A pitcher’s Earned Run Average (or ERA) is the average number of earned runs that a pitcher gives up per nine innings pitched (as the typical game lasts nine innings).
An earned run is a run that is not scored as the result of a defensive error, such as a fielding error or a passed ball.
A pitcher’s ERA is calculated by dividing the number of earned runs he has allowed by the number of innings he has pitched, then multiplying by nine. For example, if a pitcher is charged with 21 earned runs over the course of 90 innings pitched, his ERA would be 2.10.
(21/90) x 9 = 2.1
An ERA under 3.00 is generally considered to be excellent. The lower a pitcher’s ERA, the better.
The lowest all-time career ERA in baseball history was 1.82, by Ed Walsh, who pitched from 1904 to 1917. The lowest career ERA during the live-ball era (that is, post-1920), belongs to Mariano Rivera, who pitched from 1995-2013 and posted an ERA of 2.21.