We hear a lot about the importance for pitchers to keep the ball down (although down and in specifically seems like a bad idea looking at fastballs and breaking pitches). But how low is enough, and what difference does it have on hitters’ ability to put balls in play?

Regardless of the break of the pitch, one thing is certain: the lower it is thrown, the more likely it will be hit on the ground. (For the following graphs, every pitch has been remapped to an average strike zone with a bottom of 1.6 and a top of 3.5 feet high). First, the groundball % by the height of the pitch:

If nothing else, the bottom of the strike zone is worth a 70-75% ground ball rate. With a few exceptions, this effect can be seen on the groundball/flyball rates on teams around the league. For every team, here’s the average height of their pitches to their groundout/flyout rates.

Now there’s a reason to keep the ball low- home run power falls off fast as soon as the ball is below the belt but is relatively steady anywhere above it (the weird spike above the top of the zone is a lack of sample size). The same is true for doubles and triples power: a graph of slugging percentage versus height looks much the same:

So while keeping the ball down doesn’t have a big effect on the number of hits allowed unless you paint the very bottom of the zone, keeping it anywhere below the belt is the best way to sap a hitter’s power. If you really need to get someone out, you can venture into the upper half of the zone- but the hits that do come will go further.