Enter both sides of any two-way market. The calculator strips out the sportsbook's hold and returns the de-vigged fair odds — the price each side would carry if the book made zero margin.
Every two-way market a sportsbook offers includes a built-in margin (the "vig" or "hold"). If both sides were a true 50/50, fair odds would be -100 / -100 (or +100 / +100, same thing). What you actually see is something like -110 / -110 — that 10-cent difference is the book's take.
To find the true implied probability of each side as the market sees it, you have to remove the vig. The standard method: convert each side's American odds to implied probability, sum them (the sum will be more than 100% because of vig), and divide each by the total. That normalizes to 100% and gives you the fair, de-vigged probability for each side.
De-vigged fair odds are the most accurate single-source estimate of true probability available without your own model. They're what professional bettors compare against when evaluating whether a bet has edge, and they're the foundation of closing line value math.
The flow looks like this: you bet a team at +135 (your taken price). At game start, the same book's closing line on that side is +120. The closing line's de-vigged fair odds might be roughly +130. That comparison — your taken price vs. the closing line's fair price — is the most rigorous way to measure closing line value.