A Complete Guide to Baseball Statistics: From Batting Average to WAR

Baseball is the most statistically rich sport in the world. A sport that resets between every pitch — creating thousands of isolated batter-vs-pitcher matchups across a 162-game season — generates a volume and variety of data that no other league can match. For generations, fans debated batting averages and ERAs. Today, front offices evaluate players using measures that didn't exist a decade ago.

This guide covers everything from the most basic counting stats you see on every broadcast to the advanced metrics that determine how MLB teams spend hundreds of millions of dollars. By the end, you'll be able to pick up any stat line and know exactly what it tells you — and, just as importantly, what it doesn't.

Batting Statistics

Batting Average (AVG or BA)

Batting average is the oldest and most recognized baseball statistic: hits divided by at-bats. A hitter who goes 3-for-10 in a series has a .300 average for those games. League-wide batting averages have historically hovered around .260–.270, though the modern game (with its emphasis on strikeouts and home runs) has pushed that closer to .245–.250 in recent seasons.

As a benchmark: a .300 average has historically been considered excellent. A .250 average is roughly league average. Below .220 and a player is likely under significant pressure for their spot in the lineup.

The limitation: Batting average treats all hits equally. A bloop single counts the same as a line-drive home run. It also ignores walks entirely — which is a significant omission, since a walk advances a baserunner just as effectively as a single does.

On-Base Percentage (OBP)

On-base percentage measures how often a batter reaches base by any means: hits, walks, and hit-by-pitches. The formula is (H + BB + HBP) ÷ (AB + BB + HBP + SF). It is the stat that made Billy Beane and the Oakland A's famous after Michael Lewis's book Moneyball highlighted how undervalued it was in the early 2000s.

Why does this matter so much? Because outs are the scarcest resource in baseball. Each team gets 27 outs per nine innings — that's it. Every time a batter reaches base, they deny the opposing pitcher an out. Walks do this just as effectively as singles. A player with a .280 average and a .400 OBP (thanks to a lot of walks) creates more runs than a .310 hitter with a .340 OBP.

Good OBP benchmarks: .350 is roughly average; .370–.380 is good; .400+ is elite. The all-time single-season record is Barry Bonds's .609 in 2004 — the product of an unprecedented 232 walks in 617 plate appearances.

Slugging Percentage (SLG)

Slugging percentage is total bases divided by at-bats. A single is one base, a double is two, a triple is three, a home run is four. A player who hits a home run and a single in five at-bats has a slugging percentage of 1.000 (5 total bases ÷ 5 AB). The league average sits around .400–.420 in most modern seasons.

Slugging captures what batting average misses: the power and extra-base value of hits. A player batting .280 with a .520 slugging percentage is a very different offensive threat from a player batting .280 with a .380 slugging percentage — even though their averages are identical.

OPS (On-Base Plus Slugging)

OPS is simply OBP + SLG. It is the single most widely used advanced batting stat in mainstream broadcasting because it is simple to calculate and strongly correlates with run production. An OPS above .800 is considered good; .900+ is very good; .950+ is elite. The best hitters in the game regularly exceed 1.000 OPS.

OPS+ (Park- and Era-Adjusted OPS)

OPS+ adjusts a player's OPS for the ballpark they play in and the offensive environment of their era, then scales so that 100 is always exactly league average. A player with an OPS+ of 130 was 30% better than the average hitter in their park and era. This makes comparing players across decades much more meaningful — Babe Ruth's 1927 numbers look different once you account for what everyone else was doing that year, and OPS+ handles that automatically.

wOBA (Weighted On-Base Average)

Developed by sabermetrician Tom Tango, wOBA assigns different weights to each way of reaching base based on how much each event actually contributes to run scoring. A walk is worth less than a single (it can't score a runner from second on a hit), a single is worth less than a double, and so on. The weights shift slightly from year to year as run-scoring environments change.

wOBA is more precise than OPS because OPS treats OBP and SLG as equally important when they are not — OBP is actually worth about 1.8 times as much as SLG in terms of run value. A good wOBA is around .340; an elite wOBA is .400+.

wRC+ (Weighted Runs Created Plus)

wRC+ takes the run value captured by wOBA and adjusts it for the player's home park and the offensive environment of the season, then scales to 100 = league average. It is the single best measure of overall offensive performance because it handles all the context adjustments automatically.

A wRC+ of 150 means the player produced 50% more runs than a league-average hitter in a neutral environment. A wRC+ of 80 means they were 20% below average. Because it is park- and era-adjusted, you can compare a hitter at Coors Field (notoriously favorable to offense) to a hitter at Petco Park (notoriously tough) and get a fair comparison.

ISO (Isolated Power)

ISO is slugging minus batting average: SLG − AVG. It strips out the value of singles (already captured by AVG) and measures only extra-base power — doubles, triples, and home runs. A player with an ISO above .200 is considered a legitimate power hitter. Below .100 is weak power. It is a quick way to identify power hitters within a broader stat line.

