Kalshi vs Polymarket Fee Math: When Each Platform Is Actually Cheaper in 2026

The Short Answer

Kalshi (opens in a new tab) and Polymarket (opens in a new tab) US use the same kind of formula: the per-contract fee is proportional to p × (1−p), so it peaks at 50¢ and falls toward the extremes. Kalshi publishes a 0.07 taker coefficient; Polymarket US publishes 0.05. On those formulas, Polymarket US's per-contract fee is slightly lower at every price. But the fee is only part of the cost: the spread and order-book depth often matter more, especially in thin markets.

The Per-Contract Fee Table

(Calculated from the published taker formulas, before rounding: Kalshi 0.07 × p × (1−p) per contract, rounded up to the cent per order; Polymarket US 0.05 × p × (1−p) per contract, subject to its published cap. Check each platform's current fee schedule in the app before trading.) These are per-share fees, applied at trade execution. There are no withdrawal fees on either platform if you use ACH; debit card deposits on Kalshi carry a 2% fee on the deposit itself.

What This Means in Real FL Examples

Example 1 — You buy 1,000 Yes shares on the Heat to make the playoffs at 75¢. On Kalshi: 1,000 × ~$0.0131 = ~$13.13 in fees. On Polymarket US: 1,000 × ~$0.0094 = ~$9.38. The fee difference is about $3.75 on this trade; compare it with the difference in spread between the two order books before choosing.

Example 2 — You buy 1,000 Yes shares on a longshot primary candidate at 8¢. On Kalshi: 1,000 × ~$0.0052 = ~$5.15. On Polymarket US: 1,000 × ~$0.0037 = ~$3.68. At extreme prices the fee gap shrinks a lot, and available liquidity usually decides where to trade.

Beyond the Headline Fees

Fees are not the only cost. Three other line items materially affect what you actually pay:

• Bid-ask spread: On thin markets, the spread can dwarf the fee. Kalshi has tighter spreads on most US sports right now because of higher US volume.
• Slippage: Large orders move the price against you. If you're trading 5-figure positions, you may need to leg into them across both platforms.
• Funding costs: Money sitting on the platform isn't earning interest. If you're trading slow-resolving political markets, the time value matters.
• Tax friction: profits are taxable, but the IRS hasn't issued guidance on how event contracts are classified. Kalshi provides a profit-and-loss statement rather than a 1099-B for trading, so keep your own records and talk to a tax professional.

The FL-Optimal Fee Strategy

If you trade a mix of markets, the best strategy is not to pick one platform — it's to maintain funded accounts on both and route each trade to the venue with the lowest total cost (fee plus spread). For a Florida trader running, say, $25k of annual volume, routing well can save a noticeable amount each year. It's not life-changing, but it's money you keep once the setup is done.

The other reason to keep both accounts: arbitrage. When Kalshi and Polymarket have different prices on the same event, the spread is yours to capture if you can move quickly enough. Most retail won't do this profitably, but having both accounts open means you can react when the obvious mispricings appear.