About the Bristol Stool Form Scale
The Bristol Stool Form Scale sorts stool into seven types, from separate hard lumps (type 1) to entirely liquid (type 7). Types 1 and 2 indicate constipation and slow colonic transit, types 3 and 4 are normal, and types 6 and 7 indicate diarrhoea and rapid transit. It was developed in Bristol in the 1990s as a way to measure something patients describe inconsistently, and it works because stool form tracks how long the stool has spent in the colon — the longer the transit, the more water is absorbed and the harder the result.
Formula
No calculation — the scale is an ordinal description of stool form, matched by inspection.- Type 1–2
- Hard lumps and lumpy sausage. Prolonged colonic transit, so more water has been absorbed.
- Type 3–4
- Cracked sausage and smooth sausage. Normal transit; type 4 is often described as the ideal.
- Type 5–7
- Soft blobs through to entirely liquid. Progressively faster transit, so less water has been absorbed.
- The scale is ordinal, not interval: the difference between types 1 and 2 is not the same quantity as between 6 and 7, so averaging Bristol types across a week is not meaningful. Report the proportion of stools in each band instead.
- It measures form, not frequency or volume. A patient can pass type 4 stool twice a day or twice a week, and the scale says nothing about which.
Interpreting the result
Read the distribution, not the reading. A patient reporting mostly type 4 with an occasional type 6 is different from one alternating between 1 and 7, and only the second is describing the mixed pattern that Rome IV calls IBS-M. In constipation, the useful target of treatment is types 3–4 rather than a particular frequency, because form is what patients actually experience as difficulty. And a genuine change in a stable pattern matters more than the absolute type: a lifelong type 2 is far less concerning than a new type 6 in someone who has always been type 4.
| Score | Band | What it means | Action |
|---|---|---|---|
| Type 1 | Constipation | Separate hard lumps, like nuts. Markedly slow transit | Assess and treat constipation; check for red flags |
| Type 2 | Constipation | Sausage-shaped but lumpy. Slow transit | Fibre, fluid, activity; review constipating drugs |
| Type 3 | Normal | Sausage-shaped with surface cracks | No action needed |
| Type 4 | Normal | Smooth, soft sausage or snake — often called ideal | No action needed; the usual treatment target |
| Type 5 | Tending to loose | Soft blobs with clear-cut edges, passed easily | Often normal; may indicate lacking fibre |
| Type 6 | Diarrhoea | Fluffy pieces with ragged edges, mushy. Rapid transit | Assess for infective, inflammatory or malabsorptive cause |
| Type 7 | Diarrhoea | Watery, no solid pieces, entirely liquid | As above, plus assess hydration |
What the Bristol Stool Scale needs (1 inputs)
- Stool form (type 1–7)
- The form of the stool as passed, matched against the seven descriptions. Where a patient's stools vary, what matters clinically is the distribution across a period rather than the single worst or most recent one.
Units. No units — the scale is a set of seven descriptive categories, numbered 1 to 7. The numbers are labels for the categories, not measurements, so a type 6 is not 'twice as loose' as a type 3.
What it returns
- Bristol type (1–7)
- The matched type, with the transit-time and clinical interpretation attached.
- Category
- Constipation (1–2), normal (3–4), tending to loose (5), or diarrhoea (6–7).
How it is calculated
The scale is a proxy for colonic transit time. Water absorption in the colon is time-dependent, so a stool that has moved through slowly arrives dry and fragmented, and one that has moved through quickly arrives loose or liquid. Lewis and Heaton established this relationship directly, by altering transit time with senna and loperamide and observing that stool form changed with it — which is why the scale is a measurement rather than just a vocabulary. That also sets its limits: anything that changes stool water for reasons other than transit, such as an osmotic load or steatorrhoea, will move the type without the transit time having changed.
