A scale describing the looseness of human stools, from 1, hard little lumps, to 7, a complete lack of solids. At one time it was thought to be a measure of transit time, but that is not strictly true.
The web has an astounding number of entries on the Bristol stool number (I guess we are all third-graders at heart). A fine illustration of the stools corresponding to each number can be found at
and Wikipedia has a comprehensive article on the scale's history and uses.
S. J. Lewis and K. W. Heaton.
Stool Form Scale as a Useful Guide to Intestinal Transit Time.
Scandanavian Journal of Gastroenterology, vol 32, no. 9, pages 920-9248 (May 1997).
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Last revised: 6 May 2020.