Innings pitched measures the number of innings a pitcher remains in a game. Because there are three outs in an inning, each out recorded represents one-third of an inning pitched.
Just because a pitcher appears in a game, doesn't mean he will record an inning pitched or even a third of an inning pitched. In order for a pitcher's IP total to increase, he must be pitching while an out is recorded. (This includes pickoffs and caught stealings, while double plays are worth two-thirds of an inning pitched.) Pitchers are not credited for a third of an inning pitched if a batter reaches on an error because an out was never recorded.
The league leaders in innings pitched are generally some of the best pitchers in the game because they are the most adept at working deep into ballgames. They are also typically pitchers who do a good job of keeping their pitch count low, and they typically do not walk very many hitters. Of course, pitchers must also stay healthy to achieve high innings pitched totals, and younger pitchers typically have a hard time working their way onto the MLB leaderboard because of innings limits or time spent in the Minors.
Watch: Jacob deGrom records 7 2/3 innings pitched against the Orioles.
In A Call
"innings," "innings of work," "frames"