The true strike zone, the strike zone used in our accuracy calculations, is based off of the rule book strike zone.
The width of the true strike zone is constant and is equivalent to the width of home plate, or 17 inches; as such, finding out if a taken pitch was horizontally within this strike zone is trivial.
Finding out whether a pitch is within the top and bottom of the strike zone is slightly more complicated. That’s primarily because the top and bottom of the strike zone do not stay constant between at-bats, or even between pitches. According to the MLB Rulebook, the strike zone is determined from the batter’s stance as the batter is prepared to swing at a pitched ball. The Rulebook further defines the top of the strike zone as the midpoint between the top of the shoulders and the top of the uniform pants, and the bottom of the strike zone as the kneecap.
Thankfully, MLB reports the top and bottom of the strike zone for each and every pitch, as set by a Statcast operator – these are the values that our algorithm uses. And once we obtain the top and bottom of the strike zone for any given pitch, our algorithm can check if the pitch falls within the strike zone using collision geometry, making sure to incorporate the radius of a baseball.