Low BMI Reference Calculator
Calculate your BMI and see the WHO underweight severity reference band (mild, moderate, or severe thinness) for adults. This is a descriptive reference, not a diagnosis of anorexia or any eating disorder.
How It's Calculated
Formula
\text{BMI} = \dfrac{\text{weight (kg)}}{\text{height (m)}^2}This calculator computes BMI using the same formula as this site's BMI Calculator (weight in kilograms divided by height in meters squared), then applies the World Health Organization's severity sub-classification of the underweight range: severe thinness (BMI under 16.0), moderate thinness (16.0 to 16.9), and mild thinness (17.0 to 18.49). These bands are a finer-grained breakdown of the same "Underweight" category (BMI under 18.5) that this site's BMI Calculator already reports as a single bucket — this tool exists to show which part of that range a low BMI falls into, using a published, source-backed framework, not a new formula. This is an estimate for general informational and educational purposes, based on a published formula — it is not a medical diagnosis, and it is not a substitute for individualized advice from a qualified healthcare provider. This calculator does not diagnose anorexia nervosa or any eating disorder, does not calculate a weight-loss or target weight figure, and does not provide instructions for reaching a lower BMI. A low BMI has many possible causes, and severity bands here are a population-screening reference only — see a qualified healthcare provider for individual evaluation. Like BMI itself, this figure cannot distinguish muscle mass from fat mass or account for individual body composition. The adult severity bands apply to ages 20 and up, matching this site's BMI Calculator's own adult-scope rule.
Worked Examples
Height 170 cm, weight 43.35 kg (BMI 15.0)
- BMI = 43.35 / (1.70 × 1.70) = 43.35 / 2.89 = 15.0
- 15.0 is under 16.0, so the band is Severe thinness (WHO underweight severity band)
Height 170 cm, weight 52 kg (BMI ≈ 18.0)
- BMI = 52 / 2.89 ≈ 18.0
- 18.0 is between 17.0 and 18.49, so the band is Mild thinness (WHO underweight severity band)
Frequently Asked Questions
Does this diagnose anorexia or an eating disorder?
No. BMI alone cannot diagnose anorexia nervosa or any eating disorder. This calculator only reports a BMI number and a descriptive WHO severity band for low BMI values — it is not a diagnostic tool and does not assess psychological or behavioral criteria.
How is this different from the site's BMI Calculator?
The BMI Calculator reports a single flat "Underweight" category for any BMI under 18.5. This calculator uses the same BMI number but breaks that range into three WHO severity bands (mild, moderate, severe thinness) for more detail when a BMI is low.
Does this tell me how much weight to gain?
No. This calculator does not compute a target weight or provide instructions for changing your BMI. It reports where your current BMI falls on a published reference scale.
Why does age matter here?
The WHO adult severity bands apply to ages 20 and up, the same adult-scope rule this site's BMI Calculator uses. Under age 20, your BMI is still computed and shown, but without an adult severity band attached.