// FREE TOOL
Kalshi vs Polymarket arbitrage calculator
Enter the ask price on each venue and your size — the calculator applies Kalshi's taker-fee formula and your gas estimate to show the fee-adjusted winner and the edge in basis points.
Top-of-book only — real fills sweep depth, and displayed prices move while you leg. Depth-aware, fee-adjusted routing is what Mithril automates.
Why displayed prices lie
The same event often trades on both Kalshi and Polymarket at slightly
different prices — but the venue showing the lower price is not always the
cheaper fill. Kalshi charges a taker fee of
0.07 × contracts × p × (1 − p) (rounded up to the cent); Polymarket
charges no exchange trading fee but costs you the spread and, in some
setups, Polygon gas.
Worked example: a market shows 43¢ on Kalshi and 42¢ on Polymarket. For 500 contracts, Kalshi's fee adds ≈1.7¢ per contract at these prices — so Kalshi's effective price is ≈44.7¢ against Polymarket's 42¢ (plus any gas). Here the displayed gap understates Polymarket's advantage. Reverse the displayed prices and the fee can flip the answer entirely.
This calculator does the top-of-book version of that math. What it can't see:
- Depth. Your size may sweep several levels — effective price worsens with size on both venues.
- Legging risk. Prices move while you execute the second leg. A "locked" arb that takes seconds to leg is not locked.
- Resolution-criteria mismatches. The two venues sometimes define the "same" event differently. Read both rule sets before treating a spread as an arb — details in cross-venue arbitrage.
For the full treatment, see Kalshi vs Polymarket fees and smart order routing.
Fee details reflect public documentation as of July 2026 — confirm against the venues' current schedules 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.