Genesis says Noah's Ark was 300 cubits long.
Genesis 6:15 gives the ark’s principal dimensions as 300 cubits in length, 50 cubits in width, and 30 cubits in height. Because the ancient cubit was based on the length of an arm, its exact measurement varied, so modern conversions are estimates rather than certain figures.
Using a common cubit estimate of about 45 centimeters would make the ark roughly 135 meters long, but other estimates produce somewhat different results. The biblical number itself is stable even though its modern equivalent is not.
The proportions produce a long, broad vessel with a length six times its width and ten times its height. Modern naval designers have sometimes noted that these proportions would be comparatively stable for a large barge-like craft, but such observations do not establish that the biblical vessel existed historically. Genesis also describes three decks, internal compartments, a side door, and a window.