Home-run-per-fly-ball (HR/FB) rate is the rate at which home runs are hit against a pitcher for every fly ball he allows. It's as simple as the name makes it sound. The league average HR/FB rate is usually slightly below 10 percent.
For example, if a pitcher faces 25 batters -- 10 of whom hit fly balls -- and he surrenders one home run, his HR/FB rate is 10 percent.
Home runs / total fly balls, including home runs.
Why it's useful:
Research shows that the HR/FB rate of individual pitchers can vary greatly from year to year. That means pitchers with high HR/FB rate have generally -- but not always -- experienced some bad luck.
Because HR/FB rates tend to regress toward the league average, pitchers who have allowed many fly balls -- but few long balls -- are likely due for an uptick in homer allowance. The opposite holds true for hurlers who have allowed few fly balls but many homers.