The Genesis ark’s stated length was 300 cubits.
Genesis 6:15 gives the vessel’s dimensions as 300 cubits long, 50 cubits wide, and 30 cubits high. The passage presents these measurements as instructions given to Noah before the flood. A cubit was an ancient unit based approximately on the length of a forearm, but its exact value varied by culture and period.
Using a commonly cited cubit of about 45 centimeters, 300 cubits would be roughly 135 meters, or about 450 feet. That conversion is approximate because the biblical text does not define which cubit standard is intended. The important point is that 300 cubits is the stated length, not a precise modern measurement.
The dimensions produce a long, boxlike vessel with a length-to-width ratio of 6:1. Genesis also mentions three decks, a side door, and a window, although the account does not provide a complete naval blueprint.