Why Price Gaps Between Kalshi and Polymarket Aren't Free Money (Most of the Time)
Why Kalshi-Polymarket price gaps aren't always free money — and when they actually are. A classification framework for cross-platform spreads.
You've seen the 10–15¢ spreads. Here's the real classification of what they actually are.
The 5 Types of "Arbitrage"
Most price gaps between platforms fall into one of these five buckets. Only one of them is actually exploitable.
Why Fees Eat Apparent Arbitrage
Even when you see a real gap, the combined cost of opening and closing both legs often exceeds it.
Fee Rates by Platform
| Platform | Entry fee | Exit fee |
|---|---|---|
| Kalshi | Check current platform fees | None |
| Polymarket | Varies by venue and market category | None |
Worked Example: 3¢ Gap
The question: "Is a 3¢ gap — Kalshi YES at 55¢, Polymarket YES at 52¢ — real arbitrage?"
What you see: Kalshi 55¢ | Polymarket 52¢ → apparent 3¢ gap
Leg 1 — buy: Buy Polymarket YES at 52¢. Fee: varies by venue and market category; some markets may be fee-free while Polymarket US has its own fee schedule. Check the official fee schedule and the specific market before treating a spread as edge.
Leg 2 — sell: Sell Kalshi YES at 55¢. Fee: confirm the current Kalshi fee schedule before trading.
Total fee cost: Sum the verified entry and exit fees for both legs. If the total exceeds the gap, it's not arbitrage.
Net edge after fees: Only positive if the verified fees are smaller than the gap.
Possible outcomes
The 3¢ gap can be real arbitrage only if the verified total fees are smaller than the gap.
You end up long one side with open exposure plus fees paid, and no offsetting leg. What looked like a 3¢ gap was a contract-wording difference, not arbitrage.
A 3¢ gap might survive fees — but only if both legs fill at the quoted price and the contracts are truly identical. Most aren't.
Most "Arb" Is Actually Contract Mismatch
The same event, priced differently, because the resolution criteria differ in ways that matter.
Why Kalshi and Polymarket Trade at Different Prices Even on Identical Contracts
Different participant pools can sustain persistent price differences that aren't exploitable.
The Rarer Case: When a Gap Is Actually Exploitable
Real cross-platform arbitrage exists, but requires all six conditions to hold simultaneously.
Real Arb Checklist
- Same underlying contract terms
- Same resolution source (exact wording matches)
- Same resolution date
- Gap larger than combined fees on both sides
- You can fill both legs at quoted price (no slippage)
- Settlement timing aligns (you won't be stuck in one platform awaiting payout)
Honest bottom line
Real cross-platform arb exists but is rare and usually closed before manual traders can execute. Systematic traders with API access and automated execution are the primary beneficiaries. If you're reading this page, you're probably not trading fast enough to catch it. Where retail has real edge: category selection and resolution mechanics knowledge — not pure speed arb.