MLB ABS Challenge System Explained: What Overturns Reveal

MLB ABS Challenge System Explained: What Overturns Reveal

During 2026 spring training, one game produced more than eight overturned ball-strike calls, a total so unusual that 65% of games in the spring-training sample had two or fewer reversals, according to MLB.com's spring-training review. That report doesn't name the umpire, the teams, or the exact date behind the outlier, only that it happened once, in the earliest days of camp, back in February.

The MLB ABS Challenge System, introduced for the 2026 regular season at MLB ballparks, lets the batter, pitcher, or catcher, never the manager, appeal a home-plate umpire's ball or strike call, according to MLB.com's official rundown. As of MLB's late-March spring-training report, players had logged more than 1,800 challenges with a 53% overturn rate, per MLB.com.

To make sense of a number like eight overturns in one game, it helps to separate four things: how many challenges were made, how many succeeded, what share succeeded, and what fraction of all called pitches got challenged in the first place. The last number is the one that usually gets skipped, and it's the one that explains most of what a high-overturn game actually shows.

How MLB ABS challenges work in 2026

Each team starts a nine-inning game with two challenges. A team keeps its challenge when the appealed call is overturned and loses it only when the umpire's original call is confirmed. Teams that enter extra innings with no challenges left receive one per extra frame, MLB.com reports.

Only the batter, catcher, or pitcher may initiate a challenge, and the decision has to come almost instantly, roughly within two seconds of the pitch, by tapping the cap or helmet with no help from the dugout or other fielders. One exception applies: if a checked-swing appeal or a play involving a runner follows the pitch, the challenge can wait until that play ends.

Two other restrictions shape what shows up in a box score. Challenges aren't available when a position player is pitching, and they're not available immediately after a replay review, a rule meant to keep multiple review processes from stacking on top of each other.

Results appear almost instantly as an animated graphic on the videoboard and broadcast. Across 288 games in 2025 Spring Training, challenges averaged 4.1 per game and took 13.8 seconds each to resolve, per MLB.com.

This is MLB's 2026 rule, first tested in Minor League ball starting in 2022, and it runs in every 2026 regular-season game at an MLB ballpark. Three scheduled games are exceptions: the Mexico City Series, the Field of Dreams game, and the Little League Classic, none played in an MLB park and none equipped with the infrastructure to run the system, per MLB.com. Minor League and spring-training figures are useful context for how MLB ball-strike challenge rules behave, but they aren't the governing standard for regular-season MLB games. Readers following another level should check that league's own rulebook, since it may use different challenge rules.

The system operates through a small number of narrow, player-driven appeals rather than a full electronic strike zone. That design means the overturn total in any single game already reflects player decisions before it reflects umpire performance.

Why the automated strike zone doesn't match the umpire's zone

ABS uses a two-dimensional rectangle set at the plate's midpoint: 17 inches wide, with the top of the zone at 53.5% of the batter's measured height and the bottom at 27%, according to MLB.com. The rulebook's official strike zone is three-dimensional, but the ABS zone used for challenges only measures that single slice at the plate's center.

Umpires have traditionally called a rounder zone that runs more pitcher-friendly vertically, topping out around 55.6% of a batter's height and bottoming at 24.2%, a gap that exists independent of any individual umpire's skill, per MLB.com's season overview.

That gap matters in practice. A pitch an umpire calls using the traditional, rounder zone can still register as a miss against ABS's flatter rectangle. That's a definitional difference between two strike zones, not necessarily a judgment error by the umpire.

The reported tracking uncertainty is small but relevant at the edge of the zone. The Ringer, citing The Athletic, said MLB was 95% confident a tracked pitch location falls within 0.39 inches of its actual spot, and 99% confident within 0.48 inches (The Ringer). That uncertainty limits how confidently anyone should call a very close overturn an obvious miss, even though the ABS ruling stands as the official result of the challenge.

The zone's difference from tradition doesn't, by itself, explain a high overturn count in one game. That depends on which pitches actually got challenged.

