9.21 Determining Percentage Records
(a) Percentage of games won and lost, divide the number of games won by the sum of games won and games lost;
(b) Batting average, divide the total number of safe hits (not the total bases on hits) by the total times at bat, as defined in Rule 9.02(a) (Rule 10.02(a));
(c) Slugging percentage, divide the total bases of all safe hits by the total times at bat, as defined in Rule 9.02(a) (Rule 10.02(a));
(d) Fielding average, divide the sum of putouts and assists by the sum of putouts, assists and errors (which shall be called chances);
(e) Pitcher’s earned-run average, multiply the total earned runs charged against such pitcher by 9, and divide the result by the total number of innings he pitched, including fractions of an inning; and
Rule 9.21(e) Comment: For example, 9 1⁄3 innings pitched and 3 earned runs is an earned-run average of 2.89 (3 earned runs times 9 divided by 9 1⁄3 equals 2.89).
(f ) On-base percentage, divide the sum of hits, bases on balls and times hit by pitch by the sum of at-bats, bases on balls, times hit by pitch and sacrifice flies.
Rule 9.21(f) Comment: For the purpose of computing on-base percentage, ignore instances of a batter being awarded first base on interference or obstruction.