In baseball statistics, home runs allowed per nine innings pitched (HR/9IP or HR/9) or home runs allowed per nine innings (denoted by HR/9) is the average number of home runs given up by a pitcher per nine innings pitched.
It is determined by multiplying the number of home runs allowed by nine and dividing by the number of innings pitched.
[1] The career leaders in HR/9IP through 2018 were Jim Devlin (0.0448), Al Spalding (0.0468), and Reb Russell (0.0488).
[2] There were 87 single-season leaders in HR/9IP through 2018 who pitched a season without giving up a home run.
[3] The active leaders in HR/9IP through 2018 were Clayton Kershaw (0.6225), Adam Wainwright (0.6755), and Charlie Morton (0.7682).