What the shape tracks is colon residence time. Your colon spends the last hours before a movement reclaiming water, about a litre a day. The longer the bolus sits, the more water comes out and the harder and more broken-up what remains. Pellets mean it sat too long; water means it barely stopped. Stool shape correlated with measured transit time at r = −0.54 ¹, confirmed later against water content in healthy adults ².

Read it as a moderate signal you build across a couple of weeks. The obvious types are easy to call; the middle ones (Type 2, 3, 5, 6) get classified right only 55–63% of the time ², and that's exactly the band where constipated shades into fine. In people already chronically constipated the shape barely tracks transit at all ³. It's feedback for a healthy gut, not a diagnosis.

The chart has three bands. Drift toward pellets is the one most people fix; the rest is watching.

The chart's real value is the threshold: a sudden, persistent change plus any red flag is the line where you stop reading and book an appointment. Blood, even once. Unexplained weight loss. Pencil-thin stools. Diarrhoea past four weeks. A lasting change in habit over 50. These trigger urgent lower-bowel cancer referral in the national pathways ⁴. Most turn out to be haemorrhoids or an infection, but you can't tell from the outside, and the cost of missing the small fraction that isn't is years.