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Specimen guide · Fig. 1

Poop 101

Everybody does it. Here’s what yours is telling you, and what it isn’t.

Fig. 2 · The Bristol scale

Seven shapes, one scale

Doctors use the Bristol stool scale to describe what lands in the bowl, from separate hard lumps (1) to watery (7). Tap a type.

The middle for most people · types 3–5 [2]
  1. 1 Separate hard lumps
  2. 2 Lumpy sausage
  3. 3 Sausage with cracks
  4. 4 Smooth and soft

    The middle of the scale: soft, smooth and easy to pass. Around 9 in 10 adults with normal habits land between types 3 and 5 (men) or 2 and 6 (women).[2]

  5. 5 Soft blobs, clear edges
  6. 6 Fluffy, ragged pieces
  7. 7 Watery, no solid pieces
Shaded: the middle for most people, types 3–5 [2]
Illustration of Bristol type 4
Type 4 Smooth and soft

The middle of the scale: soft, smooth and easy to pass. Around 9 in 10 adults with normal habits land between types 3 and 5 (men) or 2 and 6 (women).[2]

Fig. 3 · Transit

It’s about time

Stool shape is partly a clock. The longer food takes to pass through, the more water your gut takes back, and the firmer it gets. In one study of 66 volunteers, stool form followed how long food took to pass through better than how often people went or how much they passed.[1] Another found the link in adults with constipation but not in healthy adults.[7] So it’s a rough window, not a measurement.

MOUTHSTOMACHSMALL INTESTINECOLONmost microbes live here
  1. Mouth
  2. Stomach
  3. Small intestine
  4. Colon most microbes live here
Fig. 4 · The microbiome link

A link with your microbes

In a 2016 study of 53 healthy women in Belgium, those who reported looser stools had fewer kinds of gut microbes, and Akkermansia and Methanobrevibacter were higher in those with firmer stools.[3] It’s one study so far. A difference is an association: it doesn’t mean stool form changes these microbes, or that they affect health.

Akkermansia Breaks down gut mucus. Higher in women with firmer stools, in one study. Its page → Methanobrevibacter An archaeon that makes methane. Higher in women with firmer stools, in one study. Its page →
Fig. 5 · What your type can (and can’t) tell you

A window on transit, not a test result

Across groups of people, a type goes loosely with how fast food moved through the gut: firmer with slower, looser with faster. It’s a rough window, not a measurement, and one day’s type is one reading of something that moves. It isn’t a diagnosis or a microbiome test.

What it can describe

  • Across groups of people, a type goes with how fast food moved through the gut, loosely. It’s a rough window, not a measurement.[1][7] Lewis & Heaton 1997 · 66 volunteers · Saad 2010: not seen in its healthy adults
  • Types went with how much water a stool holds, rising from hard to loose.[9] Blake 2016 · 169 healthy volunteers
  • Of 4,775 US adults who said their bowel habits were normal, 90% of men reported types 3 to 5 and 90% of women types 2 to 6.[2] Mitsuhashi 2018 · NHANES 2009–2010
  • In a 2016 study of 53 healthy women in Belgium, those who reported looser stools had fewer kinds of gut microbes.[3] Seen once ● Vandeputte 2016 · Belgium
  • Among 69 factors linked to differences between people’s gut microbes, stool consistency showed the largest effect size. Medicines accounted for the most in total.[10] Seen once ● Falony 2016 · 1,106 people, Belgium

What it can’t tell you

  • A Bristol type isn’t a diagnosis or a microbiome test.
  • In 863 healthy people, a measured transit time matched differences in gut microbes better than the Bristol type people reported.[11] Asnicar 2021 · UK, US
  • Each of these studies measured people once. They show who differs, not what changes.
  • It isn’t exact. People rating the same photo often land within one type of each other, and split most between 2 and 3 and between 5 and 6.[12] Chumpitazi 2016 · 34 providers, 35 photos
  • One type is one reading of something that moves day to day.[13] Heaton 1992 · 1,897 people, UK

A difference is an association: it doesn’t mean stool form changes these microbes, or that they affect health.

As of Oct 2026. Strength words are counted by code; ● means each person was measured once.

Fig. 6 · How Probiome reads a photo (optional)

No photo needed

  1. Pick or photoPick the closest type, or add a photo.
  2. A suggestionA model suggests the closest type.
  3. You decideYes, or pick another.
  4. Standard glassDrawn as the type, never your photo.
How we read photos

No photo needed. Picking your type is enough on its own.

With a photo, a model looks at the shape (how many pieces, the surface, the edges) and suggests the closest type, with how sure it is.

Nothing is saved until you say yes or pick another type.

Your specimen is always drawn as the type’s standard shape, never your photo.

Your photo stays private: never shown, shared or sold. Your answers help the model learn. You can delete a photo anytime.

We’re still learning. If we got it wrong, tell us.

