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Polymarket cost calculator
Enter shares, price, and an optional gas estimate to see the total cost and effective per-share price of a Polymarket trade.
Polymarket charges no exchange trading fee on most markets — the costs are the spread you cross and, in self-custody setups, Polygon gas.
Where Polymarket's costs actually are
Polymarket charges no exchange trading fee on most markets — but "no fee" is not "no cost". The real costs of a Polymarket fill are:
- The spread. If the book quotes 41¢ bid / 43¢ ask and you buy at the ask, you paid 1¢ over mid the moment you traded. On thin long-tail markets the spread is routinely several cents — the dominant cost of the trade.
- Price impact. Size that sweeps multiple book levels fills at progressively worse prices. See slippage in thin markets.
- Polygon gas. Orders settle on-chain. Depending on how you custody and operate, gas is either abstracted away or a small per-settlement cost — enter your own estimate above.
Because Kalshi's cost lives in an explicit taker fee while Polymarket's lives in spread and gas, comparing the two venues on displayed price alone gets the answer wrong routinely. The fee comparison works through real examples, and the arbitrage calculator does the cross-venue math for your numbers.
Details reflect public documentation as of July 2026 — always confirm against Polymarket's current docs before trading.
Stop doing this math by hand
Mithril compares the fee-adjusted net price on Kalshi and Polymarket on every order and routes to the cheaper venue — with an execution receipt on every fill.