What is HR/9? Definition, Formula, and Example
HR/9 (home runs allowed per nine innings) is a rate stat that measures how often a pitcher gives up home runs, normalized to a full game's workload — calculated as (HR ÷ IP) × 9.
Plain-English Definition
HR/9 is the rate at which a pitcher allows home runs, scaled to a standard nine-inning game. It strips out the noise of unequal innings totals — a starter throwing 200 innings and a reliever throwing 60 can be compared apples-to-apples. League average sits around 1.10–1.20 HR/9 in the modern juiced-ball era. Elite is under 0.90. Above 1.50 is a serious red flag, and anything over 2.00 across a full season usually ends with a demotion or a position-player call to the mound.
How HR/9 Is Calculated
The formula is simple:
HR/9 = (HR ÷ IP) × 9
Where HR is total home runs allowed and IP is innings pitched (with fractional innings counted as thirds — 100.1 IP = 100⅓, 100.2 IP = 100⅔).
Example: a pitcher who throws 180 innings and gives up 22 home runs has an HR/9 of (22 ÷ 180) × 9 = 1.10.
A relief pitcher with 65 IP and 5 HR allowed: (5 ÷ 65) × 9 = 0.69 — excellent.
HR/9 is a core input into FIP, where home runs get weighted at 13 — by far the heaviest penalty in the formula, reflecting how much damage one swing can do.
Worked Example
In 2024, Tarik Skubal won the AL Cy Young with a 0.94 HR/9 across 192 innings — only 20 home runs allowed despite working in a hitter-friendly Comerica run environment. His combination of upper-90s velocity, a wipeout changeup, and a sharp slider kept barrels off the ball.
Compare that to Lance Lynn's 2023 season with the White Sox: 119.2 IP, 30 HR allowed, for an HR/9 of 2.25 — one of the worst marks among qualified starters that decade. His fastball played up at the top of the zone but flattened into a launching pad when he missed location, and his ERA ballooned to 6.47 before a midseason trade.
Reliever example: Emmanuel Clase has run HR/9 under 0.50 in multiple seasons — his cutter generates so much weak contact that hitters rarely square it up, let alone elevate it.
Why HR/9 Matters
For front offices and pitching coaches, HR/9 is a leading indicator of regression. Pitchers running unsustainably low HR/FB rates (the percentage of fly balls that leave the park) often see HR/9 spike the following year. For fantasy and DFS, HR/9 is a primary ratio category — a high-HR/9 pitcher tanks ERA and WHIP simultaneously, since every homer is an automatic run plus a baserunner that already crossed.
For Cy Young modeling, HR/9 carries more weight in FIP than K/9 or BB/9 because home runs are the single most damaging outcome a pitcher can allow.
Limitations and Misconceptions
HR/9 is heavily park-dependent. A pitcher at Coors Field, Great American Ball Park, or Yankee Stadium will run a structurally higher HR/9 than the same pitcher at Petco, Oracle, or Tropicana. Always check park factors before drawing conclusions.
It's also noisy in small samples. A starter with 80 IP and 12 HR (1.35 HR/9) might be unlucky on a few cheap homers — look at hard-hit rate and barrel rate for the underlying signal.
Finally, HR/9 doesn't account for runners on base. A solo shot and a grand slam count identically in HR/9 — but the grand slam costs three extra runs in ERA.
Related Terms
In Legends Deck
Every pitcher card in Legends Deck carries an HR/9 attribute that drives simulated home run probability per plate appearance, modulated by the opposing hitter's barrel rate and the ballpark's park factor. A Skubal card facing a low-barrel hitter at Oracle Park will surrender homers far less often than a Lance Lynn card facing a Judge or Ohtani at Yankee Stadium — exactly the matchup math front offices run every game.