Calling balls and strikes goes to the very soul of umpiring baseball, and I devote a lot of space to the subject, both here on the UmpireBible, as well as on the UmpireBible Blog (see, for example, Calling Balls & Strikes: The Matthew Effect).
We're going to cover two topics in this article:
Important: You should read this article together with its companion article, Working the Plate. In that article we cover topics like proper setup, timing, and mechanics.
The strike zone is defined in the rule book Definitions (strike zone) as a three-dimensional area over home plate that extends from the hollow at the bottom of the knee to a point "at the midpoint between the top of the shoulders and the top of the uniform pants." The top of the zone is going to take some discussion, so let's save that for later. Let's start with the "over home plate" part.
Home plate is 17 inches wide (Rule 2.02). A baseball is a shade under three inches in diameter (2.94436644720 inches, to be exact, but let's just call it three) (Rule 3.01). Because the definition of a strike specifies that "any part of the ball passes through [touches] any part of the strike zone" (Definitions (strike zone)), we can conclude that the strike zone is 23 inches wide. Note that the black on the edge of the plate is not part of the plate.
The height of the strike zone is trickier because it's based on the physical attributes of the batter, so the dimensions vary. Not only that, but (let's be honest) the dimension of the upper limits of the strike zone vary somewhat depending on what level of baseball you're doing. If you're doing upper level youth ball (players over 14) or men's leagues, you're calling a clean zone where the top is about a ball above the belt.
But if you call that zone for ten-year-olds, you're going to be walking batter after batter. At the lowest levels of kid-pitch baseball, you're calling pretty much shin to shoulder. And you're probably also giving a ball on the outside of the plate, maybe a ball-and-a-half (but not the inside, because you don't want hit batters).
On the left is an abstracted view of a batter with the strike zone drawn in. The point is to give you a conceptual view of how to adjust for age and level. These lines are not written in stone, so think of this as a generalized guideline on how to make adjustments for player age and ability.
Here are some important take-always from the illustration:
Okay, here's where the rubber meets the road. But before we start, let me remind you that this section is indispensably tied to the article, Working the Plate. The two are inseparable. One is not very useful without the other.
Just a few paragraphs above we defined the strike zone. Knowing what the strike zone is, however, is a far cry from actually seeing it. And with that we leap from the world of measurements and rules to the realm of the umpire-occult. Stay with me, here. The strike zone isn't something that you measure. It's something that you learn to see.
It's not so much the strike zone that you learn to see. Rather, it's the edges of the zone. And you learn to see the calculus of a breaking ball that grazes (or doesn't) the zone peeling in from the top or side, or maybe just clips the bottom as it heads for the dirt. (But watch out for that last one because it will get you in trouble.)
In his insightful article, Advanced Ball & Strike Calling, Peter Osborne argues for building your zone from the bottom up. Others, including Carl Childress (see Working the Plate: Another View, Part 1) argue for building your zone (your mental image of the zone, that is) from the top down.
I'll admit: I'm with Osborne on this one – building the zone from the bottom up. In the article linked to above, there is a section entitled "The Philosophy of the Strike Zone." Far better than I can, Osborne dissects the process of building your zone. Read it. Then read it again. You don't have to agree with everything Osborne says; however, if you don't, you should have a pretty good reason why not. I'm not saying Osborne's advice is gospel; I'm just saying it's really well thought out.
Again, it's a matter of learning to see. Reference points like the knees of both batter and catcher form part of the portrait, as does the movement of the catcher's mitt, the batter's belt, the position of the batter's hands, and of course the plate itself.
See next: Working the Plate.