Common questions about cross-platform arbitrage between Polymarket and Kalshi, how the fee math works, and how the free calculator keeps your inputs private.
You buy YES on one platform and NO on the other for the same event, so your
two positions cover every outcome. If the combined cost plus fees is below
the $1.00 that one side always pays at settlement, the
difference is a guaranteed profit no matter how the event resolves.
A raw price gap can look profitable and then vanish after fees. Polymarket
charges a taker fee on a p × (1 − p) curve; Kalshi
charges roundup(0.07 × C × P × (1 − P))
rounded up to the cent. Both peak near 50¢, so thin edges on coin-flip
markets disappear. See the full
fee breakdown.
Yes. It runs entirely in your browser — no backend, no account, no network call carrying your inputs. Nothing you type leaves your device.
The Kelly tab sizes a position as a fraction of your bankroll from your fair-probability estimate and the market price, using the Kelly criterion for $1 binary contracts. The Vig tab strips the overround out of a multi-outcome market's prices to reveal the implied true probabilities.
No — treat it as a lead to verify. Confirm the two markets settle on exactly the same outcome, check live order-book depth and spread, and weigh withdrawal and timing risk before committing capital.