${\displaystyle HR/9IP=9\cdot {\frac {HR}{IP))}$

In baseball statistics, home runs allowed per 9 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] Pitchers with high fly ball rates are more likely than pitchers with high ground ball rates to have high HR/9 rates.^[1]

Leaders

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 had pitched a season without giving up a home run.^[3] All played prior to 1927.^[3]

The active leaders in HR/9IP through 2018 were Clayton Kershaw (0.6225), Adam Wainwright (0.6755), and Charlie Morton (0.7682).^[4]