What the 17.5 threshold means — and what it doesn't
The number this calculator checks against comes from clinical literature: historically, diagnostic manuals referenced a BMI below about 17.5 as one indicator consistent with anorexia nervosa in adults, alongside the WHO's broader underweight line at 18.5 and severe thinness below 16. Those numbers are screening thresholds. They are not, and have never been, a diagnosis.
That distinction matters in both directions. A low BMI can come from many causes — genetics, illness, medication, high activity with high metabolism — and plenty of naturally slim people live healthily below 18.5. Equally, eating disorders exist at every body weight: current diagnostic practice explicitly recognises that a person can be severely ill at a normal or high BMI. A number on this page can prompt a conversation with a professional; it cannot replace one.
Why a persistently low BMI deserves attention
When body weight stays well below the healthy range for a long time, the body starts making trade-offs: bone density declines, hormone production slows (periods often stop), the heart muscle can weaken, and concentration and mood suffer. These changes arrive gradually and are easy to normalise from the inside — which is precisely why an outside check-in with a doctor is worth so much. Most of these effects are reversible with restored nutrition, and the earlier that starts, the more completely they reverse.
If your result here is low and unintentional — you've been losing weight without trying — that's a medical question with many possible answers, and a GP visit is the right next step regardless of eating habits.
If food and weight have become a struggle
Some people arrive at this page out of curiosity. Others arrive because eating has become complicated and frightening, and checking numbers has become a habit. If the second one sounds familiar, please know two things: eating disorders are genuinely treatable, and reaching out earlier makes treatment shorter and easier. You don't need to be 'sick enough' to deserve help — that thought is itself a symptom.
- In the US, the National Eating Disorders Association (NEDA) offers screening tools and treatment referrals at nationaleatingdisorders.org.
- The National Alliance for Eating Disorders runs a free helpline with licensed clinicians (1-866-662-1235).
- In a crisis — fainting, chest pain, or thoughts of self-harm — contact emergency services or the 988 Suicide & Crisis Lifeline (call or text 988 in the US) right away.
- Anywhere in the world, a family doctor is a confidential, judgment-free starting point who can refer you to specialists.
For neutral context on where a weight sits relative to the full healthy range, the BMI calculator and healthy weight calculator show the whole picture rather than the low end alone.
Frequently asked questions
What BMI ranges are associated with underweight?
WHO classifies adult BMI below 18.5 as underweight, with severe thinness below 16. These are screening categories, not diagnoses.
Can this tool diagnose an eating disorder?
No. Eating disorders require clinical evaluation. Low BMI can have many causes — only a qualified professional can diagnose and treat.
When should I seek help urgently?
Seek help immediately if you have rapid weight loss, fainting, chest pain, severe restriction, or thoughts of self-harm. Contact a doctor or crisis line.
How do I use this calculator?
Enter height and weight to see your BMI and how it compares to clinical underweight thresholds.
Sources and support resources
- National Eating Disorders Association (NEDA)
- National Institute of Mental Health — Eating disorders
- World Health Organization — BMI classifications
This page is for general information only and is not medical advice or a diagnostic tool. If you're worried about your eating or weight, please talk to a healthcare professional.