How MLB probable pitchers work
A "probable pitcher" is the starting pitcher a team says it expects to use in an upcoming game. It is an announcement, not a commitment: the only binding choice is the pitcher written on the lineup card handed to the umpire just before the game.
What "probable" means
Clubs announce the starters they expect to use, and those names are published as the "probable pitchers" for each game. No league rule requires a club to name its starter in advance or sets a notice period, and the word "probable" is used for a reason: plans change.
What is binding is the lineup card. Under the Official Baseball Rules (Rule 5.10(f)), the pitcher named as the starter on the card must pitch to the first batter until that batter is out or reaches base, unless he is injured or ill. See when MLB lineups are released for how the card exchange works.
Why probables change
Common reasons a probable starter does not start include:
- Injury or illness, including a player going on the injured list.
- Postponements. A rained-out game can push every following starter back a day, or the club may skip a starter instead.
- Doubleheaders, which need an extra starter.
- Extra rest. A club may give a starter an extra day, for example after an off day or a heavy workload.
- Bullpen games and openers, where a reliever starts and pitches only briefly. The opener is still the starter on the lineup card.
How rotations actually worked in 2025
A five-man rotation suggests a simple pattern: each starter pitches every fifth game. Real seasons are messier.
| Measure | League average | Range across clubs |
|---|---|---|
| Different pitchers who started at least one game, per club | 13.2 | 8 to 19 |
| Share of starts made by each club's five most frequent starters | 75.9% | 63.0% to 88.9% |
| Rest before the start | Share of starts |
|---|---|
| 4 days or fewer | 31.0% |
| 5 days | 49.2% |
| 6 to 10 days | 14.7% |
| More than 10 days (often injury, demotion or a role change) | 5.1% |
Five days of rest was more common than four or fewer, and about one start in five came after six days or more. Even the most stable clubs used at least eight different starters over the season.
How predictable is the starter?
We tested two simple rules on every 2025 game, using only games played before each date (see our methodology). A rule that chooses the rested pitcher from the last five starters whose turn is longest overdue named the right starter 67.8% of the time. The even simpler rule "whoever started five games ago" was right 66.4% of the time. The rotation rule was right 76.2% of the time in April but only 58.2% in September, and it did badly in the first days of the season, when the most recent starts are from the previous year.
That is why announced probables matter: they carry information a rule based on past games cannot know. It is also why we will show a confidence for each probable pitcher rather than just a name.
What this means on LineupMeter
- Our probable pitchers page will show the expected starter for each team today and over the next few days, with our confidence in each.
- When a probable changes, the change will be logged with the time we recorded it.
- We grade the starter we projected against the pitcher who actually started; see our accuracy policy.
Sources: Official Baseball Rules, rule 5.10. Figures: our own calculations (scripts in our data pipeline). The information used here was obtained free of charge from and is copyrighted by Retrosheet. Interested parties may contact Retrosheet at "www.retrosheet.org".