How Fair Odds Are Calculated
Fair odds are the price at which a bet breaks even forever. Here is how to strip the vig from a book line, convert a win probability into fair American odds, and read a play-to price.
Fair odds are the price at which a bet would break even forever: no profit, no loss, over an infinite number of identical wagers. They are the benchmark every value play is measured against. Anything a sportsbook offers that is better than fair is, by definition, a positive-EV bet.
Step 1: implied probability
Every price already contains a probability. To read it out, convert the American odds. A -150 favorite implies 150 / (150 + 100) = 60%. A +150 underdog implies 100 / (150 + 100) = 40%. That is the win rate the price is quietly assuming.
Step 2: remove the vig
Add up the implied probabilities on both sides of a market and they sum to more than 100%. A standard -110 / -110 line totals 104.76%, and that extra 4.76% is the vig, the house margin baked into the price. Fair odds are what is left once that margin is stripped out and the two sides are scaled back to a true 100%.
Once you have a true win probability, convert it back to American odds:
Favorite (over 50%): −(p / (1 − p)) × 100. A 60% team is fair at -150.
Underdog (under 50%): ((1 − p) / p) × 100. A 40% team is fair at +150.
Step 3: play-to prices and the edge threshold
Fairline turns each fair price into a play-to price: the worst number you should still accept on that side. Anything better than play-to is a value bet; anything worse and the edge is gone, so you walk away. A play is flagged when the best available book price beats the model’s fair price by at least 3%, enough to clear a typical vig with room to spare. The model never reads book lines to build its projection. It prices the game on its own, then compares against the books only at the very end.