Turn a sportsbook line into a fair probability you can compare with a Kalshi price.
Enter both sides of a sportsbook line in American odds. The book's cut is removed, leaving the fair probability. Averaging two sharp books is better than one.
A total is −150 on the over and +120 on the under. Implied probabilities are 60.0% and 45.5%, which add to 105.5%: the extra 5.5 points is the vig. Dividing each by 1.055 gives a fair 56.9% over and 43.1% under. If Kalshi has the over at 52¢, that is about a 5-point gap before Kalshi's fee.
Sportsbook odds on both sides add up to more than 100% because the book takes a cut. Devigging scales them back to 100% so you see the fair probability.
The multiplicative method: each side's implied probability divided by the total. It is the standard starting point and works well for evenly matched lines.
Sharp books with low margins track the true probability best. Averaging two or more books reduces the error from any single line.