Baseball Batting Average & ERA

How Batting Average and ERA Are Calculated

Batting average and ERA are two of baseball’s most common statistics. Batting average measures how often a hitter reaches base safely; ERA measures how many earned runs a pitcher allows per 9 innings.

Key Factors to Consider

• A small sample size makes both stats noisy early in a season. A batter’s average can swing dramatically over just a handful of at-bats, and a pitcher’s ERA can look unusually good or bad over only a few innings — both stats become more meaningful and stable as the sample of at-bats/innings pitched grows over a season.
• Batting average doesn’t account for walks or extra-base power. A player who draws many walks (which don’t count as at-bats) or hits for more extra-base power isn’t fully reflected in batting average alone — modern analysis often pairs it with on-base percentage and slugging percentage for a fuller picture of offensive value.
• Unearned runs still count against a team, just not against the pitcher’s ERA. ERA specifically excludes runs that scored because of a fielding error or passed ball — a pitcher can have a strong ERA in an inning where the team still allowed runs to score due to a defensive mistake.
• Partial innings are recorded in thirds, not decimals, in official innings-pitched notation. “6.1” innings pitched conventionally means 6 and 1/3 innings (one out into the 7th), and “6.2” means 6 and 2/3 innings — not six-point-one or six-point-two innings in decimal terms. Convert to a true decimal (6.333 or 6.667) before using the ERA formula for an accurate result.

Interpreting Your Results

Both stats only mean something in the context of what’s typical. A batting average in the neighborhood of .250 is roughly league average in modern Major League Baseball; .300 or higher across a full season is widely considered a strong hitting year, and only a handful of hitters in any given season finish above .330. On the pitching side, a full-season ERA around 4.00 is close to league average, the high-2.00s to low-3.00s is typically considered very good, and an ERA below 2.50 over a meaningful number of innings is usually an ace-caliber, Cy-Young-contention season. These benchmarks shift some from year to year and era to era (offense-heavy seasons push both numbers up league-wide), so treat them as a rough compass rather than a fixed line — the same .275 average might be excellent in a low-offense season and merely solid in a high-offense one.

Useful to Know

Both batting average and ERA ultimately rest on a judgment call, not just arithmetic: the official scorer decides whether a batted ball that reaches base was a hit or a fielding error, and that single ruling changes both a hitter’s at-bat/hit count and, for the pitcher, whether any run that later scores counts as earned. Two scorers can occasionally rule the exact same play differently, and a ruling can even be changed on review days later — which is one reason batting average and ERA aren’t a perfectly precise, dispute-free measurement the way a runner’s race time is, even though the formulas themselves are exact.

The Formula

$$\text{AVG} = \frac{\text{Hits}}{\text{At-Bats}} \qquad \text{ERA} = \frac{\text{Earned Runs}}{\text{Innings Pitched}} \times 9$$

Worked Example

1. Batting average: $65 \div 200 = .325$
2. ERA: $(30 \div 78) \times 9 = 3.46$