What is HR/FB%?

If you remember our video on Fielding Independent Pitching (FIP), you may recall that pitchers have limited control over whether batted balls end up as hits. Similarly, in our video on Expected Fielding Independent Pitching (xFIP), we learned that the same concept applies to fly balls that end up as home runs.

HR/FB% Explained

Home Run to Fly Ball Rate, or HR/FB%, is a stat that tells us the percentage of fly balls that land as home runs. This metric is most commonly used as an evaluation tool for pitchers, as it is the guiding principle behind xFIP. HR/FB% is represented by the following equation:

HR/FB% = (Total Home Runs Allowed / Fly Balls Allowed) * 100

We can use a pitcher’s HR/FB% to help us understand a pitcher’s ability or whether they were lucky or unlucky on fly balls that become home runs.

For example, let’s say that in a given year, a pitcher has a HR/FB% of 5%, meaning that of all the fly balls they allowed, only 5% of them were home runs. Let’s also say that in this same year, the league average was 12%. This stat tells us that this pitcher was incredibly successful at limiting the number of home runs compared to the league average.

This could allude to the fact that he is skilled at preventing homers, or, was the recipient of some luck throughout the season. You can use other stats that incorporate batted ball contact and run prevention factors to determine the answer to this question.

The element of luck on HR/FB% is precisely why xFIP normalizes every pitcher’s HR/FB% to the league average, to correct for potential deviations. We should use HR/FB% to see whether a pitcher’s ERA may be inflated or deflated based on the deviation between their home runs on fly balls from the league average.

Summary

HR/FB% tells us how a pitcher performed in terms of giving up home runs per the amount of fly balls given up. It is one of the many stats that help us begin to break down what type of season a pitcher may have had. Based on what this stat tells us we can then turn to other metrics and dive deeper into whether his performance was skill, luck related, or both.