Odds conversion, Kelly criterion, discrepancy detection & probability calculation
Odds conversion is the process of translating one odds format into another. Bookmakers around the world use different formats to express the same probability, and understanding how to convert between them is essential for any serious sports analyst. The three most common odds formats are decimal odds, fractional odds, and American odds, each popular in different regions of the world.
Decimal odds (popular in Europe and Australia) represent the total return per unit staked. For example, decimal odds of 2.50 mean that a $1 bet returns $2.50 (including your stake), for a profit of $1.50. The implied probability is simply 1 divided by the decimal odds, so 2.50 implies a 40% chance (1 / 2.50 = 0.40).
Fractional odds (common in the UK and Ireland) express profit relative to the stake. Odds of 3/2 mean you profit $3 for every $2 staked. To convert to decimal, add 1 to the fraction: 3/2 becomes 2.50 in decimal. Fractional odds of 1/1 (known as "evens") equal 2.00 in decimal, implying a 50% probability.
American odds use positive and negative numbers. Positive odds (e.g., +150) show how much profit you make on a $100 bet. Negative odds (e.g., -200) show how much you need to bet to win $100. To convert positive American odds to decimal: (odds / 100) + 1. For negative American odds: (100 / |odds|) + 1. So +150 becomes 2.50 and -200 becomes 1.50 in decimal.
Understanding implied probability is key to odds conversion. Every set of odds implies a certain likelihood of that outcome occurring. However, bookmakers build in a margin (called the overround or vig), so the sum of implied probabilities across all outcomes will always exceed 100%. Removing this margin reveals the true market-estimated probability. Our calculator handles all these conversions instantly and also includes the Kelly Criterion calculator for optimal bet sizing and the arbitrage detector for finding guaranteed-profit opportunities across different bookmakers.