Free tool

Prediction market arbitrage calculator

Enter the prices for each side of a Kalshi, Polymarket or Polymarket US trade and see what it costs with fees, what it pays and what you keep. Free, no sign-up, and nothing you type leaves your browser.

How to use it

• Two-sided spread: YES on one platform and NO on the other for the same outcome, for example the example loaded above.
• Bundle: YES on every outcome of an event where exactly one must win. Add a leg per outcome.
• Prices are the asks you'd buy at, in cents. Polymarket legs default to no fee, since most markets don't charge one; if the market shows a fee rate, enter it.

The fee formulas it uses

• Kalshi: 0.07 × contracts × P × (1 − P), rounded up to the cent (details).
• Polymarket US: 0.0695 × contracts × P × (1 − P), rounded up to the cent; θ is set per market (details).
• Polymarket: contracts × rate × P × (1 − P), using the market's own rate (most are 0).

The result assumes exactly one leg wins, every leg fills at the price you entered, and both platforms settle the event the same way. Fee schedules as of October 2026. Not financial advice.