Anorexic BMI Calculator
Calculate BMI and see it against the thresholds used when assessing anorexia nervosa.
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About
Anorexic BMI Calculator
This calculator works out body mass index and shows where it falls against the thresholds clinicians use when they assess anorexia nervosa. It is a screening aid. It cannot diagnose an eating disorder, and a number on this page should never be treated as a verdict either way.
What BMI measures
BMI = weight / height², with weight in kilograms and height in metres.
where:
- weight is total body weight in kilograms
- height² is height in metres, multiplied by itself
Someone 1.80 m tall weighing 60 kg has a BMI of 60 / 3.24, which is 18.52. In imperial units the shortcut is weight in pounds x 703 / height in inches², so 160 pounds at 70 inches gives 22.96.
BMI takes two measurements and returns one number. It knows nothing about muscle, bone, fat distribution or what a person eats, which is the root of everything that follows.
Where the 17.5 threshold comes from
The figure of 17.5 is not arbitrary and it is not a diagnosis. It comes from ICD-10, the World Health Organization's classification, which lists a BMI at or below 17.5 as one physical criterion for anorexia nervosa, alongside weight loss that is self-induced and a distorted body image.
DSM-5, the American Psychiatric Association's manual, deliberately moved away from any fixed number. The earlier DSM-IV had required body weight below 85% of what was expected, and that criterion was removed because it excluded people who were clearly unwell but sat above the line. DSM-5 asks instead for three things together:
- restriction of energy intake leading to a significantly low body weight for age, sex and physical health
- intense fear of gaining weight or becoming fat, or persistent behaviour that prevents weight gain
- a disturbance in how body weight or shape is experienced, undue influence of weight on self-evaluation, or a persistent failure to recognise the seriousness of the current low weight
Two of those three are about thought and behaviour. No calculator can see them.
The DSM-5 severity bands
Once a clinician has made a diagnosis, DSM-5 uses BMI to grade how severe the illness currently is in adults. These bands describe severity of an existing diagnosis. They do not create one.
| Severity | BMI |
|---|---|
| Mild | 17 or above |
| Moderate | 16 to 16.99 |
| Severe | 15 to 15.99 |
| Extreme | under 15 |
The manual also states that severity can be raised above what the BMI alone suggests, based on symptoms, functional impairment and how much supervision the person needs. A clinician who sees someone deteriorating fast will not wait for a number to catch up.
Underweight and anorexic are not the same thing
The WHO grades low BMI in adults on its own scale, and it describes body size rather than any illness.
| WHO band | BMI |
|---|---|
| Severe thinness | under 16 |
| Moderate thinness | 16 to 16.99 |
| Mild thinness | 17 to 18.49 |
| Healthy range | 18.5 to 24.9 |
Most people with a BMI under 18.5 do not have an eating disorder. A fast metabolism, a naturally small frame, coeliac disease, an overactive thyroid, inflammatory bowel disease, cancer, chronic infection, some medications and simple genetics all produce a low BMI without any disordered eating behind it. A person can be lastingly, comfortably underweight and perfectly well.
The error in the other direction matters more, because it is the one that delays treatment. Anorexia nervosa occurs at every body size. When every diagnostic feature is present but weight sits in the normal or high range, DSM-5 classifies it as atypical anorexia nervosa within OSFED, and research has found that the medical complications, including bradycardia, electrolyte disturbance and bone loss, can be as severe as in people whose weight is low. Someone reading a BMI of 22 on this page and concluding that they are fine may be wrong.
Who anorexia affects
Onset peaks in adolescence and early adulthood, and the illness is more often diagnosed in women and girls. The commonly repeated claim that only about 10% of cases are male is out of date. Community surveys, including the US National Comorbidity Survey Replication, put men at roughly a quarter of people meeting criteria for anorexia nervosa across their lifetime. The gap between that figure and the clinical caseload reflects under-recognition rather than absence: symptoms in men are more often read as dedication to training or diet, and screening tools were largely written around presentations in women.
Anorexia also occurs in older adults, in every ethnic group, and across the socioeconomic range. It rarely arrives alone. Depression, anxiety disorders and obsessive-compulsive disorder frequently sit alongside it, which is part of why assessment involves a mental health evaluation rather than a set of scales.
What a very low BMI does to the body
Sustained energy restriction affects nearly every organ system. The heart loses muscle mass along with everything else, and a resting pulse below 50 is common, often with low blood pressure and fainting. Electrolyte disturbance, particularly low potassium, carries a risk of arrhythmia. Periods commonly stop. Bone mineral density falls, and adolescence is when most peak bone mass is laid down, so time spent underweight during those years has consequences that persist.
Anorexia nervosa has one of the highest mortality rates of any psychiatric illness. A widely cited meta-analysis by Arcelus and colleagues found a standardised mortality ratio of about 5.9, meaning deaths occur at roughly six times the rate expected for the same age and sex, from medical complications and from suicide.
