¢
- Polymarket: Same question priced at Yes 55¢
If the contracts resolve identically, you can buy Yes on Polymarket at 55¢ and buy No on Kalshi at 38¢ (100¢ − 62¢). Your total cost is 93¢, but one side will pay $1. That's a guaranteed 7¢ profit per contract—roughly a 7.5% return.
This is the core idea: lock in a spread where your combined cost is less than the guaranteed payout.
Why do price gaps exist?
You'd think efficient markets would eliminate gaps instantly. They don't, for several reasons:
- Different user bases: Kalshi attracts US-regulated traders; Polymarket attracts crypto-native global users. Different information and biases.
- Liquidity differences: Thin order books on one platform mean prices lag behind the other.
- Capital friction: Moving money between platforms takes time. Kalshi uses USD; Polymarket uses USDC on Polygon.
- Market structure: Kalshi is a CFTC-regulated exchange with order books. Polymarket uses an AMM-hybrid system. Different mechanics, different prices.
- News reaction speed: One platform's community may react faster to certain types of news.
How to find arbitrage opportunities
Finding gaps manually is tedious. Here's the systematic approach:
1. Match identical markets: Ensure both platforms are asking the exact same question with the same resolution criteria. "Fed rate cut by June" and "Fed rate cut in June" might resolve differently.
2. Calculate the spread: Add the cost of Yes on one platform and No on the other. If the total is less than $1, there's an arbitrage.
3. Account for fees: Kalshi charges fees on trades (and sometimes on winnings). Polymarket has lower explicit fees but you pay gas. Subtract all fees from your spread.
4. Check liquidity: A 10¢ gap means nothing if you can only fill 5 contracts. Check order book depth on both sides.
5. Use tools: Alphascope's arbitrage scanner automatically detects cross-platform price gaps on matching markets in real-time, so you don't have to check manually.
Step-by-step example
Let's walk through a real arbitrage calculation:
Market: "Will Congress pass the AI regulation bill by Q3 2026?"
- Kalshi Yes price: 45¢
- Polymarket Yes price: 38¢
Strategy: Buy Yes on Polymarket (38¢), buy No on Kalshi (55¢ = 100¢ − 45¢).
Total cost: 38¢ + 55¢ = 93¢
Outcome A — Bill passes: Polymarket Yes pays $1. Kalshi No pays $0. Net: $1 − 93¢ = 7¢ profit.
Outcome B — Bill doesn't pass: Polymarket Yes pays $0. Kalshi No pays $1. Net: $1 − 93¢ = 7¢ profit.
After fees (~2¢ combined): ~5¢ net profit per contract. That's a 5.4% return regardless of outcome.
Risks and pitfalls
Arbitrage isn't truly risk-free. Watch for these:
- Resolution differences: The biggest risk. Platforms may define "the same event" differently. Always read resolution criteria carefully.
- Execution risk: By the time you fill one side, the other side's price may have moved. Use limit orders.
- Capital lockup: Your money is tied up until the market resolves. A 5% return over 6 months is different from 5% over a week.
- Platform risk: If one platform has issues (frozen withdrawals, disputed resolution), your "guaranteed" profit isn't guaranteed.
- Fee changes: Platforms can change fee structures, eating into thin margins.
- Counterparty risk: Polymarket is unregulated for US users. Consider this in your risk calculation.
Tools for finding arbitrage
Manual monitoring is slow. These approaches help:
- Alphascope Arbitrage Scanner → Automatically detects cross-platform price gaps between Kalshi and Polymarket. Shows matched markets and current spreads.
- API monitoring: Both Kalshi and Polymarket offer APIs. Build scripts that poll prices and alert you to gaps.
- Spreadsheet tracking: For a low-tech approach, maintain a sheet of matching markets and update prices periodically.
How to maximize arbitrage returns
- Speed matters: Gaps close fast. Have accounts funded on both platforms so you can execute immediately.
- Size appropriately: Don't put more into one trade than the order book can handle without slippage.
- Focus on high-volume markets: Elections, Fed decisions, and major events have the best liquidity on both platforms.
- Track your annualized return: A 3% arb that resolves in a week is better than a 10% arb that resolves in a year.
- Diversify across opportunities: Don't put all capital in one arb. Spread across multiple to reduce platform risk.
Track arbitrage with Alphascope
Alphascope is built for prediction market traders who want an edge:
- Arbitrage → Real-time cross-platform price gap detection between Kalshi and Polymarket.
- News → See which news is moving prices before gaps close.
- Predictions → Browse all markets across both platforms in one view.
When a pricing gap opens, Alphascope shows it to you before manual traders notice.
FAQ
Is prediction market arbitrage legal?
Yes. Arbitrage is a standard trading practice. On Kalshi (CFTC-regulated), it's fully legal. On Polymarket, legality depends on your jurisdiction—US residents should check local regulations.
How much money do I need to start?
You can start with as little as $100 on each platform. But small amounts mean small absolute profits. Most active arbitrageurs use $1,000+ per platform for meaningful returns.
How often do arbitrage opportunities appear?
Price gaps appear daily, especially around major news events. Most are small (1-3%) and close within hours. Larger gaps (5%+) are rarer but occur during volatile periods.
Can I automate arbitrage?
Yes, with both platforms offering APIs. However, automated execution adds complexity—you need to handle rate limits, order failures, and ensure resolution criteria match. Most traders start manual and automate later.
What's the biggest risk?
Resolution differences. If Kalshi and Polymarket interpret an event differently, one side might win while the other loses, turning your "arb" into a double loss. Always verify resolution criteria match exactly.