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.