According to the Book of Genesis, the Floodwaters prevailed over the Earth for 150 days.
Genesis first says that rain fell for 40 days and 40 nights, but that period is not the whole duration of the Flood. The text later states that the waters prevailed for 150 days before beginning to decrease. This distinction explains why “40 days” and “150 days” are both associated with the story but answer different questions.
The narrative gives a sequence of dates: Noah enters the Ark, the Flood begins, the Ark comes to rest, and the ground eventually dries. Biblical scholars note that the chronology contains passages with different styles and details, so exact reconstructions can vary. Nevertheless, the statement that the waters prevailed for 150 days is explicit in Genesis 7:24.
The number is also separate from the total time Noah and his family spend inside the Ark, which Genesis presents through additional dated events.