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Odds & Probability

Odds Conversion

The process of converting odds between decimal, fractional, and American formats. Decimal to fractional: subtract 1, then simplify (2.50 = 3/2). Decimal to American: if odds >= 2.00, American = (Decimal - 1) x 100; if odds < 2.00, American = -100 / (Decimal - 1). Conversion is essential for comparing odds across different platforms.

Related Odds & Probability Terms

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.
Fair Odds
The odds that would be offered if the bookmaker had zero margin, reflecting the true probability of an outcome. Fair odds are calculated by removing the overround from the market. If the implied probability is 40% with margin, the fair odds are 1/0.40 = 2.50. Comparing market odds to fair odds helps identify value.
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.
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.
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.
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.
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