What Is Prediction Market Arbitrage?
Arbitrage is the simultaneous buying and selling of the same asset in different markets to profit from a price difference. In prediction markets, this means exploiting mispricings between platforms — or even within a single platform — to lock in risk-free (or near risk-free) profit.
Prediction markets are particularly fertile ground for arbitrage because they are fragmented across multiple platforms, each with different user bases, liquidity profiles, and pricing mechanisms. While stock markets have been arbitraged down to microseconds by high-frequency firms, prediction markets still have mispricings that persist for minutes or even hours.
Binary Arbitrage Explained
Every prediction market contract is binary: it resolves to either YES ($1.00) or NO ($1.00). If you can buy YES on one platform and NO on another platform for a combined cost of less than $1.00, you have a guaranteed profit regardless of the outcome.
Simple Example
Consider a market: "Will it rain in New York on March 15?"
- Polymarket: YES trading at $0.62, NO trading at $0.35
- Kalshi: YES trading at $0.58, NO trading at $0.38
Strategy: Buy YES on Kalshi at $0.58 and NO on Polymarket at $0.35. Total cost: $0.93.
- If it rains: Your Kalshi YES pays $1.00, Polymarket NO pays $0. Gross profit: $1.00 - $0.93 = $0.07
- If it does not rain: Your Kalshi YES pays $0, Polymarket NO pays $1.00. Gross profit: $1.00 - $0.93 = $0.07
Either way, you make $0.07 per contract. That is a 7.5% return on a position that you might hold for just 1-3 days. Annualized, that is an absurd number. The catch is finding these opportunities and executing them before they disappear.
Three Types of Prediction Market Arb
Type 1: YES-Side Arbitrage
This is the most common and easiest to execute. You find the same event on two platforms where the YES price on Platform A is significantly lower than on Platform B. You buy YES on Platform A and sell YES on Platform B (or equivalently, buy NO on Platform B).
Best markets for this: High-profile political events, major sports outcomes, and economic indicators that are listed on both Polymarket and Kalshi.
Type 2: NO-Side Arbitrage
The mirror image: you buy NO on the cheaper platform and YES on the more expensive platform. Mechanically identical to YES-side arb, but some traders overlook it because they instinctively focus on buying YES contracts.
Type 3: Intra-Platform Arbitrage
Sometimes, a single platform has internally inconsistent pricing. For example, if a market has three possible outcomes (A, B, C) and the prices of all three outcomes sum to less than $1.00, you can buy all three and guarantee a profit. This happens more frequently in multi-outcome markets like "Who will win the election?" with 5+ candidates.
Fee Structures: The Critical Detail
Fees determine whether an apparent arb is actually profitable. Here is the current fee structure for the major platforms:
Polymarket
- Trading fee: ~2% (varies by market maker spread)
- Withdrawal fee: Network gas fees only (Polygon network, typically $0.01-$0.05)
- Settlement: No fee on winning payouts
- Currency: USDC on Polygon
Kalshi
- Trading fee: $0.01-$0.03 per contract (varies by contract type)
- Profit fee: 7% on net profits per contract (this is the big one)
- Withdrawal fee: Free for ACH, $25 for wire
- Settlement: Profits taxed at the 7% fee rate
- Currency: USD
The Kalshi profit fee is the arb killer. A 7% fee on profits means your arb spread must be greater than ~7% after accounting for both platforms' fees to be profitable. This is why many arb opportunities that look great on paper (4-5% spreads) are actually unprofitable once you account for Kalshi's fee structure.
Fee-Adjusted Profitability Formula
net_profit = payout - cost_platform_a - cost_platform_b - fees_a - fees_b
min_spread = (polymarket_fee_rate + kalshi_fee_rate + kalshi_profit_fee) / 2
# For Polymarket + Kalshi:
# min_spread = (0.02 + 0.02 + 0.07) / 2 = ~5.5%
# Only trade arbs with spreads > 5.5%Position Sizing
Position sizing is how you avoid blowing up. Even with guaranteed arb profits, there are risks: platform insolvency, settlement delays, account restrictions, and liquidity problems. Here are the rules the top arb traders follow:
- Never put more than 20% of your capital on a single arb. Even if the spread is huge, you need to diversify across multiple opportunities.
- Size based on the thinner side. If Polymarket has $10K of liquidity at your target price but Kalshi only has $2K, your max position is $2K. You must execute both legs.
- Account for settlement timing. If one platform settles in 24 hours and the other in 7 days, your capital is locked for 7 days. Size accordingly.
- Keep 30% cash reserve. New arb opportunities appear constantly. If all your capital is deployed, you miss better opportunities.
Optimal Capital Allocation
For a $10,000 arb portfolio:
- $3,000 — Cash reserve (always available for new opportunities)
- $3,500 — Polymarket balance (for the Polymarket legs of arbs)
- $3,500 — Kalshi balance (for the Kalshi legs of arbs)
Risk Management
Prediction market arbitrage is "risk-free" in theory, but there are real risks in practice:
Platform Risk
What if one platform goes down, gets hacked, or freezes withdrawals? Your capital on that platform is at risk. Mitigation: diversify across 3+ platforms, never keep more than 30% of total capital on any single platform.
Settlement Risk
What if the two platforms resolve the same event differently? This is rare but has happened, particularly with ambiguously worded markets. Mitigation: only arb markets with identical, unambiguous resolution criteria on both platforms.
Execution Risk
What if the price moves between buying leg 1 and leg 2 of your arb? With manual execution, this is the biggest practical risk. Mitigation: use a bot that executes both legs simultaneously. If you cannot execute both legs within 5 seconds, skip the trade.
Regulatory Risk
Kalshi is CFTC-regulated and operates legally in the US. Polymarket is crypto-native and has a more complex regulatory position. Using both platforms for arbitrage is currently legal, but regulatory changes could affect access. Mitigation: stay informed on regulatory developments and be prepared to withdraw quickly if needed.
Building Your Arb Scanner
The technical setup for automated arb scanning is straightforward:
- Market mapping: Build a database that maps equivalent markets across platforms. "Will Biden win 2028?" on Polymarket maps to the equivalent Kalshi market.
- Price feeds: Connect to each platform's WebSocket API for real-time price updates.
- Spread calculator: For each mapped pair, continuously calculate the cross-platform spread after fees.
- Alert/execute: When a spread exceeds your minimum threshold (recommend 6%+), either alert you or auto-execute both legs.
- Position tracker: Track all open positions, expected settlement dates, and current P&L.
The best arb traders are not the ones with the most sophisticated algorithms. They are the ones who have mapped the most markets across platforms and have capital ready to deploy instantly when spreads appear.