A cubit is an ancient unit of measurement based on the length of a person's forearm, from the elbow to the tip of the middle finger. For most adults, that's about 18 inches, though it varied slightly depending on whose arm was being used as the standard.

Ancient civilizations like the Egyptians, Babylonians, and Hebrews used cubits to measure everything from buildings to fabric. When the Bible describes Noah's ark as being “300 cubits long,” it means roughly 450 feet. The Great Pyramid of Giza was designed using Egyptian royal cubits, a standardized version that ensured builders across the kingdom used the same measurements.

The cubit made practical sense before rulers and tape measures existed. A builder could quickly estimate measurements using their own body. However, this also created problems: your cubit might be different from mine, making trade and large construction projects tricky. That's why some cultures created official cubit rods, physical standards kept in temples or palaces, that everyone had to match.

Eventually, more precise and universal systems replaced the cubit. But for thousands of years, this simple forearm-based measurement helped humans build pyramids, temples, ships, and cities.