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Odds & Probability

Overround

Also known as: Book %

The sum of the implied probabilities of all outcomes in a betting market, expressed as a percentage. A fair market totals exactly 100%, but bookmakers add an overround (e.g., 105%) to guarantee a profit margin. Formula: Overround = (1/Odds1 + 1/Odds2 + ... + 1/OddsN) x 100. Lower overrounds indicate better value for bettors.

Related Odds & Probability Terms

Decimal Odds
The most common odds format in Europe and Australia, expressed as a single positive number (e.g., 2.50). To calculate your payout, multiply your stake by the decimal odds. A $10 bet at 2.50 returns $25 total ($15 profit + $10 stake). Decimal odds of 2.00 represent an even-money bet.
Implied Probability (IP)
The probability of an outcome as suggested by the betting odds, expressed as a percentage. Calculated as: IP = (1 / Decimal Odds) x 100. For example, odds of 4.00 imply a 25% probability. Implied probability includes the bookmaker's margin, so the total of all outcomes in a market exceeds 100%.
American Odds (Moneyline Odds)
Odds format used primarily in the United States, expressed as positive or negative numbers. Positive odds (+150) show how much profit you win on a $100 bet. Negative odds (-200) show how much you need to bet to win $100 profit. +150 means a $100 bet wins $150 profit; -200 means you must bet $200 to win $100.
Negative Expected Value (-EV)
A bet where the expected return is negative, meaning it will lose money in the long run if placed repeatedly. Most bets offered by bookmakers are -EV due to the built-in margin. A -EV bet occurs when the odds offered are lower than the true probability warrants. Recreational bettors predominantly place -EV bets.
True Probability
The actual likelihood of an outcome occurring, independent of the bookmaker's margin. True probability cannot be known with certainty, but it can be estimated through statistical models, historical data, and analysis. When your estimated true probability exceeds the implied probability, you have found a value bet.
No-Vig Odds (No-Juice)
Odds that have been adjusted to remove the bookmaker's margin (vig/juice), representing a fairer assessment of the true probabilities. To calculate no-vig odds, divide each outcome's implied probability by the total overround. No-vig lines are useful for identifying which side of a market has more value.
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