Kalshi and Polymarket are the two largest prediction market platforms for English-language event contracts. If you're running an arbitrage strategy between them, understanding exactly how fees and liquidity differ on each platform is critical to calculating your real net edge.

Fee structures compared

Kalshi fees: Kalshi charges a transaction fee of approximately 7% of the profit on winning trades (not on the notional). On a $100 position that pays $50 profit, the fee is ~$3.50. For some high-volume markets, maker rebates apply. No fee on losing legs.

Polymarket fees: Polymarket charges takers a fee of shares × category rate × price × (1 − price); makers pay nothing, and some categories such as geopolitics are fee-free. In Polymarket's October 2026 documentation, 100 shares at 50¢ cost $1.00 in politics and $1.75 in crypto. There is no separate fee on settlement.

For arbitrage, both legs incur fees. A rough estimate: 1–2% effective fee drag per leg across both platforms combined. Always calculate net edge explicitly — do not trade if gross spread doesn't cover both platforms' fees.

Liquidity depth

Polymarket liquidity: Polymarket has deeper liquidity on political and crypto markets, particularly for US and global events. On major markets (US elections, Fed decisions, Bitcoin price), order books support $5,000–$50,000+ positions without significant slippage.

Kalshi liquidity: Kalshi has strong liquidity on regulated US financial and economic events (Fed rates, CPI, unemployment). Political markets are also well-traded. Liquidity is generally thinner than Polymarket on crypto and international events.

Market coverage overlap

For cross-platform arbitrage, what matters is how many markets are listed on both platforms simultaneously. Arbitrage Agent currently tracks thousands of matched market pairs — events where the same underlying question is tradeable on both platforms.

The overlap is highest in:

US politics: Elections, congressional votes, presidential approval
Macroeconomics: Fed rate decisions, CPI, GDP, unemployment
Crypto: Bitcoin and Ethereum price milestones
Geopolitics: Major international events

Settlement speed

Kalshi: Settlement is typically same-day or next-day once the event resolves. Regulated exchange rules require prompt settlement.

Polymarket: Settlement uses UMA's optimistic oracle. Most contracts settle within 24–48 hours of the event resolving, but disputed contracts can take longer. Capital is locked until settlement.

Which platform is better for arbitrage?

Neither platform is "better" — you need both for cross-platform arbitrage. The key insight is that each platform's structural differences (different user bases, fee structures, and liquidity pools) are exactly what creates persistent price divergences. Without the differences, there would be no arbitrage.

For a single-platform strategy, Kalshi is the only option for US residents. Polymarket is more accessible globally and has deeper liquidity on some markets.

Practical implications for net edge

Both platforms charge taker fees that peak near a 50¢ price, so the gross spread you need depends on the prices and categories involved; use the calculator rather than a rule of thumb. On low-activity markets, spreads are often below this threshold. On active markets during breaking news, spreads of 3–8% are common.

Arbitrage Agent calculates net edge explicitly (gross spread minus both platforms' fees) before flagging any opportunity. Set your minimum threshold at 1.5% net — which means a gross spread of roughly 3.5%+ depending on position size and fill price.

Key terms: Spread · Cross-Market Arbitrage · Slippage

Kalshi vs Polymarket: Fees, Liquidity & Which Is Better for Arbitrage (2026)