Relief Pitcher Value Estimator: FIP, xFIP, and Save Opportunity Conversion
Evaluate a relief pitcher's true performance using Fielding Independent Pitching (FIP), Expected FIP (xFIP), and Save Opportunity Conversion Rate. These metrics strip out defense and luck to reveal a pitcher's actual skill level.
Pitching Basics
FIP Components
xFIP Components
Save Opportunities
Formulas Used
ERA: ERA = (ER / IP) × 9
FIP (Fielding Independent Pitching):
FIP = ((13 × HR) + (3 × (BB + HBP)) − (2 × K)) / IP + cFIP
where cFIP is the league FIP constant (typically ~3.10, adjusted annually to align FIP with ERA scale).
xFIP (Expected FIP):
xFIP = ((13 × (FB × lgHR/FB%)) + (3 × (BB + HBP)) − (2 × K)) / IP + cFIP
Replaces actual HR with expected HR based on the pitcher's fly ball rate and the league-average HR/FB rate (~10–11%).
Save Conversion Rate: SV% = (SV / SVO) × 100
Blown Save Rate: BS% = (BS / SVO) × 100
K/9: (K / IP) × 9 | BB/9: (BB / IP) × 9 | HR/9: (HR / IP) × 9
K/BB Ratio: K / BB
Note: IP is converted from baseball notation (e.g. 65.1 = 65⅓ innings) to decimal form for calculations.
Assumptions & References
- FIP weights are derived from linear weights research: HR = 13, (BB+HBP) = 3, K = −2 (Tom Tango, MGL, Dolphin — "The Book").
- The FIP constant (cFIP) is recalculated each season by MLB to align the FIP scale with ERA. A typical value is ~3.10; use the current season's published constant for accuracy.
- xFIP normalizes HR allowed using the league-average HR/FB rate (~10–11% in recent MLB seasons), removing short-term HR variance from the pitcher's control.
- FIP and xFIP are park- and defense-neutral metrics — they measure only what the pitcher directly controls: strikeouts, walks, hit batters, and home runs.
- A FIP significantly lower than ERA suggests the pitcher may be unlucky or facing poor defense; ERA is expected to decline. The reverse suggests possible regression upward.
- Elite closer save conversion rates are generally ≥ 85–90%; league average is approximately 80–85%.
- References: FanGraphs Sabermetrics Library (fangraphs.com/library), Tom Tango et al. "The Book: Playing the Percentages in Baseball" (2006), Baseball-Reference Glossary.