Hong Life CalculatorsUnderstanding the ERA, WHIP, K/9 and FIP calculator — concept and principle
These pitching statistics summarise how well a pitcher prevents runs and baserunners. ERA counts earned runs per nine innings, WHIP counts walks and hits per inning, K/9 and BB/9 show strikeout and walk rates, and FIP (fielding independent pitching) estimates run prevention from only the outcomes a pitcher controls most directly.
How it is calculated
First the innings are converted: baseball writes one out as .1 and two outs as .2, so 180.1 means 180⅓ innings. The calculator uses IP = whole innings + (decimal digit)/3.
- ERA = 9 × ER / IP.
- WHIP = (BB + H) / IP.
- K/9 = 9 × SO / IP and BB/9 = 9 × BB / IP.
- FIP = (13 × HR + 3 × (BB + HBP) − 2 × SO) / IP + constant.
The FIP constant (around 3.1–3.2 here) is set each season so that league-average FIP equals league-average ERA.
The principle
Nine innings is a regulation game, so per-nine rates express results "per game". ERA uses earned runs to remove runs caused by errors, but it still depends on defence, ballpark and luck on balls in play. FIP, developed by Tom Tango, keeps only home runs, walks, hit batters and strikeouts. The coefficients are roughly nine times each event's run value compared with a ball in play, rounded, so the result sits on the same per-nine scale as ERA. Because those events are much more stable from year to year than hits on balls in play, FIP tends to predict future ERA better than ERA itself.
Worked example
- Inputs: 60 ER, 180.1 IP, 160 H, 50 BB, 200 SO, 20 HR, 5 HBP, constant 3.15.
- IP = 180 + 1/3 = 180.333.
- ERA = 9 × 60 / 180.333 = 2.99; WHIP = 210 / 180.333 = 1.165.
- K/9 = 1800 / 180.333 = 9.98; BB/9 = 450 / 180.333 = 2.50.
- FIP = (260 + 165 − 400) / 180.333 + 3.15 = 0.139 + 3.15 = 3.29.
| Innings pitched (e.g. 180.1 = 180⅓) | ERA |
|---|---|
| 45 | 12 |
| 90 | 6 |
| 135 | 3.990147783 |
| 180 | 2.994454713 |
| 225 | 2.396449704 |
| 270 | 1.997533909 |
| 360 | 1.497227357 |
How to read the result
Recent MLB league averages have been roughly 4.0–4.3 for ERA and about 1.3 for WHIP, so an ERA under 3.00 and a WHIP under 1.20 are excellent. A K/9 near 10 is a strikeout pitcher. When ERA is much lower than FIP, as here (2.99 vs 3.29), the pitcher may have benefited from defence or luck, and ERA is likely to rise toward FIP.
Common mistakes
- Treating 180.1 as 180.1 innings instead of 180⅓.
- Including unearned runs in ERA.
- Using one season's FIP constant for another league or era.
Frequently asked questions
Why are the FIP weights 13, 3 and 2?
They are rounded run values of home runs, walks/HBP and strikeouts relative to balls in play, multiplied by about nine.
What is xFIP?
A variant that replaces actual home runs with an expected number based on fly balls and the league home-run rate.
Does the FIP formula include intentional walks?
Some versions subtract IBB from BB; this calculator uses total walks.
Sources: Official Baseball Rules, Office of the Commissioner of Baseball (earned runs and percentage records) · FanGraphs Library, FIP; FanGraphs Guts! (season constants)