What is HR/FB Rate? Definition, Formula, and Example
HR/FB rate is the percentage of a player's fly balls that become home runs, a measure of raw power for hitters and a key regression signal for pitchers.
What is HR/FB Rate?
HR/FB rate — home run to fly ball ratio — is the percentage of a hitter's fly balls that leave the park for home runs. It isolates one specific question: when this player puts a ball in the air, how often does it clear the fence? For hitters, a high HR/FB rate is a direct measure of raw power. For pitchers, it works the other way — the rate of fly balls allowed that turn into homers — and it is one of the most important regression signals in all of pitching analysis, because pitchers have far less control over it than fans assume. League-average HR/FB has hovered between 11% and 14% in recent seasons, spiking to roughly 15% during the juiced-ball year of 2019.
How HR/FB Rate Is Calculated
The formula is simple:
HR/FB = Home Runs / Fly Balls
The denominator is total fly balls hit (or allowed), not all batted balls. A few classification quirks matter: line drives that clear the fence are still counted as home runs in the numerator at most providers, which is why HR/FB can occasionally exceed the share of balls hit at fly-ball angles. The metric is most meaningful over full seasons; in small samples a couple of cheap home runs can distort it wildly.
Worked Example
Aaron Judge sits at the extreme top of the hitter scale. In his monster 2022 season he posted a HR/FB rate north of 30% — meaning nearly one in three fly balls he hit became a home run, more than double the league rate. That is elite, almost unrepeatable raw power, and it is the engine behind a 62-homer campaign. A league-average power hitter converts around 12–14% of fly balls; a slap-hitting middle infielder might sit at 5–7%.
On the pitching side, consider a starter who allows a 9% HR/FB rate one year and 16% the next while his fly-ball rate, strikeouts, and walks barely move. That swing is almost entirely noise and ballpark — his ERA will look much worse in the 16% season, but his underlying skill hasn't collapsed. This is exactly why xFIP normalizes every pitcher's HR/FB to league average: it strips out the part of home-run prevention pitchers can't control.
Why HR/FB Rate Matters
For evaluating hitters, HR/FB separates genuine power from ballpark or batted-ball luck. A hitter whose homer total jumped while his HR/FB held steady simply hit more fly balls — sustainable. A hitter whose HR/FB spiked to an unsustainable level is a regression candidate. For pitchers, the metric is the cornerstone of ERA-estimator logic: pitchers regress toward league-average HR/FB over time, so a low-HR season is rarely repeatable. Fantasy managers use this to buy low on unlucky pitchers and sell high on hitters riding a fluky homer rate.
Limitations and Common Misconceptions
The most common mistake is treating a pitcher's single-season HR/FB as a skill. It is mostly luck and park; only at the extremes (true ground-ball specialists, or pitchers in homer-friendly parks) does a persistent signal emerge. For hitters, HR/FB ignores fly-ball *volume* — a 20% HR/FB hitter who rarely lifts the ball will out-homer no one. Always read it alongside fly-ball rate, not in isolation.
Related Terms
In Legends Deck: HR/FB rate drives the power tier of a hitter's card and the home-run-suppression rating on a pitcher's card. Because the simulation engine treats pitcher HR/FB as partly random, two starters with identical real-world homer totals can carry different ratings — the one with the sustainable underlying profile holds his value across a simulated season instead of cratering to regression.