According to Wikipedia and UNESCO official site, the length of the Great Wall of China is 21.19618 million miles long. Converting to kilometers it is 21,196 Km. Whereas in 2012 China's State Administration of Cultural Relics announced that its length is 13,170 miles.
The length of the Great Wall reconstructed and extended during the Ming Dynasty (1368-1644) is 8,851.8 kilometers (5,500.3 miles). About 526 kilometers (327 miles) area of the Great Wall of China is lying in Beijing. This particular part is the most popular tourist destination.
The wall was constructed with bricks and stones mainly and different parts have different architectural styles because the available resources were utilized to strengthen the wall. The geographical description will show it stretching from China Sea port of Shanhaiguan over 3,000 miles west into Gansu province.
To maximize security, some sections of the Great Wall of China are overlapping. These were the places where invaders were expected to attack. So these strategic areas were given special attention during the construction.
As during different dynasties the length of the wall keeps on changing due to damages and extensions so the lengths are summarized in a chart below.