The cubit is approximately 20 inches (from the elbow, to the tip of the fingers).
The cubit arm is used throughout the Palermo Stone of the 24th to 23rd century BC, (the annals and history of the previous ~700 years, ~3200 to 2382 BC). It is used in the stone in the recording of the Nile River flood levels, in specific year-registers, in "sub-registers" below various King (Pharaoh) entries.