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To compare starting pitchers, use ERA and WHIP to describe what happened, then add advanced stats to examine the components behind those results. ERA measures earned runs per nine innings; WHIP measures walks and hits per inning. FIP, xFIP, SIERA and xERA each add a different lens, so the best comparison uses the same time span, includes innings pitched and treats disagreements as clues to investigate—not as a verdict from one number.
Choose the stat that answers your question
These measures overlap, but they are not interchangeable. Start with the question you want to answer, then compare the relevant stats over the same season or other comparable period.
| Stat | What it measures | Useful question |
|---|---|---|
| ERA | Earned runs allowed per nine innings | How many earned runs did the pitcher allow? |
| WHIP | Walks and hits allowed per inning | How often did the pitcher allow a walk or hit? |
| FIP | Results from home runs, walks, hit batters and strikeouts, with a season-calibrated constant | How did the pitcher perform on components that do not depend on balls-in-play outcomes? |
| xFIP | FIP with home runs estimated using the season’s league-average home-run-per-fly-ball rate | How does the comparison look if an unusually high or low home-run rate is smoothed toward the league rate? |
| SIERA | A run-prevention estimator that accounts for strikeouts, walks and batted-ball types | How does the pitcher’s batted-ball profile affect the estimate? |
| xERA | Expected wOBA translated to the ERA scale, including contact quality | What does Statcast’s expected performance suggest when contact quality is included? |
The definitions and formulas below come from MLB’s ERA, WHIP, FIP, xFIP, SIERA and xERA glossaries.
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Read ERA and WHIP as results, not full explanations
ERA: earned runs per nine innings
ERA is calculated as 9 × earned runs ÷ innings pitched. It summarizes earned-run outcomes, but does not isolate the pitcher from every factor around him. MLB notes that defense, league context and ballpark can affect ERA comparisons. Runs scored by runners a pitcher leaves on base may still be charged to him.
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WHIP: walks and hits per inning
WHIP is (walks + hits) ÷ innings pitched. Hit batters, errors and fielder’s choices are excluded. WHIP tells you how frequently a pitcher allowed baserunners by walk or hit; it does not say whether those runners later scored.
What FIP, xFIP, SIERA and xERA add
FIP: focus on selected pitcher-controlled outcomes
MLB’s FIP formula is ((13 × HR) + (3 × (BB + HBP)) − (2 × K)) ÷ IP + a season-calibrated constant. It uses home runs, walks, hit batters and strikeouts, leaving outcomes on balls in play out of the calculation. The constant places the result on a scale comparable to league ERA.
xFIP: adjust the home-run assumption
xFIP replaces actual home runs with an estimate based on the season’s league-average home-run-per-fly-ball rate. This can temper an unusually high or low home-run rate, but it is an assumption—not a correction that proves what the pitcher’s future home-run rate will be.
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SIERA incorporates batted-ball types alongside strikeouts and walks. The type of contact matters: MLB’s example notes that a pitcher generating many grounders and popups rather than line drives can have a lower SIERA than xFIP.
xERA: include contact quality
xERA converts expected wOBA to the ERA scale. Its expected wOBA inputs include strikeouts, walks, hit batters and contact quality, such as exit velocity and launch angle. MLB cautions that xERA is not necessarily predictive, so treat it as another description of performance rather than a forecast.
Compare two starters step by step
- Set the comparison period. Use the same season or equivalent time spans for both pitchers; do not compare one pitcher’s full season with another’s brief stretch without labeling the difference.
- Check innings pitched. Rate stats can move substantially in a short sample. There is no universal minimum innings threshold established by these definitions, so avoid treating an arbitrary cutoff as a rule.
- Compare ERA and WHIP. These show earned-run outcomes and walk-plus-hit frequency, respectively.
- Add FIP for component performance. It can help distinguish the listed strikeout, walk, hit-batter and home-run outcomes from results on balls in play.
- Add the specialized measures only when useful. Use xFIP to examine the home-run-rate assumption, SIERA for batted-ball profile, and xERA for expected contact quality.
- Keep context visible. Note league, park and defense when they differ. The definitions do not establish that every listed metric is adjusted for all of these conditions.
- Investigate gaps rather than declaring a winner by one stat. A lower ERA than FIP or xERA is a reason to ask what differs in the outcomes or context; it is not proof of luck or of future regression.
How to interpret disagreement between stats
If one pitcher has a better ERA but a worse FIP, start by remembering that ERA includes earned-run outcomes while FIP uses selected events and excludes balls-in-play outcomes. Differences may invite a closer look at defense, park, league context and the specific events in the comparison period. They do not, on their own, establish a cause.
If xFIP differs from FIP, the home-run estimate is a key distinction: xFIP substitutes a league-average home-run-per-fly-ball assumption for actual home runs. If SIERA differs from xFIP, batted-ball types may help explain why. If xERA diverges from either, its inclusion of contact quality provides another perspective. No one of these differences proves which estimate is correct or predicts what will happen next.
Use comparison tools without mistaking rank for explanation
Baseball Savant’s Player Comparison Tool lets readers compare players or multiple years of one player using Statcast percentiles, and choose and order statistics. Percentiles show relative standing; they do not by themselves explain why a pitcher ranks where he does. Pair the display with the underlying metric definitions and the same-period, innings and context checks above.
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