Implied probability is a concept derived from betting odds that expresses the market's estimation of the likelihood of a given outcome. It is calculated by converting odds into a percentage, and it serves as the foundation for identifying value in betting markets.

For decimal odds, the formula is straightforward: implied probability equals one divided by the decimal odds. Odds of 3.00 imply a probability of 33.3%, meaning the market estimates the event will occur roughly one time in three. For American odds, the conversion differs depending on whether the line is positive or negative. A moneyline of +200 translates to an implied probability of 33.3%, while −150 implies 60%.

The overround, also known as the vig or juice, represents the bookmaker's built-in margin. It is revealed when the implied probabilities of all possible outcomes in a market are summed. In a fair market, they would total exactly 100%. In practice, the sum typically ranges from 102% to 110%, with the excess representing the bookmaker's theoretical profit margin. Markets with lower overrounds are generally considered more favorable for bettors.

Identifying value requires comparing the implied probability from the odds to one's own estimated probability of the outcome. If a bettor estimates a team's win probability at 50% but the odds imply only 40%, the bet carries positive expected value. This principle underlies most systematic approaches to sports betting, including arbitrage and sharp betting strategies.

Free odds and probability tools enable bettors to convert between formats, calculate implied probabilities, and assess overround across multiple bookmakers simultaneously, streamlining the process of market comparison.