According to Genesis, the Flood waters rose 15 cubits above the mountains. Genesis 7:20 says that the waters prevailed 15 cubits upward, and the mountains were covered.
This figure appears in the narrative after the rain and rising waters have overwhelmed the land. It is separate from the Ark’s dimensions: 300 cubits long, 50 cubits wide, and 30 cubits high. It is also separate from the 40 days and nights of rain and the 150 days during which the waters prevailed.
The passage is part of an ancient flood narrative whose chronology and geography have been interpreted in different ways by religious readers and scholars. The text itself gives the 15-cubit figure without explaining whether it is measured from a particular local elevation or how it relates to the Ark’s draft and height.
Because several numbers occur close together in Genesis 6–8, this detail is easy to confuse with the Ark’s 30-cubit height. The Flood-water figure is 15 cubits above the mountains.