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Odds & Probability

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.

Related Odds & Probability Terms

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.
Expected Value (EV)
The average amount a bettor can expect to win or lose per bet if the same bet were placed many times. Formula: EV = (Probability of Winning x Profit) - (Probability of Losing x Stake). A positive EV (+EV) indicates a profitable bet over the long term. EV is the single most important concept in professional betting.
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.
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.
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%.
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.
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