ERA+ (ERA-plus) adjusts a pitcher's ERA relative to the league average and ballpark factors. The formula is ERA+ = 100 × (League ERA ÷ Pitcher ERA). An ERA+ of 100 is exactly average, above 100 means the pitcher is better than average, and below 100 means worse. A pitcher with an ERA+ of 150 is 50% better than the league average.

What is K/9 in baseball pitching?

K/9 (Strikeouts per 9 innings) measures a pitcher's strikeout rate normalized to a nine-inning game. The formula is K/9 = (Strikeouts ÷ Innings Pitched) × 9. A K/9 of 10+ is considered excellent in modern baseball. Top power pitchers can reach 12–14 K/9.

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Earned runs

Innings pitched

Outs pitched

ERA

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Innings in a game

Formulas Quick Reference

ERA

(ER ÷ IP) × 9

ERA+

100 × (lgERA ÷ ERA)

WHIP

(BB + H) ÷ IP

K/9 & BB/9

(K or BB ÷ IP) × 9

What is ERA in Baseball?

ERA (Earned Run Average) is the most widely used statistic to measure a baseball pitcher's effectiveness. It represents the average number of earned runs a pitcher allows per nine innings pitched. Earned runs are runs scored that can be attributed to the pitcher — they exclude runs that scored due to fielding errors by the defense.

First introduced in the late 19th century, ERA became the official standard pitching statistic in Major League Baseball in 1912. It remains the go-to metric for comparing pitchers across teams, seasons, and even eras of the game.

ERA Formula & Worked Example

The ERA formula is straightforward:

ERA = (Earned Runs ÷ Innings Pitched) × 9

Worked example: A pitcher throws 7.1 innings (7⅓ innings = 7.333 decimal), allowing 3 earned runs.

Decimal IP = 7 + (1 ÷ 3) = 7.333
ERA = (3 ÷ 7.333) × 9 = 0.4091 × 9 = 3.68

The result, 3.68, is a solidly average ERA in the modern run environment.

Innings Pitched (IP) Format

Baseball records innings pitched in a unique format: X.Y, where Y is the number of additional outs (0, 1, or 2). To convert to decimal innings, use: IP_decimal = X + (Y ÷ 3). So 6.2 IP = 6.667 decimal innings.

ERA Rating Benchmarks

ERA Range	Rating	Description
< 2.50	Elite	Cy Young-level, top 1–2% of starters
2.50 – 3.50	Excellent	All-Star caliber, ace-of-rotation quality
3.50 – 4.50	Average	Solid MLB starter, league average range
> 4.50	Poor	Below average, rotation risk or bullpen arm

What is ERA+?

ERA+ (ERA-plus) normalizes a pitcher's ERA to the league average and accounts for ballpark factors. The formula is:

ERA+ = 100 × (League ERA ÷ Pitcher ERA)

An ERA+ of 100 is exactly league average. Above 100 means better than average — a pitcher with ERA+ 150 is 50% better than the league average. Below 100 means worse. This makes ERA+ a great cross-era comparison tool: Clayton Kershaw's best ERA+ was 196 (2014), meaning he was nearly twice as good as the average pitcher that season.

WHIP Explained

WHIP (Walks plus Hits per Inning Pitched) = (BB + H) ÷ IP. It measures a pitcher's ability to prevent base runners. Unlike ERA, WHIP counts all base runners allowed (including unearned situations resulting from hits/walks), giving a cleaner picture of command and contact-prevention. A WHIP below 1.00 is elite; the best single-season WHIPs in history are around 0.78–0.83.

K/9 and BB/9 Explained

K/9 (Strikeouts per 9 innings) is a rate stat for strikeout ability. Modern power starters average 9–11 K/9. BB/9 (Walks per 9 innings) measures command — elite starters are at or below 2.0 BB/9. A high K/BB ratio (K÷BB ≥ 4.0) strongly correlates with future success, as it signals a pitcher who misses bats while throwing strikes.

Frequently Asked Questions

What is a good ERA in baseball?

A good ERA in baseball is generally below 3.50. An ERA under 2.50 is considered elite (Cy Young-level), 2.50–3.50 is excellent, 3.50–4.50 is league average, and above 4.50 is below average. The league ERA fluctuates year to year — in a high-run era, a 4.20 ERA may be above average, while in a pitcher-dominant era the same ERA could be below average. ERA+ accounts for this by comparing the pitcher to the current league average.

How is ERA calculated?

ERA is calculated with the formula ERA = (Earned Runs ÷ Innings Pitched) × 9. The multiplication by 9 normalizes the stat to a full nine-inning game. Only earned runs count — runs that score because of fielding errors are unearned and excluded. Innings pitched is expressed in thirds: 7.2 IP means 7 full innings and 2 additional outs (7.667 decimal).

What is ERA+?

ERA+ adjusts a pitcher's ERA relative to the league average and park factor. ERA+ = 100 × (League ERA ÷ Pitcher ERA). An ERA+ of 100 is exactly average. A score above 100 means the pitcher performed better than average — e.g., an ERA+ of 135 means the pitcher was 35% better than average. It is one of the best ways to compare pitchers from different eras or different ballparks.

What is WHIP in baseball?

WHIP stands for Walks plus Hits per Inning Pitched. The formula is WHIP = (Walks + Hits) ÷ Innings Pitched. It measures how many baserunners a pitcher allows per inning. A WHIP below 1.00 is elite, 1.00–1.20 is excellent, 1.20–1.40 is average, and above 1.40 is poor. A WHIP of 1.00 means on average exactly one base runner reaches per inning — the pitcher is allowing the minimum to score.

What is K/9 in baseball?

K/9 (Strikeouts per 9 innings) measures a pitcher's strikeout rate on a per-nine-innings basis. Formula: K/9 = (Strikeouts ÷ Innings Pitched) × 9. The modern MLB average for starters is around 8–9 K/9. A K/9 of 10+ is excellent, and elite power pitchers like Jacob deGrom or Gerrit Cole have posted seasons above 13 K/9. K/9 is widely used in fantasy baseball to evaluate starting pitchers.