Batting Average on Balls In Play (BABIP) measures the amount of balls put into play that fall in for hits (does not include home runs). Normally 30% — meaning a normal BABIP is .300 — of balls in play go for hits, but there are two main factors that cause BABIP to vary: defense and luck.
Pitchers with better defenses will have their balls in play converted to outs more frequently, resulting a lower BABIP. Luck is self explanatory: bloop singles, hits that should be rule hits, etc.
BABIP is a red flag when it comes to looking for fluke seasons from pitchers and hitters. Pitchers with a high BABIP (above .300) will have a higher ERA, although should be expected to improve their overall performance following season. And pitchers with a low BABIP (below .300) will have a lower ERA, although should be expected to regress the following season.
Still, it’s important to compare a pitcher or hitter’s career BABIP to their season BABIP when looking for possible improvement or regression. If a pitcher or hitter has maintained a high or low BABIP throughout their career, then they should be expected to continue posting that BABIP instead of one closer to .300.
Pitchers have less control over their BABIP than batters because they cannot control what their defense does behind them. However, hitters can directly affect what goes on after a ball is put in play; for example, faster players will generally have higher BABIPs because they turn in more infield hits.
Formula: (H – HR)/(AB – K – HR + SF)