Pitching Stats Glossary: ERA, WHIP, FIP, K/9 & More

July 15, 2026

Box scores throw a lot of pitching abbreviations at once. Here’s what the most common ones mean, in plain English, alongside the stats the ERA calculator computes directly.

ERA (Earned Run Average)

The average number of earned runs a pitcher allows per standard game, calculated as (Earned Runs ÷ Innings Pitched) × Game Length. Lower is better. It’s the most widely cited pitching stat, but it only counts runs charged as earned — runs that scored because of a fielding error or passed ball don’t count against ERA. See earned run vs. unearned run below.

WHIP (Walks + Hits per Inning Pitched)

Baserunners allowed per inning, calculated as (Walks + Hits) ÷ Innings Pitched. WHIP and ERA are companion stats — a pitcher can post a good ERA with a mediocre WHIP by stranding baserunners effectively (or with help from good defense), or the reverse. The ERA calculator computes both from the same outing.

Earned Run vs. Unearned Run

An earned run is a run that scored without the help of a defensive error or passed ball — it’s charged to the pitcher as a run they were fully responsible for allowing. An unearned run scored because of a fielding mistake (an error, or in some cases a passed ball) that extended the inning or advanced a runner beyond what clean defense would have allowed. Only earned runs count toward ERA, which is why a pitcher can allow, say, 5 total runs in an inning but have only 3 of them count toward their ERA if the other 2 scored following an error.

Innings Pitched (IP) Notation

Box scores write innings pitched as, for example, “6.1” — this is not a decimal. The number after the period is outs recorded in the partial inning (out of 3 possible): .1 = 1 out (⅓ inning), .2 = 2 outs (⅔ inning). See common ERA calculation mistakes for how often this gets misread as a literal decimal.

FIP (Fielding Independent Pitching)

A pitching metric designed to strip out the effect of team defense and luck on balls in play, using only the outcomes a pitcher directly controls — home runs, walks, hit-by-pitches, and strikeouts. FIP is scaled to look like ERA (same rough numeric range) but can diverge from actual ERA when a pitcher’s defense or luck on batted balls is unusually good or bad in a given season. It’s a more predictive, less noisy stat than ERA over a small sample, though ERA remains the standard for measuring runs actually allowed.

K/9 and BB/9

K/9 (strikeouts per 9 innings) and BB/9 (walks per 9 innings) measure a pitcher’s strikeout and walk rates independent of how many innings they’ve thrown, making it possible to compare a starter’s rate stats against a reliever’s on an even footing. Both are calculated the same way ERA is scaled — (total ÷ innings pitched) × 9 — just with strikeouts or walks in place of earned runs.

Quality Start

An unofficial but widely tracked benchmark: a starting pitcher throws at least 6 innings and allows 3 or fewer earned runs. It’s a quick way to flag a “good enough” outing without needing the final ERA math, though it’s a cruder measure than ERA itself since it doesn’t distinguish between allowing exactly 3 earned runs in 6 innings (4.50 ERA-pace) and allowing 0 in 9 (0.00 ERA-pace) — both qualify as a “quality start.”

Save / Hold

Relief-pitcher-specific stats tracking games finished with a lead protected (save, under specific inherited-lead-size rules) or a lead protected without finishing the game (hold). Neither factors directly into ERA or WHIP, but both are commonly tracked alongside them for relief pitchers specifically.

Ready to calculate ERA and WHIP for a real outing? Use the ERA calculator, or see ERA calculator examples from Little League to MLB for the math applied to real innings and earned-run totals.