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Odds & Probability

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.

Related Odds & Probability Terms

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%.
Positive Expected Value (+EV)
A bet where the expected return is positive, meaning it will be profitable in the long run if placed repeatedly. A +EV bet occurs when the odds offered are higher than the true probability warrants. For example, getting odds of 3.00 on an outcome with a 40% chance of occurring. Professional bettors exclusively seek +EV opportunities.
Probability Distribution
A mathematical function that describes the likelihood of all possible outcomes in a betting event. Common distributions used in sports betting include the Poisson distribution for goals/points, the normal distribution for point spreads, and the binomial distribution for win/loss sequences. Understanding distributions helps bettors build predictive models.
Fractional Odds
Traditional odds format used primarily in the UK and Ireland, expressed as a fraction (e.g., 5/2). The first number represents potential profit and the second represents the stake required. So 5/2 means you win $5 for every $2 staked. Fractional odds of 1/1 (evens) are equivalent to decimal odds of 2.00.
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.
Overround (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.
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