Fig. 7 · A buoyancy test

Floaters vs sinkers

A floater usually holds trapped gas, made by your microbes fermenting fiber, beans and other foods. Squeeze the gas out and it sinks. It isn’t usually fat.[4]

The exception: greasy, pale, foul-smelling stools that float, leave an oily film and are hard to flush can mean fat isn’t being absorbed. If that keeps happening, it’s worth mentioning to a doctor.[6]

Fig. 8 · Unconfirmed sighting

The ghost poop

Folk wisdom

An internet term: a poop that leaves nothing on the paper, or one you felt but can’t find. No study has counted ghosts. A clean wipe usually just means a smooth, well-formed stool, the middle of the scale.

Fig. 9 · Reference swatches, not your specimen

Fifty shades of brown (and a few others)

Bile starts green-yellow. Gut bacteria turn its pigment brown on the way through, so color tells you about bile, food, how fast things moved and, sometimes, bleeding.[6]

  • Medium brown Normal. Gut bacteria turn bile’s pigment brown on the way through.
  • Light brown Normal.
  • Dark brown Normal.
  • Green Fast transit (bile had less time to turn brown), leafy greens, green coloring, iron.
  • Yellow Fatty meals. Repeatedly greasy, foul and hard to flush: worth mentioning to a doctor.
  • Pale Barium after an X-ray study, some antacids. Red, black or very pale poop can be worth checking with a doctor.
  • Black Iron, bismuth (pink stomach remedies), black licorice, blueberries. Red, black or very pale poop can be worth checking with a doctor.
  • Red Beets, red coloring, tomato skins. Red, black or very pale poop can be worth checking with a doctor.

In babies, pale stools are an important warning sign. Some countries give parents a stool color card to check against.[5]

Fig. 10 · Frequency

Three and three

Among US adults who said their habits were normal, 96% went somewhere between three times a week and three times a day.[2] Your normal matters more than the average.

Fig. 11 · Myth vs fact

Things people say

  • “You have to go every day.” Myth Anywhere from three times a week to three times a day is common.[2]
  • “Floating means fat.” Mostly myth Floaters usually hold gas from your microbes fermenting food.[4]
  • “Green means something’s wrong.” Usually not It’s usually food, iron, or things moving quickly.[6]
Fig. 12 · Worth a doctor’s look

When to see a doctor

Stool changes from day to day; medicines[1] and fiber[8] are among the things studies have found can move it. These are worth checking with a doctor:[6]

  • Black, tarry stools, or blood in or on your stool.
  • Pale or clay-colored stools.
  • A change in your usual pattern that lasts weeks, not a day.
  • Diarrhea for more than a couple of days.
  • Severe belly pain.
  • Weight loss you can’t explain.
This page describes; it doesn’t diagnose. For curiosity, not medical advice.
Sources

Where this comes from

  1. Lewis SJ, Heaton KW. Stool form scale as a useful guide to intestinal transit time. Scand J Gastroenterol 1997. PMID 9299672 ↗
  2. Mitsuhashi S, et al. Characterizing normal bowel frequency and consistency in a representative sample of adults in the United States (NHANES). Am J Gastroenterol 2018. PMID 28762379 ↗
  3. Vandeputte D, et al. (title: “Stool consistency is strongly associated with gut microbiota richness and composition”). 53 women. Gut 2016. PMID 26069274 ↗
  4. Levitt MD, Duane WC. Floating stools: flatus versus fat. N Engl J Med 1972. PMID 5015442 ↗
  5. Matsui A. Screening for biliary atresia. Pediatr Surg Int 2017. PMID 28983697 ↗
  6. Plain-language pages from the NIH. MedlinePlus: Bowel movements ↗MedlinePlus: Floating stools ↗MedlinePlus: Pale or clay-colored stools ↗MedlinePlus: Black or tarry stools ↗NIDDK: Constipation ↗NIDDK: Diarrhea ↗
  7. Saad RJ, et al. Do stool form and frequency correlate with whole-gut and colonic transit? Results from a multicenter study in constipated individuals and healthy controls. Am J Gastroenterol 2010. PMID 19888202 ↗
  8. Balk EM, et al. Fiber intake and laxation in people with normal bowel function: a systematic review. Am J Clin Nutr 2026. PMID 41611088 ↗
  9. Blake MR, Raker JM, Whelan K. Validity and reliability of the Bristol Stool Form Scale in healthy adults and patients with diarrhoea-predominant irritable bowel syndrome. Aliment Pharmacol Ther 2016. PMID 27492648 ↗
  10. Falony G, et al. Population-level analysis of gut microbiome variation. Science 2016. PMID 27126039 ↗
  11. Asnicar F, et al. Blue poo: impact of gut transit time on the gut microbiome using a novel marker. Gut 2021. PMID 33722860 ↗
  12. Chumpitazi BP, et al. Bristol Stool Form Scale reliability and agreement decreases when determining Rome III stool form designations. Neurogastroenterol Motil 2016. PMID 26690980 ↗
  13. Heaton KW, et al. Defecation frequency and timing, and stool form in the general population: a prospective study. Gut 1992. PMID 1624166 ↗