Vigorish


Vigorish is the fee charged by a bookmaker for accepting a gambler's wager. In American English it can also refer to the interest owed a loanshark in consideration for credit. The term came to English usage via Yiddish slang, which was itself a loanword from the original Russian.
As a business practice it is an example of risk management; by doing so bookmakers can guarantee turning a profit regardless of the underlying event's outcome. As a rule, bookmakers do not want to have a financial interest creating a preference for one result over another in any given sporting event. This is accomplished by incentivizing their clientele to wager offsetting amounts on all potential outcomes of the event. The normal method by which this is achieved is by adjusting the payouts for each outcome as imbalances of total amounts wagered between them occur.
Within the mathematical disciplines of probability and statistics this is analogous to an overround, though the two are not precisely synonymous but rather bijective reciprocals of one another. Overround occurs when the sum of the implied probabilities for all possible event results is above 100%, whereas the vigorish is the bookmaker's percentage profit on the total stakes made on the event. For example, an overround of 20% results in 16.% vigorish. The connecting formulæ are:

Proportionality

It is simplest to assume that vigorish is factored in proportionally to the true odds, although this need not be the case. Under proportional vigorish, a "fair odds" betting line of 2.00/2.00 without vigorish would decrease the payouts of all outcomes equally, perhaps to 1.95/1.95, once it was added. More commonly though, disproportional vigorish will be applied as part of the efforts to keep the amounts wagered balanced, such as 1./2.00, making the outcome with fewer dollars wagered appear more attractive due to the larger payout.

Examples

The simplest wager

Two people want to bet on opposing sides of an event and agree to "fair odds", also known as evens. They are going to make the wager between each other without using the services of a bookmaker. Each person agrees to risk $100 for the chance to win $100. The person who loses receives nothing and the winner receives both stakes. Rather than pay vigorish to someone who will guarantee that the winner will be paid, they both assume the opportunity cost in the event the backer of the losing side refuses to pay the winner at the event's conclusion.
By contrast, when using a sportsbook with the odds set at 1./2.00 with vigorish factored in, each person would have to risk or lay $110 to win $100. The extra $10 per person is, in effect, a bookmaker's commission for taking the action. This $10 is not in play and cannot be doubled by the winning bettor; it can only be lost. A losing bettor simply loses their $110. A winning bettor wins back their original $110, plus his $100 winnings, for a total of $210. From the $220 collected, the sportsbook keeps the remaining $10 after paying out the winner.

Theory vs. practice

Vigorish can be defined independent of the outcome of the event and of bettors' behaviors, by defining it as the percentage of total dollars wagered retained by the bookmaker in a risk-free wager. This definition is largely theoretical in practice as it makes the assumption that the bookmaker has balanced the wagers perfectly, such that they makes equal profit regardless of the contest result.
For a two outcome event, the vigorish percentage, v is
where the p and q are the decimal payouts for each outcome.
This should not be confused with the percentage a bettor pays due to vigorish. No consistent definition of the percentage a bettor pays due to vigorish can be made without first defining the bettor's behavior under juiced odds and assuming a win-percentage for the bettor. These factors are discussed under the debate section.
For example, 1./2.00 pricing of an even match is 4.55% vigorish, and 1.95/1.95 pricing is 2.38% vigorish.
Vigorish percentage for three-way events may be calculated using the following formula:
where, and are the decimal payouts for each outcome. For comparison, for overround calculation only the upper part of the equation is used, leading to slightly higher percentage results than the vigorish calculation.

Other kinds of vigorish

Casino games

More generically, vigorish can refer to the bookmaker/casino's theoretical advantage from all possible wagers on any game they offer. The term may also refer, and be applied in specific ways, to particular casino games.