How MLB's Automated
Ball-Strike System Works
The Automated Ball-Strike Challenge System is the biggest change to baseball officiating in generations. Starting March 25, 2026, players can challenge a ball-strike call and get a definitive answer in 15 seconds. Here's everything you need to know.
What Is ABS?
A Camera-Based Strike Zone
The Automated Ball-Strike Challenge System — ABS — is MLB's technology-based solution to ball-strike disagreements. Twelve Hawk-Eye cameras installed throughout each ballpark track every pitch with millimeter precision.
The system doesn't replace home plate umpires. They still call every pitch in real time. But now, when a batter, pitcher, or catcher believes a call was wrong, they can challenge it — and the camera system delivers a definitive, objective ruling in about 15 seconds.
It's similar to replay review in baseball, but faster, more precise, and resolved on the field rather than by a video room. The result is announced over the PA, and the original call stands or is overturned.
Debuts Opening Day
ABS Challenge System launches across all 30 MLB ballparks on March 25, 2026. The WBC semifinal on March 15 was played without the system — making the Perdomo call one of the last major controversies of the pre-ABS era.
The Zone
How the Strike Zone Is Defined
ABS uses a personalized strike zone for every batter — calculated from their height.
Strike Zone Specifications
| Parameter | Value / Rule |
|---|---|
| Zone width | 17 inches (width of home plate) |
| Zone top | 53.5% of batter's height (midpoint between shoulders and belt) |
| Zone bottom | 27% of batter's height (just below the kneecap) |
| Zone depth | 2D at front edge of home plate midpoint |
| Tracking system | Hawk-Eye (12 cameras per park) |
| Resolution | Sub-millimeter pitch location |
The Rules
How Challenges Work
Everything you need to know about using the ABS challenge — who can do it, when, and what happens.
How to Challenge
Tap your helmet. That's it. The batter, pitcher, or catcher can initiate a challenge by tapping their helmet immediately after a questionable ball-strike call. No verbal argument needed.
2 Challenges Per Team Per Game
Each team gets 2 challenges. If your challenge is successful — the call is overturned — you keep the challenge. If unsuccessful, you lose it. Starting in extra innings, each team gains one additional challenge per extra frame.
~15 Second Resolution
The Hawk-Eye system processes the challenge and delivers a verdict in approximately 15 seconds. The result is announced over the PA system and displayed on the scoreboard. No video room, no long delay.
The Call Is Final
Once the ABS system rules on a challenge, the ruling is final. No appeal, no manager argument. The system's verdict is the verdict. The human umpire accepts the outcome either way.
Who Can Challenge
Only the batter, pitcher, or catcher may challenge a ball-strike call — not the manager from the dugout. The challenge must be immediate, before the next pitch is thrown.
Why It Matters
The 6% Problem
MLB umpires are remarkably accurate — roughly 94% correct on called pitches. That sounds high. But across a full season, it adds up fast.
Most missed calls are inconsequential — a 1-2 pitch that should have been 2-2. But some happen in the 9th inning, with the tying run on third, full count, two outs. Those are the calls that define seasons. That's why ABS exists.
See It In Action
The 2026 WBC semifinal showed exactly what happens when a game-deciding call goes wrong with no recourse. See how ump.bot analyzed the Perdomo and Soto calls.
Read the WBC analysis →ump.bot + ABS
ABS Gives Teams Challenges.
ump.bot Gives Fans Transparency.
ABS is a powerful tool for teams in the moment — a check on the most critical calls when it matters most. But it doesn't give fans a picture of how their umpire is performing over the course of a game, a series, or a season.
ump.bot fills that gap. We track every called pitch — not just the challenged ones — and publish real-time scorecards for every home plate umpire. Accuracy by inning. Accuracy by pitch location. Historical trends. Run impact of missed calls. The full picture, for every fan.
Get Early Access for Opening DayFAQ
Common Questions About ABS
Does ABS replace the home plate umpire?
No. Human umpires still call every pitch in real time. ABS only comes into play when a challenge is made. Umpires retain their role and authority — the challenge system is a check, not a replacement.
What happens if I use both challenges and they're both wrong?
You have no more challenges for the game (until extra innings, when you get one more per frame). Teams should use challenges strategically — ideally for full-count pitches in high-leverage situations where a wrong call changes the outcome most dramatically.
Can a pitcher challenge a ball call?
Yes. The pitcher, catcher, or batter can all challenge any ball-strike call. If a pitcher thinks a borderline pitch was called a ball when it was a strike, they can challenge. The challenge comes from the team's shared allotment — not individual player counts.
How does ABS handle a pitch that's on the exact edge of the zone?
The system uses millimeter-precision tracking. A pitch that's half a millimeter outside the zone is a ball; a pitch that's half a millimeter inside is a strike. There's no "borderline" judgment — the system gives a binary yes/no answer. This is different from the human eye, which struggles at the margins.
Was ABS used at the 2026 WBC?
No. The WBC was played under traditional umpiring rules without the ABS Challenge System, despite MLB adopting it for the regular season starting March 25, 2026. The two missed calls in the USA-DR semifinal underscored why international competition should adopt the technology.
How is ump.bot different from the ABS system?
ABS is a challenge mechanism for teams during games. ump.bot is a public analytics platform that scores every umpire on every called pitch — whether challenged or not — and publishes those results for fans. We use the same strike zone definition and tracking data that ABS uses, applied retroactively to build a complete picture of umpire performance.