This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
The small intestine begins at the pylorus and ends at the ileocaeal valve. It has an average length of 6.75 metres (22 ft. 6 in.) in the adult, independent of the age, weight, and height of the individual (Treves). Jonnesco gives its length as 7.5 metres (24 ft. 7 in.) and Sappey as 8 metres (26 ft. 3 in.). The duodenum is about 25 to 30 cm. (10 to 12 in.) long, and two-fifths of the remainder, or about 8 1/2 ft., is jejunum, and three-fifths, or about 12 1/2 ft., is ileum.