On June 5th, 2012, the State Administration of Cultural Heritage announced that the official length of the Great Wall is 21,196.18 km (13,170.7 mi).
Most of the Great Wall visited and seen on photos was built in the Ming Dynasty. The Ming Great Wall’s Length: 8,851 km (5,500 mi)
Who Built the Great Wall? – From emperors to peasants
Great Wall Protection – threats, restoration, and saving measures
Best Times to Visit the Great Wall of China – different scenery in different seasons