Home Run to Fly Ball rate (HR/FB) is the ratio of how many home runs are hit against a pitcher for every fly ball they allow. Home runs are obviously not good for a pitcher, and a pitcher can reduce the number of home runs hit against them in two ways: by increasing their ground ball rate (therefore lowering their fly ball rate), or by reducing their HR/FB ratio.
While pitchers can control (to a certain extent) the type of batted balls hit against them, there is less skill involved when considering whether a long fly ball is hit into the seats or to the warning track. For example, pitchers who throw in a home ballpark with short fences will tend to have a higher HR/FB ratio than pitchers who throw in large ballparks. Pitcher HR/FB ratios have also been shown to vary considerably from year to year, meaning they have limited predictive value. The only surefire way to limit home runs is to limit fly balls.
HR/FB is one of the easier sabermetric calculations. You simply take the number of home runs allowed and divide by the number of fly balls allowed (and then multiply by 100 to turn it into a percentage for presentation purposes):
HR/FB = (Total Home Runs Allowed / Fly Balls Allowed)*100
Please note that the home run value used is all home runs, not just home runs hit on fly balls. Home runs can occur on line drives and occasionally via ground ball. All home runs are included.
HR/FB is very important because it offers insight into how “lucky or unlucky” a pitcher’s home run rate might be. Home runs kill pitchers, but because they’re a relatively rare event a few lucky or unlucky moments one way or the other can dramatically alter a pitcher’s season. HR/FB gives you some information about those home runs allowed.
Specifically, pitchers generally don’t have much control over how often fly balls leave the ballpark. They play a major role in allowing those fly balls to begin with, but the difference between a ball clearing the fence and dying on the warning track is largely out of their control and it generally takes several hundred fly balls allowed for that luck to balance out. As such, HR/FB matters because it tells us if the pitcher is allowing more home runs than we might expect given their batted ball profile. If a pitcher has allowed 200 fly balls during the season, but has allowed just 10 home runs, it’s very likely that he’s been a little bit lucky given that almost no one can consistently run a 5% HR/FB. League average is around 10% and true talent for almost every pitcher is about 8-12%.
This isn’t to say that HR/FB% isn’t a skill, but rather that the true gap is much small the observed values you might see over the course of a single season worth of data. It wouldn’t be surprising for a 9% true talent HR/FB to have an 11% HR/FB for a season or for an 11% true talent to have a 9%.
As a result, HR/FB can help us better forecast a pitcher’s future innings. If a pitcher has been allowing a lot of home runs with an average HR/FB, it means he’s more likely to keep allowing home runs than if a pitcher has been allowing a lot of home runs with a 14-15% HR/FB. In the long run, we expect most pitchers to regress toward league average, or perhaps their career average if there is something about them that is unusual.
HR/FB allows us to get a better sense of how legitimate a pitcher’s home run rate might be, but that doesn’t mean there isn’t some underlying ability to suppress HR/FB, it’s a just a relatively small range of possible true talents,
How to Use HR/FB:
Using HR/FB is quite easy. First, it’s important to acclimate yourself with league average, which is roughly 10%. In the long run, most pitchers will end up very close to average, so if you see a pitcher with a great or terrible HR/FB over a 50 or 100 innings, you can pretty much bank on them moving closer to average in the future.
Some great pitchers can limit their HR/FB and some pitchers without good stuff or great command can run higher than average HR/FB. The key is that it takes a large number of fly balls (~400+) to be confident in a pitcher’s ability to diverge from the pack. If you see a 12% HR/FB 40 innings into a season, you’re going to want to bet on regression toward the mean, but if you see that same 12% over 500 innings, you’re going to expect much less regression.
Stats like xFIP don’t do a great job with pitchers who routine sit better or worse than league average in HR/FB because xFIP regresses their HR/FB to league average. If you have a pitcher with a HR/FB that is noticeably different from league average, you should investigate why that is as you analyze that player. Often, it’s just small sample random variation and you will see it even out in short order. This leads you to expect the player’s current run (good or bad) is going to end, but if the HR/FB difference is consistent, you have a pitcher who might be onto something (or not!). Additionally, parks play an important role. Pitchers at Coors Field will have higher HR/FB than those at Petco, so you want to mentally account for that.
In short, when the HR/FB for a pitcher deviates from average, look closely at the rest of their game to see if this is something you expect to continue.
Please note that the following chart is meant as an estimate, and that league-average HR/FB rate varies on a year-by-year basis. To see the league-average HR/FB rate for every year from 2002 to the present, check the FanGraphs leaderboards.
Remember, extreme home run rates in either direction are likely unsustainable. Certain pitchers can consistently post lower than average home run rates, though, so if trying to determine if a pitcher’s HR/FB rate is unsustainable, be sure to also compare it to their career rate while making note of the number of innings and the ballparks involved.
Things to Remember:
● Taking a glance at a pitcher’s HR/FB ratio can help tell you if a player had an over- or under-inflated ERA. Pitchers with HR/FB ratios much higher or lower than league average will normally regress towards league average in the future, which will have a corresponding effect on their ERAs and FIPs.
● One limitation of the HR/FB ratio is that home runs can also come off of line drives. Generally speaking though, the main principles and implications of a pitcher’s HR/FB ratio remain the same.
● HR/FB is not park adjusted.
Links for Further Reading: