The human rectum is approximately 12 to 15 centimeters long, or roughly 5 to 6 inches. This makes it the final straight segment of the large intestine, sitting just above the anal canal and serving as a temporary holding area for stool before a bowel movement.
Where the Rectum Starts and Ends
The rectum begins at the rectosigmoid junction, where the S-shaped sigmoid colon straightens out. Anatomically, this transition sits at the level of the third sacral vertebra (S3), which is partway down the back of your pelvis. Surgeons use a slightly different landmark, placing the start of the rectum at the sacral promontory, a bony ridge a bit higher up. MRI studies have confirmed that the rectum typically begins at least 10 millimeters below where the first and second sacral vertebrae meet, based on where the supporting tissue of the sigmoid colon ends.
At its lower end, the rectum transitions into the anal canal at a point called the anorectal junction. The anal canal itself is only about 3 to 4 centimeters long. The cell lining changes at this boundary, shifting from the intestinal-type tissue of the rectum to the skin-like tissue of the anal canal. This distinction matters in medicine because diseases behave differently depending on which tissue type they affect.
Why the “15 cm Rule” Exists
In clinical practice, the length of the rectum is conventionally set at 15 centimeters measured from the anal verge, which is the outer edge of the anal opening. This is somewhat arbitrary. The actual length varies from person to person depending on height, body proportions, and pelvic anatomy. But the 15 cm standard gives doctors and surgeons a consistent reference point, particularly when staging rectal cancers or planning surgery. A tumor found within 15 cm of the anal verge is generally classified as a rectal tumor rather than a colon tumor, which changes how it’s treated.
Shape and Structure
Despite its name (from the Latin “rectum intestinum,” meaning “straight intestine”), the rectum isn’t perfectly straight. It follows the curve of the sacrum and coccyx (tailbone), bending gently backward and then forward. Along its interior walls, three lateral folds called rectal valves project inward. These folds help support the weight of stool and prevent it from pressing constantly against the anal canal.
The rectum is wider than most of the colon and can expand significantly. Its lower portion, sometimes called the rectal ampulla, is the most distensible part. When stool accumulates here, stretch receptors in the rectal wall trigger the urge to have a bowel movement. This is why the rectum functions less as a passageway and more as a reservoir.
How Rectal Length Affects Medical Procedures
The rectum’s dimensions come up in several everyday medical contexts. For rectal thermometers in infants and young children, the Mayo Clinic recommends inserting the tip only 1.3 to 2.5 centimeters (about half an inch to one inch) into the rectum, never forcing past resistance. Adult suppositories are typically inserted about 2 to 3 centimeters, just past the anal canal, so the medication stays in contact with the absorbent rectal lining rather than slipping back out.
During a digital rectal exam, a doctor’s gloved finger reaches roughly 6 to 8 centimeters into the rectum. This is enough to feel the prostate gland in men or check for masses in the lower rectum, but it covers only about half the rectum’s total length. For a more complete view, a rigid proctoscope can visualize the full 15 cm, while a flexible sigmoidoscope extends well beyond the rectum into the sigmoid colon.
Variation Between Individuals
Not everyone’s rectum is the same length. Cadaveric and imaging studies show a range from about 10 to 15 centimeters in most adults. Taller individuals tend to have slightly longer rectums, and men’s rectums are on average a bit longer than women’s, partly because the male pelvis is typically narrower and deeper. Conditions like chronic constipation can stretch and elongate the rectum over time, a situation sometimes called megarectum, while pelvic surgery or radiation therapy can shorten or stiffen it.