HR/FB% (home run rate) is the percentage of home runs a pitcher gives up out of the number of fly balls they gave up. League average is normally around 10.6% (the number used in xFIP).
So if a pitcher’s HR/FB% is below 10.6%, you should expect them to see a slight uptick in home runs eventually. The same goes for a HR/FB% that’s higher than 10.6%: you should expect that pitcher’s home rate to decrease at some point.
But remember it’s important to first look at a pitcher’s career HR/FB% before checking if their current home run rate is unusual. Pitchers who pitch in small or big ballparks generally have higher and lower home run rates, respectively. Pitchers whose home park is big leave their fielders have more space to convert fly balls into outs. Of course, for smaller parks there is less space to convert fly balls into outs.
HR/FB% can also be used to determine if a hitter is due to increase or decrease their home run production. The best way to check is to first look at their career HR/FB% and compare it with their current season HR/FB%. If their HR/FB% is below their career average, expect that hitter to increase his home run output. If their HR/FB% is above their career average, expect that hitter’s home run output to decrease. When looking for trends with hitters remember that you should account for age. If a hitter is getting into his late 30s you should expect his HR/FB% to drop — that’s expected.
Hitters also have more control over their HR/FB% than pitchers, so you shouldn’t look at a hitter’s HR/FB% and compare it to 10.6%. Compare it to their career HR/FB%.