BABIP (Batting Average on Balls in Play)

BABIP measures how often a batted ball that stays in the field of play (excluding home runs and strikeouts) becomes a hit. The formula is (H − HR) ÷ (AB − K − HR + SF). The league average is typically around .300.

Why does this matter? Because BABIP is influenced heavily by luck and defense — factors outside a hitter's direct control. A batter with a .400 BABIP is likely getting lucky; their average will probably drop. A batter with a .220 BABIP is probably getting unlucky; a rebound is likely. BABIP is one of the best tools for identifying which batting average changes are real versus which are flukes.

Pitching Statistics

ERA (Earned Run Average)

ERA is the number of earned runs a pitcher allows per nine innings pitched. A run is "earned" if it scores without the help of an error. ERA has been the standard pitching metric since the 1800s. League ERA typically runs between 4.00 and 4.50 in modern baseball, somewhat higher than historical rates due to the increased prevalence of home runs.

ERA benchmarks: Under 3.00 is elite. 3.00–3.75 is excellent. 3.75–4.25 is good. 4.25–5.00 is average to below average. Above 5.00 is poor.

WHIP (Walks Plus Hits per Inning Pitched)

WHIP = (Walks + Hits) ÷ Innings Pitched. It measures how many baserunners a pitcher allows per inning. A WHIP of 1.00 means one baserunner per inning, which is very good. League average is typically around 1.30. Below 1.10 is excellent; above 1.50 is poor. WHIP is particularly useful for quick assessment because it is simple to calculate and directly captures how often a pitcher puts himself in trouble.

K/9 and BB/9 (Strikeouts and Walks per Nine Innings)

K/9 and BB/9 normalize a pitcher's strikeout and walk rates to a nine-inning baseline. They are useful for comparing pitchers who throw different numbers of innings. A high K/9 (above 9.0 is excellent, above 12.0 is elite) signals a pitcher who dominates hitters with stuff. A low BB/9 (below 2.5 is good, below 2.0 is excellent) signals strong command.

The K/BB ratio (strikeouts divided by walks) is a single number that captures both: a ratio above 3.0 is good, above 4.0 is excellent, and anything below 2.0 is a warning sign about a pitcher's long-term viability.

FIP (Fielding Independent Pitching)

ERA has a fundamental problem: it credits or punishes pitchers for things their fielders do. A groundball pitcher with a terrible infield will post a worse ERA than their actual pitching warrants. FIP fixes this by estimating what a pitcher's ERA would be if they had league-average defense behind them. It uses only the outcomes a pitcher directly controls: home runs, walks, hit-by-pitches, and strikeouts.

The FIP formula is: ((HR × 13) + ((BB + HBP) × 3) − (K × 2)) ÷ IP, plus a constant (around 3.10) that scales the result to look like ERA. Because FIP is park- and context- independent, it often predicts future ERA better than current ERA does.

xFIP (Expected Fielding Independent Pitching)

xFIP takes FIP one step further by normalizing the home run rate. It replaces actual home runs with the number of home runs the pitcher would be expected to allow based on their fly ball rate and the league-average HR/FB (home run per fly ball) rate. This removes the effect of a pitcher getting lucky or unlucky on fly balls leaving the yard.

A pitcher with a 3.50 ERA but a 2.80 xFIP is pitching better than their ERA shows. A pitcher with a 3.50 ERA and a 4.20 xFIP may be due for some negative regression. xFIP is one of the best indicators of future pitching performance available.

The All-Encompassing Metrics

WAR (Wins Above Replacement)

WAR attempts to answer a single question: how many more wins does this player give his team compared to a freely available replacement-level player (a fringe roster player anyone could sign off the waiver wire)? It accounts for batting, baserunning, fielding, position adjustments, and for pitchers, pitching performance.

There are two main versions — fWAR (from FanGraphs, using FIP for pitchers) and bWAR or rWAR (from Baseball Reference, using RA/9 for pitchers). They usually agree within 0.5–1.0 wins per player per season, with occasional larger disagreements for pitchers with unusual groundball profiles.

WAR's most important use is for comparing players across positions — a shortstop producing 4.0 WAR is more valuable than a first baseman producing 4.0 WAR because shortstop is a much more demanding defensive position, and WAR's positional adjustment accounts for that.

Putting It All Together

Modern player evaluation doesn't rely on any single stat. Instead, analysts triangulate: they use wRC+ or OPS+ for a quick offensive picture, BABIP to flag potential regression, FIP or xFIP alongside ERA to assess pitching sustainability, and WAR as the overall summary. Each stat captures something slightly different.

The best way to develop intuition for these numbers is to use them regularly. Baseball Nerd displays full batting and pitching stat lines for every active MLB player, updated daily from the official MLB Stats API. Check the Stats leaderboards to see where today's best players rank across every category — and check any player's full profile to see how these numbers stack up over their career.