In Genesis, Noah’s Ark was 300 cubits long.
Genesis 6:15 gives the ark’s dimensions as 300 cubits in length, 50 cubits in width, and 30 cubits in height. A cubit was an ancient unit based broadly on the length of a forearm, but its exact standardized length varied. That means the biblical text provides a ratio and a number in cubits, not a universally agreed modern measurement.
Using a common estimate of about 45 centimeters per cubit would make the ark roughly 135 meters long, but translations and calculations may differ. The ark’s length-to-width-to-height ratio is 10:1:0.6, and naval historians sometimes discuss how that broad, box-like form would affect stability.
The 300-cubit length is separate from the number of decks. Genesis names three levels, while this question concerns only the vessel’s longitudinal measurement. The text gives dimensions for the story’s ark, not measurements confirmed by an excavated shipwreck.