The Great Pyramid was originally 280 royal cubits high. That corresponds to approximately 146.6 metres, although the pyramid is now shorter because its pyramidion and most of its smooth outer casing have disappeared.
The royal cubit was the principal Egyptian unit used for monumental construction. Its exact length varied slightly across periods and physical standards, so converting 280 cubits into modern units is an approximation. The pyramid’s reconstructed dimensions are commonly given as 280 cubits in height and 440 cubits along each side of its square base.
This proportion produced a slope of about 51°50′40″, described in Egyptian surveying terms by a seked. The original height made the Great Pyramid the tallest human-made structure in the world for more than 3,700 years. The frequently quoted modern height—about 138.5 metres—describes the surviving structure, not the complete monument built for Pharaoh Khufu.