What no-vig probability is
The raw implied probabilities of a two-sided market sum to more than 100%. The amount over 100% is the book's margin. No-vig probability removes that margin, giving you the closest publicly available estimate of the book's view of true probability.
The math
Compute each side's raw implied probability from American or decimal odds. Sum them. Divide each side's raw probability by the sum. The result is each side's vig-free probability. The two vig-free probabilities sum to exactly 100%.
Example: -110 / -110 implies 52.38% / 52.38%, summing to 104.76%. Divide each by 1.0476 and you get 50.00% / 50.00%. Hold = 4.76%.
Why this matters
Different books have different holds on the same market. Comparing books at their no-vig probabilities (rather than raw odds) lets you see which book is pricing each side as the favorite, on a level field. It also helps when you're estimating value: compare your own probability estimate to the no-vig probability, not the raw implied.
For more on hold and how it compares across books, try our Sportsbook Hold Calculator or read about why odds differ between sportsbooks.