Arb Calculator

Plug in the YES prices on two markets covering the same outcome, set a bankroll, and get the optimal split that locks in equal payoffs on either resolution.

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. There are two possible orientations: (1) YES on A + NO on B, or (2) NO on A + YES on B. The calculator tries both, picks whichever has the cheaper combined cost (must be < 1.00 per $1 of payoff), and splits your bankroll so both legs return the same payoff. If neither orientation is < 1.00, there's no arb at these prices.

Why four prices, not two. On a real order book, YES + NO often doesn't sum to exactly 1 — that gap is the bid/ask spread. Entering all four prices lets the calculator use what you'd actually pay to execute, not a theoretical mid.

Caveats not included. Trading fees (Kalshi/Polymarket ~3%, PredictIt ~12%) and order-book depth — the displayed price may not be the full size you can fill. A tight arb on paper may not survive either.