In Genesis’s description of Noah’s Ark, the ark was 300 cubits long.
Genesis 6:15 gives the vessel’s dimensions as 300 cubits in length, 50 cubits in width, and 30 cubits in height. The text uses the ancient unit called a cubit, traditionally associated with the length from an adult’s elbow to the end of the fingers. Because cubit standards varied, the exact modern measurement cannot be stated with complete certainty.
Using a commonly cited cubit of about 45 centimetres, 300 cubits would equal roughly 135 metres, or about 450 feet. That conversion is an estimate rather than a measurement explicitly provided by Genesis.
The proportions are often discussed because they make the ark a long, rectangular vessel rather than a ship designed for speed. Genesis also says it had three decks, rooms, a roof, and a door in its side. These details distinguish the biblical description from later artistic reconstructions.