According to Genesis, how many days does rain fall at the start of Noah’s Flood?

According to Genesis, rain falls on Noah’s Flood for 40 days and 40 nights. Genesis 7:12 uses that exact phrase when describing the beginning of the Flood narrative.

The forty-day period is not the same as the entire duration of the Flood. Genesis separately says that the waters prevail on the earth for 150 days, and the narrative continues until the ground becomes dry. This distinction is a common source of confusion when the story is summarized.

The number 40 also appears repeatedly in biblical literature as a period associated with testing, transition, or judgment. Examples elsewhere include Moses’ time on Mount Sinai and Israel’s years in the wilderness, although those episodes are separate from Noah’s story.

Genesis presents the rain as one part of a larger event involving subterranean waters and the opening of the “windows of heaven.” The text therefore distinguishes the initial rainfall from the later recession of the floodwaters.