What is Arm Strength? Definition, Formula, and Example
Arm strength is the Statcast metric measuring a fielder's maximum throwing velocity in miles per hour, used to grade outfield arms and infielder throwing tools.
Plain-English Definition
Arm strength is the Statcast metric that captures how hard a defender throws the baseball, expressed in miles per hour. Specifically, it reports the maximum throwing velocity a player reaches on competitive throws during a season — not the average. Statcast began publicly tracking outfielder arm strength in 2020 and now publishes leaderboards for outfielders, infielders, and catchers. It is the defensive analogue to a pitcher's fastball velocity: a raw, physical tool that can be measured directly and grade on a familiar mph scale.
How It's Measured
Statcast's Hawk-Eye optical tracking system records the ball's velocity off the fielder's hand on every throw to a base or cutoff. Only competitive throws count — generally throws over a minimum distance with intent to retire a runner. Soft tosses, relays from short range, and pre-pitch warmups are filtered out.
The published arm strength figure is the average of a player's top 90th-percentile throws over the season — roughly, the speed they reach when air-mailing it home, not their average flip to the cutoff. This avoids penalizing fielders for the many slow, accurate throws they make on routine plays.
For catchers, arm strength is reported alongside pop time, which combines arm and transfer speed.
Worked Example
In the 2023 season, Lourdes Gurriel Jr. of the Diamondbacks led MLB outfielders with an average max-effort throw of 98.7 mph. Other leaders included Hunter Renfroe, Jose Siri, and Jorge Soler, all in the 96–98 mph range. The league average outfielder arm strength sits around 87 mph.
For context, a typical MLB starting pitcher's fastball averages 94 mph — meaning the hardest-throwing outfielders are launching the ball back to the infield at velocities that rival the man on the mound. On the infield, Oneil Cruz posted the hardest tracked throw in MLB history, a recorded 97.8 mph strike from shortstop in 2022.
Why It Matters
Arm strength feeds three downstream evaluations:
- Outfield assist projection. A strong arm deters runners from tagging up or taking the extra base. The deterrent effect — runners who *don't* run — never appears on a stat line but shows up in DRS outfield arm components.
- Position fit. Right field traditionally houses the strongest arms because of the long throw to third base. A 95+ mph arm with average range is often more valuable in right than in left, where the throws are shorter.
- Catcher value. Combined with pop time and framing, arm strength drives caught-stealing rate.
- Scouting grades. The 20-80 scale for arm strength now maps directly to mph thresholds: a 70 outfield arm is roughly 95+ mph, a 50 (average) is ~87 mph, a 40 arm is ~83 mph.
Limitations and Common Misconceptions
- Accuracy is not measured. A 100 mph throw 20 feet up the line is worse than an 85 mph strike to the cutoff. Arm strength tells you the ceiling, not the result.
- Sample size on max-effort throws is small. Outfielders may attempt only 20–40 competitive max-effort throws per year. Year-over-year variance can be large for the same player.
- It does not capture release speed. A quick, accurate transfer matters as much as raw velocity — pop time captures that for catchers; outfielders have no equivalent public metric yet.
- It is not the same as exit velocity. Exit velocity measures the ball off a hitter's bat. Arm strength measures the ball off a fielder's hand.
Related Terms
In Legends Deck
In Legends Deck, every position player card carries a Throwing rating derived directly from Statcast arm strength. Outfielders with 95+ mph arms (Gurriel Jr., Renfroe, Cruz when he played short) earn 80-grade Throwing tools that suppress simulated extra-base advancement against them. Combined with the Sprint Speed of the baserunner, the engine resolves each tag-up and first-to-third attempt against the fielder's arm rating — meaning your right fielder's mph matters every time the ball is hit to the corner.