About the Anorexic BMI Calculator
This calculator assesses BMI against the severe underweight thresholds that clinicians use when weight loss becomes a medical concern. It is intended as an information tool for understanding those thresholds — it cannot diagnose an eating disorder, and a normal BMI does not rule one out.
The formula
BMI = weight (kg) / height (m)²The same BMI formula, read against clinical underweight bands: below 18.5 is underweight, below 17 indicates moderate concern, and below 16 is classed as severe underweight by the World Health Organization.
How to use this calculator
- 1Enter your Weight (kg). The field starts at
40, which you can overwrite. - 2Enter your Height (cm). The field starts at
165, which you can overwrite. - 3Read the result straight away — it recalculates as you type, so there is no button to press. Use Share to copy a link that reopens the page with your exact numbers filled in.
Worked example
| Input | Value |
|---|---|
| Weight (kg) | 40 |
| Height (cm) | 165 |
Result
BMI: 14.7
Status: Severely Underweight (Anorexic range)
Healthy weight for your height: 50.4–68.1 kg
⚠ If BMI < 18.5, please consult a healthcare professional.
Those are the values the page loads with, so you can reproduce this result yourself and then change one field at a time to see what drives the outcome.
Understanding your result
Very low body weight carries direct physical consequences regardless of cause: loss of bone density, disrupted menstrual cycles, cardiac arrhythmias, impaired immune function, and reduced concentration. The WHO thresholds at 17 and 16 exist because these risks escalate sharply below them, which is why they inform decisions about the level of care someone needs.
BMI is only one part of any clinical assessment, and relying on it alone is actively misleading. Anorexia nervosa, bulimia, and binge eating disorder all occur across the full weight range, and most people with an eating disorder are not underweight. Diagnosis rests on behaviour, thoughts about food and body, and physical signs — not on a number.
Things worth knowing
- If you are worried about your own or someone else's eating, contact a doctor or an eating disorder helpline rather than tracking numbers.
- Rapid weight loss can be medically serious even when the resulting BMI is still in the normal range.
- Eating disorders have the highest mortality of any psychiatric condition, and early treatment substantially improves outcomes.
- Frequent weighing and calorie tracking can worsen disordered eating. Many clinicians advise stopping both during recovery.
- Recovery is genuinely possible with proper support, and it does not depend on reaching a particular number.
Frequently asked questions
What BMI is considered severely underweight?+
The World Health Organization classifies a BMI below 16 as severe underweight, 16 to 16.9 as moderate, and 17 to 18.4 as mild. These bands inform clinical decisions but are not diagnoses.
Can BMI diagnose an eating disorder?+
No. Diagnosis depends on eating behaviour, thoughts about food and body image, and physical signs. Most people with eating disorders are not underweight, so BMI misses the majority of cases.
What health risks come with being severely underweight?+
Reduced bone density, loss of muscle including cardiac muscle, arrhythmias, hormonal disruption and loss of menstruation, weakened immunity, hair loss, cold intolerance, and difficulty concentrating.
Where can I get help?+
Start with a doctor, who can assess physical health and refer you to specialist care. National eating disorder helplines offer free, confidential support and can advise on next steps for you or someone you are concerned about.