Who challenges MLB umpire calls, and how selection affects overturn rates

Only 1.4% of all pitches were challenged during 2026 spring testing, but among close, edge-of-the-zone takes, where challenges cluster, that rate jumped to 7%, per MLB.com's late-March report. That's the central fact for interpreting any overturn rate above 50%: players are selectively appealing borderline pitches, not challenging at random, so a high overturn rate doesn't mean anywhere close to half of all called pitches were wrong.

Fielding-side challenges succeeded more often than hitter challenges, 60% against 45%, and catchers initiated 95% of those fielding-side appeals during spring testing, according to MLB.com. Several MLB clubs have already adopted internal no-pitcher-challenge policies, per MLB.com's season overview. A catcher-heavy challenge total in a given game may reflect team policy as well as the catcher's closer view of the pitch, rather than a surprise.

Challenge frequency also shifts with the count. Batters challenged just 4.9% of challengeable pitches overall this spring, but that rate rose to 17% on two-strike counts and 20% on full counts, per MLB.com. The spring data shows that batters challenged more often in two-strike and full counts, rather than spreading appeals evenly through an at-bat.

Individual skill can vary widely once a season is underway. In ESPN's mid-May snapshot of the 2026 season, Cubs catcher Carson Kelly had gone 21-for-25 challenging while catching, an 84% success rate that Baseball Savant translates to roughly 2.3 runs above expectation, the highest individual total tracked through that point in the season, according to ESPN. Kelly's results illustrate how individual challenge skill can affect a game's total, a variable separate from how accurately the umpire behind the plate called the rest of that game's pitches.

What automated ball-strike system overturned calls actually measure

Reading a box score with a high overturn count comes down to that fourth measure: how many of all called pitches actually got challenged. The 1.4% versus 7% split shows why it matters. Teams contested a tiny fraction of all pitches overall but a much larger share of the tightest, edge-of-zone calls, so overturns concentrate on a narrow slice of borderline pitches rather than spreading across everything the umpire called that night.

A simple example makes the point. Eight overturns out of ten challenges in, say, a 140-pitch game is a strong success rate for the players involved, but it still covers a small share of the night's total calls. Eight overturns out of twenty challenges would tell a different story, one suggesting a wider, less selective mix of pitches got contested.

A real example from spring testing shows how one player alone can move a total. Cardinals pitcher Matthew Liberatore challenged three calls in the first three innings on March 20 and won all three, according to MLB.com. That's a strong run of challenges from one pitcher, but it still says nothing about the many pitches he didn't challenge that day.

A few factors help put any single high-overturn game in context:

• Compare total challenges to total pitches called. A handful of challenges out of a full game's worth of pitches is a small sample.
• Note who initiated each challenge, since fielding-side appeals succeeded more often than hitters' during spring testing, and catchers accounted for the large majority of fielding-side appeals (MLB.com).
• Note the count when the challenge happened, since batters challenge far more often on two-strike and full counts than earlier in an at-bat (MLB.com).
• Remember the roughly 0.4-inch tracking uncertainty, which limits how confidently a close miss can be sized up, even when the challenge result itself is official (The Ringer).

The spring data also showed different overturn rates by inning group, so the timing of challenges is another useful piece of context: 60% in innings one through three, 51% in innings four through six, 43% in innings seven and eight, then back up to 46% in the ninth, according to MLB.com. That pattern doesn't say why rates shifted, only that when a game's overturns happened is worth checking alongside how many occurred.

Reading a high-overturn game

MLB's Challenge System runs in every 2026 regular-season game at MLB ballparks, aside from the three scheduled games this year played outside MLB parks without the infrastructure to support it. Within that system, an overturn count is a record of which pitches players chose to contest, not a review of every call the umpire made that night.

Before treating a high number as a bad night for an umpire, check who challenged, on what count, and how many total pitches were called that game. A catcher with a sharp eye at the edges of the zone, or a lineup that saves its appeals for full counts, can push a game's total higher without saying anything about the umpire's overall accuracy. That's the distinction worth checking the next time a box score shows a stack of overturned calls in one game.