A starting pitcher must pitch a minimum of five innings (of a full nine-inning game) in order to be awarded a win.
Of course, there are a few other bits to the formula to qualify the starter for a win:
- His team must win the game.
- If he leaves the game after five innings, his team must be leading and must not fall behind in subsequent innings. The exception is if the pitcher is lifted for a pinch runner or pinch hitter and his team takes the lead in that half of the inning (i.e., before another pitcher takes the mound for his team).
- The official scorer does not award the win to another pitcher.
That last bit is sort of wonky, as the scorer can award a win to any pitcher in the game whom he deems to have been especially effective on the mound.
Otherwise, and in most cases, the win goes to the pitcher of record.