What is the traditional biblical measurement of Noah's Ark's length?

The traditional biblical measurement of Noah's Ark's length is 300 cubits.

Genesis 6:15 gives the ark's dimensions as 300 cubits long, 50 cubits wide, and 30 cubits high. This proportion produces a vessel six times as long as it is wide and ten times as long as it is high. The text gives the dimensions as instructions to Noah before the Flood.

A cubit was an ancient unit based broadly on the length of a forearm, so its exact value was not identical in every culture or period. That is why modern conversions vary, often placing the ark's length somewhere around 135 to 157 metres depending on the cubit used. The number 300 is the direct biblical measurement; any metre or foot equivalent is an interpretation.