在统计和概率理论中，一个事件或者一个陈述的发生比（英语：Odds）是该事件发生和不发生的比率，公式为：（是该事件或陈述的概率）。

例如，如果一个人随机选择一星期7天中的一天，选择星期日的发生比是：。不选择星期日的发生比是6。

发生比其实是一种相对概率。 一般来说，普通大众不太使用发生比来描述概率。

发生比可以用小数表示，也可以用比率表示。例如0.25或者1:4。

博彩术语赔率，并不是发生比，而是投资报酬率的一种概念。 是管理机构或者博彩公司会从总赌资中收取手续费后剩下的部分才当作奖金。 通常是固定的比率，也有依中奖人数平均的。 例如：你花1元钱买了一张彩票，1:6的赔率指的是：如果你输了，你损失1元；如果你赢了，你赢6元、并且将你原先的1元钱拿回，所以你总共拿回7元。

Odds are a numerical expression, usually expressed as a pair of numbers, used in both gambling and statistics. In statistics, the odds for or odds of some event reflect the likelihood that the event will take place. Odds against reflect the likelihood that a particular event will not take place. In gambling, the odds are the ratio of payoff to stake, and do not necessarily reflect exactly the probabilities. Odds are expressed in several ways (see below), and sometimes the term is used incorrectly to mean simply the probability of an event. Conventionally, gambling odds are expressed in the form "X to Y", where X and Y are numbers, and it is implied that the odds are odds against the event on which the gambler is considering wagering. In both gambling and statistics, the 'odds' are a numerical expression of the likelihood of some possible event.