In Genesis, after how many days of rain did the Flood waters rise?

In Genesis, rain fell on the earth for forty days and forty nights.

Genesis 7:12 states that the rain fell for forty days and forty nights after Noah and the animals entered the ark. The same chapter describes the waters rising and covering the mountains, making the forty-day period one of the best-known numbers in the Flood narrative.

This figure is sometimes confused with the total duration of the Flood. Genesis also describes the waters prevailing for 150 days and records a much longer sequence before the ground became dry. Thus, forty days and forty nights refers specifically to the period of rain, not necessarily to the entire time Noah remained aboard the ark.

The number forty appears elsewhere in biblical narratives as a period associated with testing or transition. In the Noah story, however, the specific fact is simply the stated duration of rainfall in Genesis 7:12.