A contract price and a moneyline are the same bet.
One is quoted in cents, the other in American odds with vig baked in. Convert between them, then strip the vig out of the book’s line so you are comparing like with like.
Why the two prices aren’t directly comparable
A Kalshi or Polymarket contract at 47¢ pays $1 if it resolves YES, so the price is the market’s probability: 47%. A sportsbook quote is not. Add up the implied probabilities of both sides of a book’s line and you will get more than 100% — often 104–110%. That excess is the vig, and it is the book’s margin. Comparing a raw book price to a contract price without removing it makes the exchange look cheap every single time.
What “no-vig” actually does
It rescales both sides so they sum to exactly 100%, leaving the book’s implied view with its margin removed. That fair number is what you compare against the exchange. If the book’s no-vig fair value is 51% and Kalshi is at 47¢, the exchange is offering the same outcome four points cheaper — before fees.
When this calculator refuses
If the two prices sum to less than 100%, the calculator returns nothing rather than a number. That is deliberate. A complete two-way book always sums to more than 100%, so a sum below it proves the pair doesn’t span every outcome — there is a draw, or a third runner, that these two prices never accounted for. De-vigging anyway divides by a short denominator, inflates fair value, and produces a large, attractive edge that does not exist. Most calculators will compute it for you regardless. This one tells you why it won’t.
A fair price is only half of it — both venues charge to trade. See what the round trip costs →
Informational only, not trading advice.
The terminal runs this on every market on both exchanges continuously, and prints the gap between them on the row.