Genesis uses the cubit as the unit of length for the Ark’s dimensions.
Genesis 6:15 gives the Ark’s measurements as 300 cubits long, 50 cubits wide, and 30 cubits high. A cubit was an ancient unit based broadly on the length of a forearm, but its exact value varied by culture and historical period. Because of that variation, converting the Ark’s dimensions into modern metres or feet is only approximate.
Using a commonly cited cubit of about 45 centimetres would make the Ark roughly 135 metres long, 23 metres wide, and 14 metres high. Other estimates differ because ancient cubits were not standardized across all societies.
The cubit is also used in measurements throughout the Hebrew Bible. It is therefore more accurate to report Genesis’s original unit than to claim one exact modern conversion.