What a batting average leaves out

Batting average is the oldest number most people learn in baseball, and the one most likely to mislead them. It is not wrong: it answers a narrow question precisely. The trouble starts when that answer is read as a general measure of reaching base, because the denominator deliberately leaves things out.

Here is the formula taken apart, the arithmetic done by hand, and the size of the gap shown with a worked example.

The formula, exactly

Batting average is hits divided by at-bats (MLB Glossary — Batting Average). The glossary defines the components separately. A hit is a ball struck into fair territory that lets the batter reach base without an error or a fielder’s choice (MLB Glossary — Hit). An official at-bat comes when a batter reaches base on a fielder’s choice, a hit or an error, or is put out on a non-sacrifice; a plate appearance is each completed turn batting, regardless of the result (MLB Glossary — At-bat). Walks are not at-bats.

That second definition is the whole story. An at-bat is not a trip to the plate; a trip to the plate is a plate appearance, and one that ends in a walk is not an at-bat. A walk puts a runner on first base and leaves a batter’s batting average exactly where it was. The glossary itself notes that batting average does not take into account walks or hit-by-pitches, nor hit type (MLB Glossary — Batting Average).

A worked example

Take a hypothetical batter with a clean, round season line, so the arithmetic stays visible:

• Plate appearances: 600
• Walks: 60
• Hits: 160
• At-bats: 540

The at-bat figure is not independent; it is what remains after the walk is removed from the plate appearance. Batting average is 160 ÷ 540 = .296. Now compare with the measure that keeps the walk in the denominator. On-base percentage credits the batter for hits and walks — and for being hit by a pitch, which we set at zero here to keep it simple — over plate appearances:

• Numerator: 160 hits + 60 walks = 220
• Denominator: 600 plate appearances
• On-base percentage: 220 ÷ 600 = .367

The same season reads as .296 by one measure and .367 by another. Neither number is a mistake. They are answers to different questions. The gap between the rounded figures is 71 points (70.4 before rounding), and it comes from the 60 walks: they are absent from batting average but sit in both the numerator and the denominator of on-base percentage.

Why the gap is not a rounding issue

The gap is not a decimal-place curiosity; it is structural, and its size depends on how often a batter walks. A patient hitter who walks in one plate appearance out of ten loses a tenth of his plate appearances from the batting-average denominator. A free swinger who almost never walks loses almost none.

Here is the effect with numbers. Take two invented hitters, each with 600 plate appearances, no hit-by-pitches and no sacrifices, and each reaching base 220 times:

Both reached base exactly as often, and their on-base percentages are identical, yet their batting averages are 64 points apart. The batting averages differ because the patient hitter’s walks left his at-bat count and the free swinger’s hits filled his. The comparison depends on the mix of walks and hits, not on hitting skill: it shows that a batting average cannot be read as a measure of reaching base.

What each outcome is worth to each measure

The table below is our summary of the definitions on the source pages (the On-base Percentage page counts hits, walks and hit-by-pitches as times on base), not from any season’s data. It ignores sacrifices and special cases such as errors. It records how a single outcome of a plate appearance is treated by three common measures. Read it as a map of what counts where.

Two rows carry the argument. A walk never appears in a batting average, in either the numerator or the denominator, but it counts as a time on base in on-base percentage. A strikeout is an at-bat without a hit, so it lowers batting average, and it also counts against on-base percentage; a walk helps one and does nothing to the other.

Where slugging fits

Slugging percentage uses the same denominator as batting average — at-bats — but weights each hit by the number of bases it produced. A double counts twice, a triple three times, a home run four times. This makes slugging a measure of power rather than of contact, and it inherits the same blind spot: walks do not count.

The common fix in modern stat lines is to add the two together as on-base plus slugging. That sum keeps every component visible, but it mixes a ratio with a plate-appearance denominator and a ratio with an at-bat denominator, so the total is not a single proportion of anything.

How to use this when you read a stat line

Three habits we suggest:

1. Check the denominator before you compare. If two numbers are called “percentage,” ask what is on the bottom. A difference in denominator is a difference in question.
2. Avoid ranking batters by average alone. A .300 average and a .360 on-base percentage describe two different skills. Both are in the line for a reason.
3. Look at plate appearances, not at-bats, when you want volume. At-bats undercount how often a batter came to the plate, and the shortfall is exactly the events that batting average ignores.

What this measure is still good for

None of the above makes batting average useless. It is a well-defined record of one specific ability: getting a hit when a hit was the outcome available. It is stable and it is published everywhere. The error is not in the number — it is in reading a narrow number as a broad verdict.

Method

Both hitters and the season line are invented, and every ratio in the tables was recomputed with a script. We opened the seven glossary entries below on 2026-10-31 to check the definitions.

Sources

• MLB Glossary — Batting Average (AVG) — the hits-over-at-bats definition.
• MLB Glossary — At-bat (AB) — what is excluded from an at-bat, including walks and sacrifices.
• MLB Glossary — Plate Appearance (PA) — the wider unit a walk does count toward.
• MLB Glossary — Walk (BB) — the outcome at the centre of the gap.
• MLB Glossary — Hit (H) — what counts as a hit: fair territory, no error or fielder’s choice.
• MLB Glossary — On-base Percentage (OBP) — the formula that keeps walks in the denominator.
• MLB Glossary — Slugging Percentage (SLG) — base-weighted hits over at-bats.