Robot umpires are coming to MLB. Here’s how they work

NEW YORK– NEW YORK (AP) — Robot referees will arrive in the big leagues this year.

The automated ball/strike system will be introduced as a challenge system in which the human referee makes each decision, which can be appealed to the computer. Robot umpires have been tested in the minor leagues since 2019, with recent tests conducted at Triple-A since 2022, at MLB spring training last year and at the 2025 All-Star Game in Atlanta.

Here’s what you need to know about MLB’s robot umps.

Stadiums are equipped with cameras that track every pitch and determine whether it crossed home plate into the strike zone. In early tests, referees wore headphones and heard “ball” or “strike,” then relayed them to players and fans with traditional hand signals.

The challenge system adds a wrinkle. Human referees call every pitch, but each team has the opportunity to challenge two calls per game. Teams that waste their challenges get an additional challenge in each additional round. A team retains its challenge if successful, similar to the rules for big league teams with video reviews, which were first used for home run calls in August 2008 and widely expanded to many calls for the 2014 season.

Only a batter, a pitcher or a catcher can contest a call by signaling it with a simple tap of the helmet or cap; and assistance from the canoe is not authorized. A challenge must be made within 2 seconds, and the pitch and strike zone graphic is displayed on the scoreboard and broadcast feed. The referee then announces the updated count.

Last year, challenges during spring training averaged 13.8 seconds.

A Hawk-Eye pose-tracking camera system tracks pitches and indicates whether they are in a strike zone based on each hitter’s height, which is measured without shoes. Each player will be measured for their strike zone from 10 a.m. to noon on an ongoing basis during spring training – the time of day to maintain uniformity, as size decreases later in the day. The data will be verified by the Southwest Research Institute. MLB estimated the calibration process to take less than a minute for each player.

While the strike zone actually called by big league umpires tends to be oval shaped, the ABS strike zone is a rectangle, just like in the rule book.

Developing consensus on what a computer strike zone is was a problem.

MLB has changed the shape of the ABS strike zone several times.

It started with a width of 19 inches in 2022, then brought it back to 17 inches – matching the width of home plate. The shrinking strike zone has led to an increase in walks and only slight changes in strikeout rates.

The top of the strike zone made up 51% of a hitter’s height in 2022 and 2023, then increased to 53.5% in 2024 after pitchers complained. The bottom of the strike zone is 27% since 2022 after initially being set at 28%. The position of the batter is not taken into account.

The ABS makes the ball/strike decision in the middle of the plate, 8 1/2 inches from the front and 8 1/2 inches from the back. This contrasts with the rulebook zone called by referees, which says the zone is a cube and a strike is a throw that crosses any part. Big league umpires call about 94 percent of pitches correctly, according to UmpScorecards.

ABS, which uses Hawk-Eye cameras, has been tested in the minor leagues since 2019. The independent Atlantic League tested the system during its 2019 All-Star Game and MLB installed the technology for that year’s Arizona Fall League for top prospects. ABS was tried in eight of nine stadiums in the Low-A Southeast League in 2021, then moved to Triple-A in 2022.

At Triple-A at the start of the 2023 season, half of the games used bots for ball/strike calls and half had a human making decisions subject to team appeals to the ABS.

MLB transitioned Triple-A to an all-challenge system on June 26, 2024, then used the challenge system last year in 13 spring training stadiums hosting 19 teams for a total of 288 exhibition games. Teams won 52.2% of their ball/strike challenges (617 out of 1,182).

During last year’s MLB All-Star Game, four of five challenges made by home plate umpire Dan Iassogna were successful.

The success rate hovers around 50% among minors. At Triple-A last season, the success rate dropped from 50.6% to 49.5%. Defenses – usually catchers – were more successful, winning 53.7% of challenges compared to 45% for hitters. Challenges increased from 3.9 to 4.2 per game.

In 2024 at Triple-A, only 1.6% of first pitches were contested, but that number rose to 3.9% for two-strike pitches, 5.2% for three-ball pitches and 8.2% for full counts.

Challenge percentages were higher later in the game. While 1.9% of pitches were challenged in the first three innings, 2.5% were challenged in the fourth through sixth, 2.8% in the seventh and eighth, and 3.6% in the ninth.

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