Prediction Market Arbitrage Calculator

Enter the two prices you'd actually take, set a bankroll, and get the optimal split that locks in equal payoffs on either resolution.

Inputs

Fill any two opposing prices (e.g. Side A YES and Side B NO). The other two are optional, for handling bid/ask spread.

Read before betting. The math here ignores trading fees (Kalshi and Polymarket are ~3% per fill) and order-book depth. The displayed price may not be the full size you can fill, and a tight arb on paper may not survive once fees are taken out.

How it works. An arb means buying one outcome on market A and the opposite outcome on market B, so one of the two legs always pays. Fill in the two opposing legs you'd actually take (either Side A YES + Side B NO, or Side A NO + Side B YES), plus your bankroll, then hit Calculate. If the combined cost-per-$1 is less than 1.00 there's an arb; the calculator splits your bankroll so both legs return the same payoff.

Optional: all four prices. On a real order book YES + NO usually doesn't sum to exactly 1 (that gap is the bid/ask spread). If you fill all four prices, the calculator considers both orientations and picks whichever is cheaper.