Kalshi vs sportsbook odds: how to compare prices across venues

A sportsbook and a prediction market are quoting the same event in two different languages. The book says -135; Kalshi says 54¢. Until both are translated into the same unit, you can't see which one is offering the better deal. This guide covers the translation, the same math SportsParity's live screener runs every sixty seconds.

Step one: American odds to implied probability

Negative odds: probability = odds / (odds + 100), using the absolute value. So -135 implies 135/235 ≈ 57.4%. Positive odds: 100 / (odds + 100). So +120 implies 100/220 ≈ 45.5%. A Kalshi contract needs no conversion, a 54¢ ask is a 54% implied probability, because the contract pays $1 on a win.

Step two: remove the vig

Add both sides' implied probabilities at a sportsbook and you'll get more than 100%, typically 102–105% on a moneyline. That excess is the book's margin, the vig. The simplest correction divides each side by the total: if the raw numbers are 57.4% and 46.6% (sum 104%), the fair probabilities are 55.2% and 44.8%. That devigged figure is the fair line, the price the market actually believes.

Step three: read the gap in cents

Now both venues speak cents. If the devigged consensus says 58.5¢ and the Kalshi ask is 54¢, the contract is trading 4.5¢ under fair value, a positive expected-value entry before fees. When the venues cross far enough that opposite positions at each lock a profit either way, the market is arbitraged; those windows are rare and brief, which is precisely why screening them by hand doesn't work.

What to watch out for

Three honest cautions. Exchange fees: Kalshi charges trading fees that scale with price, net edge is always smaller than gross. Liquidity: a great price on a thin book may not fill at size. Speed: gaps close in minutes; a screenshot of an edge is not an edge. Verify the live quote at the venue before acting, and treat any single number as a snapshot, not a promise.

The board does this arithmetic continuously across NFL, NBA, and MLB, see today's edges or join early access for alerts when a gap clears your threshold.