The share of a hitter's fly balls that become home runs — or, for a pitcher, the share allowed.
Home runs require fly balls, so HR-per-fly-ball measures how often a hitter's airborne contact clears the fence. It connects a hitter's fly-ball tendency with their raw power, and on the pitching side flags arms who give up damage when they allow balls in the air.
Like any rate built on home runs, it carries sample noise over short spans, so it is most useful alongside the contact-quality inputs — barrel rate, exit velocity — that drive it.
Pairing a hitter who hits the ball in the air with a pitcher who allows a high HR-per-fly-ball rate is a recurring setup in home-run research and a factor inside HR DNA.
It links a hitter's fly-ball rate to their power and, for pitchers, identifies who is vulnerable when they allow air contact. It is one input among several, since home-run rates are noisy in small samples.