In a box score, 5.1 innings pitched is not five-and-a-tenth innings — it's five innings plus one out, which is 5⅓ innings. The digit after the point counts outs recorded (0, 1, or 2), each worth a third of an inning, per the MLB glossary entry for innings pitched. Treating that digit as a normal decimal is the single most common error in do-it-yourself pitching math.
How the notation works
Every completed inning is three outs, so partial innings advance in thirds:
- .0 — no outs in the current inning
- .1 — one out recorded (⅓ of an inning)
- .2 — two outs recorded (⅔ of an inning)
There is no .3 — the third out completes the inning and rolls the total to the next whole number. The conversion to real numbers is:
True innings = Whole innings + (Outs digit ÷ 3)
Total outs = (Whole innings × 3) + Outs digit
So a season line of 182.1 IP is 182 + 1/3 ≈ 182.333 true innings, or 182 × 3 + 1 = 547 outs. The Innings Pitched Calculator does this conversion in both directions of the formula — true decimal innings and total outs — from any box-score IP value.
Why the mistake actually matters
Feed the raw notation into any formula that divides by innings and the answer comes out wrong — always biased against the pitcher, because 6.1 read as a decimal understates 6⅓ innings of work.
Worked example: a starter allows 3 earned runs over 6.1 innings pitched.
- Wrong (notation as decimal): (3 ÷ 6.1) × 9 = 4.43 ERA
- Right (6⅓ = 6.333 innings): (3 ÷ 6.333) × 9 = 4.26 ERA
That's a 0.17 ERA error from a single start's arithmetic, and the gap compounds in every per-inning stat: WHIP, K/9, and BB/9 all divide by innings pitched. SportStatNow's pitching calculators accept the box-score notation directly and convert it internally, so 6.1 means what the box score meant.
Converting outs back to notation
Scorekeepers work the other direction: total the outs, divide by 3, and the remainder becomes the digit. A pitcher credited with 29 outs has 29 ÷ 3 = 9 innings with remainder 2 — written 9.2 IP. This is also why innings pitched can be summed across a staff cleanly in outs but not in raw notation: 5.2 + 3.2 is not 8.4, it's 17 outs + 11 outs = 28 outs = 9.1 IP.
The 0.0 IP edge case
A pitcher who enters the game, faces batters, and records no outs before being pulled is credited with 0.0 innings pitched. His runs allowed still count, but there's no denominator to divide them over — which is why a blown outing with no outs shows an "infinite" or blank ERA in the next day's stats. The notation isn't broken there; the rate stat just has nothing to stand on until an out is recorded.