The odds in favor of an event is defined as the number of favorable outcomes divided by the number of unfavorable outcomes. Similarly, the odds against an event is defined as the number of unfavorable outcomes divided by the number of favorable outcomes.

Probability can be viewed as a ratio of part/whole, which is used to describe the likelihood of an event. Odds can be viewed as a ratio of part/part. The following formula can be used to convert a probability into an odds: