Anorexic BMI Calculator · Practical guide
What Is a Dangerously Low BMI?
A BMI below 16 is classified as severe thinness and signals critically low body weight.
At a glance
The Short Answer
A BMI below 16.0 is considered dangerously low. The World Health Organization labels this range severe thinness, and it indicates a critically low body weight that may pose serious health risks. A BMI between 16 and 16.99 (moderate thinness) is also a warning sign that deserves medical attention.
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Anorexic BMI Calculator
Enter your own values, then compare the result with the examples below.
Results
WHO Thinness Thresholds
| BMI Range | Category | Risk Level |
|---|---|---|
| Below 16.0 | Severe Thinness | Critical — seek medical advice |
| 16.0 – 16.99 | Moderate Thinness | Significantly underweight |
| 17.0 – 18.49 | Mild Thinness | Underweight |
| 18.5 – 24.9 | Normal Weight | Healthy range |
What These Numbers Look Like in Practice
For someone who is 5 foot 5 (165 cm), weighing 90 lb produces a BMI of 15.0 — severe thinness. At 100 lb the BMI is 16.6, which is moderate thinness. The healthy weight range for that height starts at about 111 lb and runs to roughly 150 lb, so anything below 111 lb falls into one of the underweight categories above.
What To Do If Your BMI Is Very Low
A low BMI is a screening signal, not a diagnosis. If your result falls in the severe or moderate thinness range, seek medical or nutritional support — early intervention matters for recovery from underweight conditions. This calculator is an awareness tool only and does not replace evaluation by a healthcare professional.
Frequently Asked Questions
Is a BMI of 17 dangerous?
A BMI of 17 falls in the mild thinness category (17-18.49). It is below the healthy range but not classified as dangerously low; still, it is worth discussing with a doctor.
What is the lowest healthy BMI?
The healthy range starts at a BMI of 18.5. Anything below that is considered underweight by WHO standards.
Do these thresholds apply to teenagers?
No. The standard WHO BMI categories apply to adults aged 18-65. Children, teens, and elderly adults should be assessed with different reference data.
Can someone be naturally thin with a low BMI?
Some people are naturally lean without health problems, but a persistently low BMI combined with fatigue, hair loss, or missed periods warrants a medical evaluation.