What is Overround?

The overround, also called the book percentage, is the sum of the implied probabilities of every outcome in a market. A perfectly fair market sums to exactly 100%; anything above that is the bookmaker's built-in margin, closely related to the vig and to the concept of Implied Probability. The excess is what guarantees the book a long-term return regardless of which outcome occurs. Work through a simple example. A fair coin toss priced at true odds would be 2.00 on each side, and 1/2.00 + 1/2.00 = 50% + 50% = 100%. A bookmaker instead offers 1.91 on each side. The implied probabilities are 1/1.91 = 52.36% each, summing to 104.7%. That 4.7% above 100 is the overround, and it represents the theoretical hold on a perfectly balanced book. Three-way markets stack the margin across more outcomes. If a football match is priced 2.40, 3.40 and 3.10 for home, draw and away, the implied probabilities are 41.7%, 29.4% and 32.3%, totalling roughly 103.4% for an overround of about 3.4%. Comparing overrounds across sportsbooks tells you which offers the better value overall: a 102% book is markedly more generous than a 108% one. The common mistake is comparing only a single price you fancy rather than the whole market. A book can post a tempting figure on one selection while loading the margin elsewhere. To judge true value, convert every price to its implied probability and add them up; the lower the total, the more of your stake is being returned to bettors over time.