What is SIERA? Definition, Formula, and Example
SIERA (Skill-Interactive ERA) is an ERA estimator that predicts a pitcher's true run-prevention skill from strikeouts, walks, and batted-ball mix while accounting for how those skills interact.
What is SIERA?
SIERA — Skill-Interactive Earned Run Average — is an ERA estimator that strips out defense, sequencing, and ballpark luck to measure what a pitcher's actual skill profile *should* produce in runs allowed. Developed by Matt Swartz and Eric Seidman at Baseball Prospectus in 2010, SIERA goes beyond FIP and xFIP by including ground-ball, fly-ball, and pop-up rates and by recognizing that pitcher skills compound — a pitcher who strikes out 30% of hitters *and* gets ground balls suppresses runs at a rate greater than the sum of those two skills measured independently. SIERA is scaled to ERA, so 3.50 is good, 4.00 is league average, and anything under 3.00 is elite.
How SIERA is Calculated
SIERA uses a multivariate regression with interaction and squared terms. The current FanGraphs formulation looks like this:
SIERA = 6.145 − 16.986·(SO/PA) + 11.434·(BB/PA) − 1.858·((GB − FB − PU)/PA) + 7.653·(SO/PA)² ± 6.664·((GB − FB − PU)/PA)² + 10.130·(SO/PA)·((GB − FB − PU)/PA) − 5.195·(BB/PA)·((GB − FB − PU)/PA)
The sign on the squared batted-ball term flips depending on whether the pitcher is ground-ball-leaning or fly-ball-leaning. The interaction terms are the whole point — high-strikeout ground-ballers (think a heavy-sinker, plus-slider profile) get an extra credit because those skills magnify each other on the run-prevention curve.
Worked Example
In 2024, Tarik Skubal posted a 2.49 SIERA over 192 innings — best among qualified AL starters. The inputs: 30.3% K-rate, 4.6% BB-rate, and a 43.7% ground-ball rate. Plug those into SIERA and you get a profile that suppresses runs through three independent skills simultaneously: he misses bats, he doesn't walk hitters, and when contact happens it stays on the ground. His actual ERA was 2.39, meaning SIERA basically validated the season — no fluke BABIP, no smoke-and-mirrors strand rate. Contrast that with a pitcher running a 3.20 ERA and 4.30 SIERA: that gap signals defense, sequencing, or HR/FB luck propping up the surface number.
Why SIERA Matters
Front offices use SIERA as a regression flag. A pitcher whose ERA sits a run below his SIERA is overperforming his peripherals; expect ERA to climb. The reverse — ERA above SIERA — flags buy-low candidates whose underlying skills haven't caught up to the box score yet. Fantasy and DFS players use SIERA to project second-half ERA more accurately than first-half ERA itself, because SIERA stabilizes faster than ERA (roughly 60-70 innings vs. 200+). Pitching-development groups use SIERA to grade trades and free-agent contracts because it isolates skill from the noise of defensive support.
Limitations and Common Misconceptions
SIERA is not a defense-adjusted ERA — it ignores defense entirely by assuming league-average BABIP on the batted balls it tracks. It cannot capture pitchers who genuinely suppress hard contact (it doesn't see exit velocity), so soft-tossing weak-contact specialists like Kyle Hendricks tend to outperform their SIERA year after year. It also can't model relievers cleanly in low-inning samples; usage and leverage distort the inputs. And SIERA is not predictive of injury or velocity decline — it's a skill snapshot, not a health gauge.
Related Terms
In Legends Deck
Pitcher cards in Legends Deck use SIERA as a primary anchor for the "Stuff Floor" rating — the run-prevention skill that survives regardless of the defense behind the card. A starter with a 2.80 SIERA and a 3.40 ERA grades higher in our simulation than his real-life ERA suggests, because the engine resolves balls in play against league-average fielders. That's why SIERA-darlings often outperform their MLB stat lines once they hit the deck.