Anorexic BMI Calculator: WHO Thinness Grades
Calculate BMI and see the WHO thinness grade for a low BMI, with the BMI bands clinicians use to describe severity. BMI alone cannot diagnose an eating disorder; the page points to professional help.
About this low BMI calculator
This page calculates body mass index (BMI) and shows where a low result sits on the World Health Organization (WHO) grades of thinness. Enter weight and height in metric, US or other units and the result shows the BMI, the broad category and the WHO class, with the three thinness grades listed and the matching one marked.
People often search for an "anorexic BMI" when they are worried about their own weight or about someone close to them. This page gives the numbers in plain, neutral terms. It cannot say whether anyone has an eating disorder.
BMI alone cannot diagnose an eating disorder. Anorexia nervosa and other eating disorders are diagnosed by a qualified professional from behaviour, thoughts and physical health, not from a single measurement. Many people with an eating disorder have a BMI in the normal range, and some people have a low BMI for other reasons, such as illness or their natural build.
This is an estimate, not medical advice.
Getting support
If you are worried about your eating, your weight or someone else's, you do not need to wait until a number looks serious. A doctor or other health professional can help, and most countries have a national eating disorder helpline offering free, confidential support by phone, chat or email. If someone is in immediate danger, contact your local emergency services.
WHO thinness grades
Adult grades
- Mild thinness: BMI 17.0 to 18.49
- Moderate thinness: BMI 16.0 to 16.99
- Severe thinness: BMI below 16.0
A BMI from 18.5 to 24.9 is the WHO normal range. These grades apply to adults; children and teenagers are assessed with BMI-for-age charts instead.
DSM-5 severity bands
When a clinician has already diagnosed anorexia nervosa, the American Psychiatric Association's DSM-5 manual uses BMI to describe the current level of severity in adults:
DSM-5 bands
- Mild: BMI 17 or more
- Moderate: BMI 16 to 16.99
- Severe: BMI 15 to 15.99
- Extreme: BMI below 15
The manual notes that the level may be raised to reflect other symptoms and the degree of functional disability. These bands describe severity after a diagnosis; they are not a test for one, and this calculator does not apply them.
How BMI is calculated
For example, 50 kg at 1.75 m gives 50 ÷ 3.0625 = 16.3, which the WHO grades as moderate thinness.
Privacy
Nothing is uploaded. Every calculation runs in your browser, and what you enter never leaves your device or gets saved.
Related tools
The standard BMI Calculator shows every WHO class, and the Healthy Weight Calculator shows the healthy weight range for a height in kilograms and pounds. The Mass Converter converts between kilograms, pounds and stones.
Sources
- World Health Organization. Physical status: the use and interpretation of anthropometry. WHO Technical Report Series 854, 1995.
- World Health Organization. Obesity: preventing and managing the global epidemic. WHO Technical Report Series 894, 2000.
- American Psychiatric Association. Diagnostic and Statistical Manual of Mental Disorders, fifth edition (DSM-5), 2013, anorexia nervosa severity specifiers.
Open-source credits
The calculations are implemented in-house in TypeScript, with no third-party library. They follow the standard BMI formula and the WHO adult thinness grades listed above.