Genesis gives the ark's length in cubits, an ancient unit of measurement.
Genesis 6:15 specifies a length of 300 cubits, a width of 50 cubits, and a height of 30 cubits. A cubit was traditionally related to the length of a forearm, but its exact value varied across cultures and historical periods. Because of that variation, converting the ark's dimensions into modern metres produces estimates rather than one certain figure.
Using a common approximation of about 45 centimetres per cubit would make the ark roughly 135 metres long, 22.5 metres wide, and 13.5 metres high. Other cubit standards lead to somewhat different measurements.
The text also describes three decks, a roof, a side door, and rooms. These measurements belong to the biblical narrative and should not be confused with dimensions established by archaeological evidence, since no verified ark remains have been found.