According to Genesis, Noah’s Ark was 300 cubits long.
Genesis 6:15 gives the Ark’s three principal dimensions as 300 cubits in length, 50 cubits in width, and 30 cubits in height. A cubit was an ancient unit based approximately on the length of a forearm, so the exact modern conversion is uncertain. Using common estimates, 300 cubits would be roughly 135 metres, or 450 feet.
The dimensions are presented as part of God’s instructions to Noah before the Flood. They describe a huge rectangular vessel rather than a conventional sailing ship. The biblical account does not provide enough information to establish a precise modern measurement, because ancient cubits varied by culture and period.
A common mix-up is to treat 300 cubits as the Ark’s height or width. In the Genesis measurements, it is specifically the length. The same passage also says the Ark had three decks and was coated with pitch to make it watertight.