What Was A Cubit?

The royal (and the short) cubit were units of measurement widely used in Ancient Egypt, to calculate distance.  The royal cubit corresponded to the distance from the elbow to the extended fingertips, around 21 inches or 53 centimetres.  Each cubit was then divided into 28 djeba or digits each one about the width of a man's finger. Four digits equalled one shesep, which was the width of a palm.  There were seven sheseps in a royal cubit.  In a short cubit there were 6 sheseps.  
The Egyptians actually had standard measuring rods like tape measures which were made from palm wood and could be used when building or measuring a reasonable distance.  For longer distances, the Egyptians used a khet (or rod) which was equal to 100 royal cubits and the iteru (or river) which was equal to 20,000 cubits, a distance of some 6.5 miles or 10 kilometres.