cubit. The latest approximation gives Genesis 17:5 inches. But this is the "profane" cubit. The sacred cubit was probably about 25 inches.
Related Commentaries of Genesis 6:15
Genesis 6:15
15 And this is the fashion which thou shalt make it of: The length of the ark shall be three hundred cubits, the breadth of it fifty cubits, and the height of it thirty cubits.