In Genesis, the floodwaters prevailed over the Earth for 150 days.
The number appears in the flood narrative in Genesis 7:24 and is part of the account’s chronological structure. Genesis says that the waters increased for 40 days and 40 nights, but that is not the same as the total period during which the waters prevailed. The 150-day figure describes the extended dominance of the floodwaters.
Readers sometimes confuse the 40 days of rain with the 150 days of prevailing waters. Genesis later describes the waters receding, after which the ark came to rest on the mountains of Ararat. Biblical chronologies can be read in more than one way because the narrative contains overlapping time references, but 150 days is the explicit figure associated with the waters prevailing.
The flood story is one of the best-known sections of Genesis and has parallels in older Mesopotamian flood literature, including the Epic of Gilgamesh and the Atrahasis tradition.