Facts & figures
| Subtype | Rule | Predominant types |
|---|---|---|
| IBS-C | More than 25% of bowel movements type 1–2, and 25% or less type 6–7 | 1–2 |
| IBS-D | More than 25% type 6–7, and 25% or less type 1–2 | 6–7 |
| IBS-M | More than 25% of both type 1–2 and type 6–7 | Both extremes |
| IBS-U | Meets IBS criteria but neither pattern predominates | Variable |
Assessed only on days with at least one abnormal bowel movement. This is why prospective recording matters — the rule is a proportion, and proportions cannot be recalled accurately.
Evidence
Derivation — Bristol, transit-time correlation
1997Developed at the University of Bristol and published in 1997, establishing that stool form corresponds to intestinal transit time. Transit was manipulated pharmacologically and stool form changed accordingly, which is what makes the scale a proxy measurement rather than a naming convention.
Stool form correlated with measured whole-gut transit time, supporting its use as a surrogate where formal transit studies are impractical.
Validation — healthy adults and diarrhoea-predominant IBS
2016Prospective assessment of validity and reliability in healthy adults and in patients with diarrhoea-predominant irritable bowel syndrome, published in Alimentary Pharmacology & Therapeutics in 2016.
Supported the scale's reliability for recording stool form, while noting that agreement is best when patients record at the time of the bowel movement rather than recalling it later.
How it compares
Bristol Stool Scale vs Rome IV criteria for irritable bowel syndrome
They work together rather than competing: Rome IV decides whether it is irritable bowel syndrome, and Bristol decides which subtype — you cannot complete the Rome IV subtyping step without stool form.
Rome IV sets the diagnostic criteria: recurrent abdominal pain at least one day a week for three months, with at least two of three pain characteristics, six months since onset. Once those are met, the subtype comes from the proportion of bowel movements at Bristol types 1–2 versus 6–7 on days with abnormal habit. So Bristol is not an alternative to Rome IV, it is a component of it.
Bristol Stool Scale vs Stool frequency alone
Form predicts colonic transit better than frequency does, which is why the scale exists — someone opening their bowels daily can still be constipated.
Frequency is the more intuitive measure and the one patients volunteer, but it is a poor proxy for transit and a poor treatment target. A patient passing type 1–2 stool every day is straining daily and experiencing constipation, and a frequency-based assessment records them as normal. The pair together — form and frequency — is more informative than either alone.
Pearls & pitfalls
- It measures form only. Frequency, volume, urgency, blood and mucus all matter clinically and none of them appear on the scale.
- One reading is close to useless. The clinically meaningful output is the distribution over one to two weeks, recorded prospectively.
- Type 4 is the usual treatment target in constipation, not a particular number of bowel movements per week — form is what patients experience as difficulty.
- The scale is ordinal, so averaging types is not valid. A patient alternating 1 and 7 does not have an average of type 4; they have a mixed pattern.
- A new change in a previously stable pattern is more significant than the absolute type, particularly over the age of 50.
- Recall is unreliable. Patients asked to remember last week's stool form agree with their own contemporaneous records much less well than clinicians expect.
- Types 6–7 are not automatically functional. Bile-acid diarrhoea, coeliac disease, microscopic colitis, pancreatic insufficiency and inflammatory bowel disease all present this way and all have specific treatments.
Critical actions
- Screen for alarm features regardless of type: onset over 50, rectal bleeding, unexplained weight loss, nocturnal symptoms, iron-deficiency anaemia, a family history of colorectal cancer or inflammatory bowel disease, or a palpable mass.
- For persistent types 6–7, check faecal calprotectin and coeliac serology as a minimum, and consider bile-acid malabsorption — it is common and specifically treatable.
- For persistent types 1–2, review every drug the patient takes: opioids, anticholinergics, calcium-channel blockers and iron are common culprits.
- Ask for a prospective two-week record before subtyping irritable bowel syndrome, rather than assigning a subtype from a retrospective description.
- Do not treat a stool chart in isolation — the scale supports a history, it does not replace one.
Why this score exists
The scale came out of Ken Heaton's group at Bristol, and its purpose was measurement rather than education: the team needed a way to quantify stool form for transit-time research, because the words patients and clinicians used were not comparable. The seven categories were chosen to span the range that pharmacological manipulation of transit actually produced. Its later career as a patient-facing chart on clinic walls was a side effect of it being unusually easy to understand, not the original design goal — which is worth knowing, because it explains why the scale says nothing about frequency, volume, blood or urgency. Those were never what it was built to measure.
