Did the flood last forty days or 150 days?

The two numbers measure different stages of one flood, and Genesis dates each stage. The flood began on the seventeenth day of the second month (7:11). For forty days the rain fell (7:12), and “the flood was forty days upon the earth” (7:17): forty days of downpour and rising water, until the ark was lifted and floated. The waters then ‘prevailed’ (Hebrew gavar, ‘be strong, have the upper hand’) over the earth for 150 days (7:24). By the end of the 150 days the fountains of the deep and the windows of heaven had been stopped, and the waters had begun to go down (8:2–3), and “the ark rested in the seventh month, on the seventeenth day of the month” (8:4).

The dates show how the numbers fit together. From the seventeenth day of the second month to the seventeenth day of the seventh is exactly five months, which at thirty days a month is 150 days. So the 150 days are counted from the start of the flood and include the forty days of rain, as Gill noted. Neither number is the length of the whole flood: Noah left the ark on the twenty-seventh day of the second month of the next year (8:14–16), a year and ten days after the flood began. Forty days of rain, 150 days of prevailing water and a little over a year in all are parts of one account, not rival claims.

150 days after the forty

A few readers count the 150 days from the end of the forty days of rain. That does not fit the dates in 7:11 and 8:4, which put five months between the start of the flood and the ark’s coming to rest.

Two sources

Critical scholars often assign the forty-day figures and the dated 150-day chronology to different sources. The final text, by giving dates, shows how its writer understood the numbers to fit.

Christians are not agreed that the months of the flood account had thirty days each. Danny Faulkner (Answers Research Journal), following Umberto Cassuto, allows that the 150 days may be a round figure for five lunar months of about 147 days, and argues that the flood most likely lasted 365 days, exactly one year. The Septuagint dates the start and end of the flood to the twenty-seventh day of the second month, which would make it exactly a year. On any of these views the 150 days run from 7:11 to 8:4.