This is more of an informative post, since most of you know what the cubit is and how much is it. But I wanted to summarize and add a few points.
Cubit or Amah (אמה) is first mentioned in Genesis 6:16. Mostly, cubit is important for Tabernacle construction.
Torah defines all units of measurements based on the human body or other biological measurements. Omer, for example, is defined as a handfuls (also a human body measurement).
So is cubit, is the length of the forearm(from the tip of the elbow to the tip of the middle finger):
Cubit is a variable constant and varies from person to person and between genders and ages.
Tabernacle was probably based on a cubit of Moses, Bezalel or Aaron.
Google defines cubit as 18 inches. I checked, and my cubit is 18.5 inches (I am 5' 11") and my female friend's cubit is 16.5 inches.