About the creator
First author, 1997 scale validation
Published the seven-type stool form scale as a surrogate measure of colonic transit time.
Senior author
Co-developed the scale at the University of Bristol, from which it takes its name.
Limitations
- Captures form only, so it cannot substitute for a full bowel history.
- Ordinal rather than interval, so types cannot be averaged and parametric statistics on raw types are inappropriate.
- Retrospective reporting is substantially less reliable than contemporaneous recording, which limits its accuracy in ordinary consultations.
- Validated principally in adults; paediatric use generally relies on modified versions with age-appropriate images.
- Anything that changes stool water independently of transit — osmotic laxatives, steatorrhoea, high-volume secretory diarrhoea — moves the type without the transit time having changed, so the transit inference breaks down.
- Cultural and dietary differences in baseline stool form mean the 'normal' band is a population statement, not an individual one.
If you are the patient
The Bristol Stool Chart is a set of seven pictures used to describe what your stool looks like, so that you and your doctor mean the same thing. Types 1 and 2 are hard and lumpy and mean things are moving through slowly — that is constipation. Types 3 and 4 are smooth and soft and are considered normal, with type 4 often called ideal. Types 5, 6 and 7 get progressively looser, and 6 and 7 mean diarrhoea. It is genuinely useful to keep a note for a week or two of which type you have each time, because doctors find that far more reliable than trying to remember. Two things worth telling your doctor about rather than waiting: any lasting change from what is normal for you, and any blood, unexplained weight loss, or being woken at night by your bowels.
Frequently asked questions
What is the Bristol Stool Scale?#
It is a seven-point scale describing the form of stool, from type 1 (separate hard lumps) to type 7 (entirely liquid). It was developed at the University of Bristol and published in 1997 as a way to measure stool form reliably, because it corresponds to how long the stool has spent in the colon.
Which Bristol stool type is healthy?#
Types 3 and 4 are considered normal, and type 4 — a smooth, soft sausage — is often described as ideal. Type 5 is frequently normal too. Types 1 and 2 indicate constipation and types 6 and 7 indicate diarrhoea.
What does Bristol type 6 mean?#
Type 6 is mushy stool in fluffy pieces with ragged edges, indicating rapid colonic transit — diarrhoea. Persisting type 6 should not be assumed to be functional: coeliac disease, bile-acid malabsorption, microscopic colitis, pancreatic insufficiency and inflammatory bowel disease all present this way and each has a specific treatment.
How is the Bristol Stool Scale used to diagnose IBS?#
It supplies the subtyping step of the Rome IV criteria. Once the Rome IV symptom criteria for irritable bowel syndrome are met, the subtype is assigned from the proportion of bowel movements at types 1–2 versus 6–7 on days with an abnormal bowel habit: more than 25% at 1–2 gives IBS-C, more than 25% at 6–7 gives IBS-D, more than 25% of both gives IBS-M.
Can you be constipated and still go every day?#
Yes, and this is one of the main reasons the scale is useful. Constipation is about difficulty and stool form as much as frequency. Someone passing type 1 or 2 stool daily is straining every day and is constipated, even though a frequency-based assessment would record them as normal.
Should I record my stool type or just remember it?#
Record it. Validation work found that agreement is considerably better when patients note the type at the time of the bowel movement than when they recall it afterwards. Since the Rome IV subtyping rule is a proportion over time, a prospective one- to two-week record is what makes it usable.
Is the Bristol Stool Scale valid in children?#
The original scale was developed and validated in adults. Modified versions with age-appropriate images exist for paediatric use, and those are what should be used rather than the adult chart.
Does the Bristol type tell you the cause of diarrhoea?#
No. It describes the consistency and implies fast transit, but it does not distinguish an infection from coeliac disease, bile-acid malabsorption, inflammatory bowel disease or a functional disorder. It is the starting point of the assessment, not the conclusion.