What is HR/9?
Home runs allowed per nine innings, abbreviated HR/9, is a standard baseball pitching statistic. It measures how many home runs a pitcher gives up for every nine innings pitched, putting pitchers with different workloads on the same scale. A lower HR/9 is better. This is a universal baseball metric — the math is identical in every league, with no region-specific rules.
How to use this calculator
Enter the whole innings pitched, choose any partial inning recorded in outs (each out is one third of an inning), and enter the number of home runs allowed. The calculator converts the partial inning to a decimal, sums the total innings, and computes the rate. The result is shown rounded to two decimal places (the third decimal digit is rounded half-up), matching how HR/9 is typically displayed.
The formula explained
$$\text{HR/9} = \frac{\text{Home Runs Allowed}}{\text{Innings Pitched} + \text{Partial Inning}} \times 9$$ Innings pitched is recorded in thirds because a half-inning ends after three outs: one out is \(\frac{1}{3}\), two outs are \(\frac{2}{3}\). So 7 full innings plus two outs equals 7.667 innings. Dividing home runs by innings gives a per-inning rate, and multiplying by nine projects it across a complete game.
Worked example
Suppose a pitcher throws 15 innings and allows 2 home runs. $$\text{HR/9} = \frac{2}{15} \times 9 = \frac{18}{15} = 1.20$$ With a partial inning — 7 innings and two outs (7.667 IP) and 3 home runs — $$\text{HR/9} = \frac{3}{7.667} \times 9 = \frac{27}{7.667} = 3.52$$
FAQ
What is a good HR/9? As a rough guide, a value under about 1.0 is excellent, around 1.0 to 1.3 is league-average, and above 1.5 is high. Standards vary by era and ballpark.
Why count partial innings in thirds? A pitcher can leave the mound mid-inning. Each out they record is credited as one third of an inning, so the rate reflects exactly how long they pitched.
What if innings pitched is zero? HR/9 is undefined because you cannot divide by zero. Enter at least a partial inning to get a result.
