Has the Strike Zone Expanded?

Lately, there have been complaints about the size of the strike zone coming from offensive players. The declining offense throughout Major League Baseball may be caused by a larger strike zone. Ian York answers the question: Has the strike zone expanded?

One of the things we are watching closely this year is the size of the strike zone. It is well established now that the strike zone has been increasing in size, especially to right-handed batters, since the universal use of the PITCHf/x system in 2008. It seems probable that the approximately 10% increase in strike zone size is at least partially responsible for the increased strikeout rate, and the reduced offense overall, in the same period.

I previously looked at the size of the 2015 zone in mid-May, at which point I concluded that:

the differences between 2014/2015 are quite minor. It appears that the top of the zone has been slightly less likely to have a strike called in it and that the bottom has been slightly more likely to have one called there, but the key word is “slightly”.

That was with a very small amount of data, so we need to be cautious about those conclusions.

As reported by Michael Silverman of the Boston Herald, among others, John Farrell and Mike Napoli have recently commented about the 2015 strike zone, suggesting that it is again significantly larger:

“There are some nights you feel the strike zone goes from the chalk line of the right-handed batter’s box to the chalk line of the left-handed batter’s box … Whether it’s been an edict or a mandate that’s been handed down from Major League Baseball, we see it consistently, a much wider, much lower strike zone across the board.”

As stated by Nick Cafardo at the Boston Globe, MLB officials disagree with Farrell:

According to MLB spokesman Michael Teevan, based on data MLB has accumulated, “the strike zone this season has been consistent with the past few seasons. There has not been significant change in the numbers.”

MLB’s statement here is clearly misleading, in that it implies that the strike zone has been consistent for the past few seasons. It has not. Between 2013 and 2014, the strike zone increased in size significantly, almost entirely by expanding the bottom of the zone.

Here is a chart I have shown before. This shows the probability of a strike being called in 2014 in specific places above home plate, relative to the same locations in 2013. Areas where a strike is more probable are red; those where a strike is less probable are blue. The grey polygon outlines the 2013 strike zone: it represents the region where a pitch was equally likely to be called a strike or a ball:

It is obvious that the 2014 zone is larger than the 2013 zone.

However, just because the MLB spokesman is wrong doesn’t mean that Farrell is right. With an additional three weeks worth of called-strike data, how does the 2015 strike zone compare to the 2014 zone? Here is the same kind of chart as above (this time, with the 2014 strike zone shown as the polygon):

Farrell is right but only barely. This year, umpires are more likely to call a strike at the bottom of the zone to both left- and right-handed batters, and they are also more likely to call a strike on the outside edge to right-handed batters. (Right-handed batters have taken the brunt of the expanded zone since 2011; their strike zone has consistently been slightly larger than that of a left-handed batter.) This may be the first year that the strike zone has become wider, even slightly. In previous years, the increase in zone size has come from expanding the bottom, while the sides of the zone, if anything, became more narrow.

However, the differences are not huge. The colorbar at the right of the image shows the relative probability associated with the various shades of red and blue, and for most of this year’s expanded zone the increase in strike probability is about 0.5 or less – that is, a pitch in one of the pinkish patches at the bottom of the zone is no more than 50% more likely to be called a strike than it was in 2014. Compare that to the larger, more intense patch of pink in the 2013-2014 comparison.

If you look at the strike zone on its own (that is, without specifically looking at probability differences vs the previous year) it’s very difficult to see any change. Here is a chart showing the probabilities of a pitch being called a strike in particular regions. Blue means very unlikely to be a strike, green means almost certainly a strike, and the red zone, where a pitch could be called either way, represents the de facto edge of the strike zone. The white polygon overlaid on the chart is the 50% probability outline for the 2014 zone, and it is very hard to see a definite difference between the two this way:

Some umpires have larger or smaller strike zones than the average. However, Farrell made his comments after a doubleheader on Wednesday, June 3, when no walks were issued. The home plate umpires were Mike Winters and Mark Wegner. There isn’t enough data this year to evaluate umpires individually yet, but both of those umpires have consistently had average-sized strike zones in previous years. It seems unlikely that either of the umpires is consistently calling a larger zone than average this year, although it certainly is possible that one or both had a bad day.

Nevertheless, Farrell clearly does have a legitimate point to his complaint, and has a very good reason to complain given the considerable trend to a much larger strike zone year after year since 2010. Not only does a larger strike zone inevitably lead to more strikeouts for every team, it must specifically target teams whose offense philosophy has focused on grinding out at-bats, taking pitches, and knowing the strike zone. It must be exceptionally frustrating to players who pride themselves on their ability to identify a strike vs. a ball to have that ability minimized at the beginning of every year, until they are able to identify the new edges of the shifting zone.

Follow Ian on Twitter @iayork.


  1. Why wouldn’t you compare today’s strike zone against one of the steroid era. It’s not just the bodies, the strike zone used to be a postage stamp.

    • The PItchf/x system was not installed at major league parks and working on a regular basis until 2007. It would be very difficult to make any quantitative comparisons to strike zones before that time.

    • Mike is right that without PITCHf/x we can’t accurately measure the strike zone size before 2007, so we can only guess about the size during the steroid era.

      That said, we can see that the strike zone size remained pretty much constant from 2007 to 2010, after which it began to change year to year. We also know that in 2010, the umpires agreed to a contract that included video evaluation of their strike calls, and that immediately following that the zone began to expand to get closer to the rulebook zone.

      The simplest explanation is that umpires were calling the same zone for years, until video evaluation showed that it was too small. That implies that the 2008 zone was about the same as it was in the heart of the steroid era.

      And you’re right, the 2008 zone was a lot smaller than the one today. Not a postage stamp, but at least 10% smaller than today’s zone, which correlates with the increase in strikeout rate since that period.

      So we don’t know for sure what the earlier strike zones were. They probably underwent many changes over the years (for example, I’ve heard anecdotally that changes in umpires’ protective gear changed the zone size, because it changed they way they could comfortably stand). But it’s likely that the strike zone in the early- to mid-2000s was similar to the 2008 zone.