What is a Tag Up in Baseball? Definition and Rules

What is a Tag Up?

A tag up is the baserunning rule that requires a runner to re-touch their current base after a fly ball is caught before they can legally advance to the next base. The runner must remain in contact with the base until the fielder makes the catch; if they leave early, they can be called out on appeal. The tag-up rule applies to all fly balls, including line drives and pop-ups, as long as the ball is caught in the air. The runner is allowed to leave the base the instant the ball touches the fielder's glove — not when the ball is secured or when the throw begins.

How is a Tag Up Measured?

The tag-up is a timing event, not a measured metric. The key variables are:

• Catch Timing: The runner must be on the base when the catch is made. Statcast tracks the fielder's catch probability and the ball's hang time, which determines how far a runner can advance.
• Tag-Up Distance: The distance from the base to the next base, typically 90 feet. A runner who tags up on a shallow fly ball to right field has little chance of scoring; a runner on third tagging on a 300-foot fly ball has a much better chance.
• Pop Time of the Catcher: If the runner is trying to score, the catcher's pop time — the time from catch to tag at home plate — determines whether the runner is safe.

The "tag-up rule" is binary: the runner either re-touches the base legally or they don't. The strategic question is whether to attempt the advance, which is determined by the depth of the fly ball, the runner's speed, and the outfielder's arm strength.

Worked Example: The Sacrifice Fly

The most common tag-up is the sacrifice fly. In 2023, the league-average sacrifice fly was hit to a distance of 315 feet from home plate, with an average hang time of 4.2 seconds. A runner on third with a sprint speed of 28 ft/s can cover 90 feet in roughly 3.2 seconds, leaving about 1 second of margin for the outfielder's catch and throw. A runner with 30 ft/s sprint speed (elite) has a nearly 95% success rate on a 300-foot fly ball to center field, while a runner with 26 ft/s (below average) succeeds only about 60% of the time. The rule itself is simple — the runner must stay on the bag until the catch — but the decision to tag and go is a game-theory problem.

Why It Matters

Tag-ups are one of the highest-leverage baserunning plays in baseball. A runner who tags from third on a medium-depth fly ball scores a run without a hit — the single most efficient way to score in baseball. Teams that execute tag-ups well score more runs per fly ball than teams that don't. In advanced analytics, the tag-up is part of "baserunning runs," which Statcast measures as the expected runs added by a runner's decisions on the basepaths. A runner who consistently tags up and scores on sacrifice flies adds 2–3 runs per season — roughly 0.2 wins. In fantasy baseball, the tag-up is invisible in traditional categories, but in simulation games, it's a key input for scoring probability.

Limitations and Misconceptions

The tag-up is often confused with the infield fly rule, but they are unrelated: the infield fly rule protects the offense from an easy double play on a pop-up with runners on first and second, while the tag-up is a universal rule for all caught fly balls. A common misconception is that the runner must wait until the ball is caught and secured — the rule states the runner may leave the instant the ball is touched by the fielder. Another misconception is that a runner who tags up and is thrown out is charged with a caught stealing; in reality, it's recorded as a "sacrifice fly attempt" and the out is a "fielder's choice" or "tag out" depending on the scorer's judgment. The tag-up rule also applies to runners on first and second — a runner on first can tag and advance to second on a deep fly ball, though this is less common.

Related Terms

• What is a Sacrifice Fly?
• What is Sprint Speed?
• What is Pop Time?
• What is a Fielder's Choice?
• What is Run Expectancy?

In Legends Deck:

In Legends Deck, tag-ups are simulated automatically based on the runner's sprint speed, the ball's hang time, and the outfielder's arm strength. A runner with 90+ sprint speed on third base will score on a fly ball to medium depth with high probability, while a slower runner may be held at third. The game also models the risk: a runner with low baserunning IQ — represented by a "baserunning" rating — may attempt to tag up on a shallow fly ball and get thrown out. This makes baserunning ratings a meaningful differentiator on card value, especially for speed-focused players.