How Innings Pitched Works (.1 and .2)

A score sheet that reads 6.2 innings pitched does not mean 6 and two-tenths of an inning. It means 6 and two-thirds of an inning. The decimal is a shorthand for thirds, not tenths. Under MLB Rule 9.02(b), one out equals one-third of an inning and is written as .1; two outs equal two-thirds and are written as .2. There is never a .3 because three outs complete the inning, the pitcher is credited with a full inning and the partial mark resets. This innings pitched decimal convention is the single most common source of miscalculation when adding innings or computing ERA. If you treat 5.2 as 5.2 rather than 5.667, you undercount the pitcher’s work by nearly half an inning per entry.

6.2 innings means 6 full innings plus 2 outs. Since each out is one-third of an inning, 2 outs equal 2/3, so 6.2 = 6 + 2/3 = 6.667 innings in decimal form. The official scorer records a partial inning only when the pitcher is replaced or the game ends before the side is retired. Under Rule 9.02(a), an inning is complete only when the pitcher retires the side, records the third out. If a pitcher records 1 out before being pulled, that is .1 inning (0.333). If they record 2 outs, that is .2 inning (0.667). A pitcher who retires the side with zero outs gets 0 innings pitched for that appearance.

Decimal Confusion

The most common error is reading 5.2 as 5.2. A scorekeeper who adds 5.2 + 4.0 as 9.2 instead of 9.667 undercounts the pitcher’s total by 0.467 innings. Over a season of 20 partial-inning entries, that is a loss of more than 9 innings, enough to shift an ERA by roughly 0.20 runs. Always convert to outs before adding.

To add innings pitched correctly, convert everything to total outs first. Every full inning is 3 outs. A partial inning of .1 is 1 out; .2 is 2 outs. Add the outs, then divide by 3 to get total innings in decimal form. Use that decimal, not the .1/.2 form, for any ERA calculation.

Worked example: A pitcher has three appearances: 5.2, 3.1, and 7.0 innings.
5.2 = (5 × 3) + 2 = 15 + 2 = 17 outs.
3.1 = (3 × 3) + 1 = 9 + 1 = 10 outs.
7.0 = 7 × 3 = 21 outs.
Total outs = 17 + 10 + 21 = 48 outs.
Total innings = 48 ÷ 3 = 16.000 innings.

Now apply the ERA formula: ERA = (Earned Runs Allowed ÷ Innings Pitched) × 9. If that pitcher allowed 12 earned runs, the calculation is (12 / 16.000) × 9 = 0.75 × 9 = 6.75 ERA.

How This Affects ERA, WHIP, and K/9

The conversion error ripples through every rate stat that uses innings pitched as the denominator. ERA, WHIP (walks per innings pitched), and K/9 all require the correct decimal innings. If you use 5.2 as 5.2 instead of 5.667, your K/9 for that appearance will be overstated by about 8% (since you divide by a smaller number). For WHIP, the error is the same magnitude. For ERA, the error is slightly smaller because the numerator includes earned runs, but the misclassification of a few partial innings over a season can shift a reliever’s ERA by 0.10 to 0.30 runs, enough to move a pitcher from “average” to “above average” on the 2023 MLB scale. The 2025 MLB league-average ERA is 4.33 (source: Baseball-Reference), so a 0.20-run error is material.

A pitcher’s season log shows these entries: 7.0, 5.2, 6.1, 8.0, and 4.2. Convert each to outs.

7.0 = 7 × 3 = 21 outs.
5.2 = (5 × 3) + 2 = 15 + 2 = 17 outs.
6.1 = (6 × 3) + 1 = 18 + 1 = 19 outs.
8.0 = 8 × 3 = 24 outs.
4.2 = (4 × 3) + 2 = 12 + 2 = 14 outs.

Total outs = 21 + 17 + 19 + 24 + 14 = 95 outs.
Total innings = 95 ÷ 3 = 31.667 innings.

Now compute ERA if the pitcher allowed 28 earned runs: (28 / 31.667) × 9 = 0.884 × 9 = 7.96 ERA. That is a high ERA because 28 earned runs in 31.667 innings is poor performance. For context, the 2025 NCAA Division I baseball average ERA is 4.50 (source: NCAA statistics archive), so 7.96 is well below average.

ERA formula: (Earned Runs ÷ Innings Pitched) × 9. If you enter 5.2 as 5.2 instead of 5.667, the denominator is too small and the ERA comes out too low. Over a 200-inning sample, a misrecorded 5.2 entry that should be 5.667 costs 0.467 innings per entry. For a pitcher who has 20 such entries, that is 9.

WHIP formula: (Walks ÷ Innings Pitched) × 9. Same error pattern. A 5.2 recorded as 5.2 inflates WHIP by roughly 9% for that game.

K/9 formula: (Strikeouts ÷ Innings Pitched) × 9. Strikeouts are the component a pitcher most controls, so getting K/9 right matters for projection. A 0.467-inning error per partial game shifts K/9 by the same fraction.

Reliever-Specific Risk

Relievers accumulate more partial-inning appearances than starters. A reliever who enters with inherited runners (Rule 9.16(c)) often records .1 or .2 innings. Misrecording those partials compounds the inherited-runner effect: the reliever’s ERA already looks worse than a starter’s because runs scored by inherited runners are charged to them. Adding a 0.467-inning undercount per entry makes the ERA look even worse. The minimum reliable sample size for a reliever is 30 innings (FanGraphs); at 15 innings with multiple partial misentries, the ERA is essentially noise.

MLB Rule 9.02(b) states that a pitcher is credited with a fraction of an inning for each out they record: 1 out = 1/3 inning, 2 outs = 2/3 inning. Rule 9.02(a) defines that an inning pitched is recorded when the pitcher retires the side (third out) or when the inning ends while the pitcher is still in the game. A pitcher who is replaced after two outs has not retired the side, so they get .2 IP. A pitcher who enters with zero outs and gets the third out receives 1.0 IP for that inning.

This partial-innings system applies across MLB, NCAA (see the NCAA Baseball Statistics Manual), and NFHS (see NFHS Baseball Rules). The rule is identical in substance for softball: a 7-inning softball game uses the same .1/.2 notation. The only difference is the game length, 7 innings instead of 9, but the partial-inning recording method is the same.