防御率又称自责分率（ERA，earned run average）是棒球术语，指投手平均每场球所失的自责分。投手因为被打安打或四坏而让人上垒（因为野手失误而上垒的不算），然后又因任何原因让这上垒的人回来得分（就算换投手，惟垒上那人还是原投手要负责的），此时的得分则为自责分。

若某野手失误让人上垒，而该守备足以结束此局者，之后只要有失分，都不算自责分；因为若没发生失误，此局可以提早结束，所以不是投手的错。

防御率的算法：「自责分×9÷所投局数」

举例来说，假设美国职棒大联盟堪萨斯皇家投手王建民本球季累计主投102又2/3局(102局又2人次)，责失31分，所以该球季防御率就是31×9÷102.667=2.72

In baseball statistics, earned run average (ERA) is the mean of earned runs given up by a pitcher per nine innings pitched (i.e. the traditional length of a game). It is determined by dividing the number of earned runs allowed by the number of innings pitched and multiplying by nine. Runs resulting from defensive errors (including pitchers' defensive errors) are recorded as unearned runs and are not used to determine ERA.