Small intestine
It is approximately 1.5 cm in diameter in newborns after 35 weeks of gestational age, and 2.5–3 cm (1 inch) in diameter in adults. On abdominal X-rays, the small intestine is considered to be abnormally dilated when the diameter exceeds 3 cm. On CT scans, a diameter of over 2.5 cm is considered abnormally dilated. The surface area of the human small intestinal mucosa, due to enlargement caused by folds, villi and microvilli, averages 30 square meters.
Large intestine
The average inner diameter of sections of the colon in centimeters (with ranges in parentheses) are cecum 8.7 (8.0-10.5), ascending colon 6.6 (6.0-7.0), transverse colon 5.8 (5.0-6.5), descending/sigmoid colon 6.3 (6.0-6.8) and rectum near rectal/sigmoid junction 5.7 (4.5-7.5).