Refeeding needs medical supervision at low weights, because restoring nutrition too quickly can cause refeeding syndrome, a dangerous shift in phosphate and other electrolytes. NICE guidance treats a BMI under 16, or minimal intake for more than ten days, as high risk. This is one of several reasons that recovery at a very low weight is not something to attempt without support.
Children and teenagers
Fixed BMI cut-offs do not apply under 18. A healthy BMI changes throughout growth, so a figure that is normal at 16 may be very low at 12. BMI has to be plotted as a percentile for age and sex, and the CDC growth charts linked on this page do that. The CDC treats below the 5th percentile as underweight for age.
In young people, the rate of change matters more than any single reading. A child tracking along the 50th percentile who drops to the 15th over a few months has lost a great deal of expected growth, even though the second reading still sits inside the normal range. Faltering growth in height is another signal that a weight-only measure misses entirely.
What this calculator cannot tell you
It cannot see behaviour, and behaviour is where anorexia lives: how food is being restricted, what happens around eating, whether exercise has become compulsory, what the fear of gaining weight feels like. It cannot see purging, laxative or diuretic use, or the amount of mental space this occupies each day.
It also cannot see composition. A muscular athlete and a person who has lost a great deal of muscle can share a BMI. That is why a clinical assessment includes physical examination, blood tests, an ECG at low weights, and a conversation, rather than a single ratio.
If this page brought you here
The number matters far less than whether food, weight or body shape has started taking up a large share of your thinking. If it has, that is worth raising with a doctor whatever your BMI says, and you do not need to be underweight, or unwell enough, or anything else, to ask for help. Early treatment is the single strongest predictor of recovery, and most people do recover.
In the United States, the National Eating Disorders Association runs a free screening tool and a helpline. In the United Kingdom, Beat provides the same. Both are confidential, and both are used routinely by people who are unsure whether what they are experiencing counts.
If you are supporting someone else, comments about their weight or appearance tend to make things harder regardless of intent. Concern about specific behaviour and how they seem to be feeling lands better than concern about a number.
Common questions
Frequently asked questions
No BMI is, on its own. ICD-10 lists a BMI at or below 17.5 in adults as one physical criterion for anorexia nervosa, but a diagnosis also requires self-induced weight loss, a fear of gaining weight and a distorted view of body shape. DSM-5 removed fixed weight thresholds from its criteria altogether.
No. Underweight is a BMI under 18.5, a description of body size. Anorexia nervosa is a psychiatric diagnosis involving restriction, fear of weight gain and body image disturbance. Most people with a low BMI have no eating disorder, and anorexia can be present at a completely normal weight.
Yes. When every feature of anorexia nervosa is present but weight sits in or above the normal range, DSM-5 classifies it as atypical anorexia nervosa under OSFED. Research has found the medical complications, including slow heart rate, electrolyte disturbance and bone loss, can be just as severe. A healthy BMI rules nothing out.
For adults: mild is a BMI of 17 or above, moderate 16 to 16.99, severe 15 to 15.99, and extreme below 15. These grade an existing diagnosis rather than making one, and a clinician can raise the severity above what BMI suggests based on symptoms and functional impairment.
Divide weight in kilograms by height in metres squared. Someone 1.80 m tall at 60 kg has a BMI of 18.52. In imperial units, multiply weight in pounds by 703 and divide by height in inches squared, so 160 pounds at 5 feet 10 inches gives 22.96.
Not as a fixed cut-off. Healthy BMI changes throughout growth, so it has to be plotted as a percentile for age and sex using growth charts, and the CDC treats below the 5th percentile as underweight for age. In young people the direction of travel across months matters more than any single reading.
Yes, considerably more than the often-quoted 10% suggests. Community surveys including the US National Comorbidity Survey Replication put men at around a quarter of people meeting lifetime criteria. The gap with clinical caseloads reflects under-recognition, since symptoms in men are more often read as dedication to training or diet.
Nearly every system is affected. Resting heart rate below 50, low blood pressure and fainting are common, as is low potassium, which risks arrhythmia. Periods often stop and bone density falls, which matters most in adolescence when peak bone mass is being laid down. Anorexia has among the highest mortality rates of any psychiatric illness.
No. It computes BMI and shows where that sits against published thresholds. Diagnosis requires a clinician assessing behaviour, thinking and physical health together, typically with a physical examination, blood tests and an ECG at low weights. Use the number as a reason to have a conversation, not as an answer.
Start with a GP or primary care doctor, who can examine you and refer on. In the US, the National Eating Disorders Association runs a free confidential screening tool and helpline; in the UK, Beat does the same. You do not need to be underweight or unwell enough to ask, and early treatment is the strongest predictor of recovery.