BMI Calculator (with WHO classification)
Calculate Body Mass Index with WHO categories. Built by a Registered Nurse working in Australia.
The formula
BMI is a screening tool, not a diagnosis. It does not account for muscle mass, fat distribution, age or ethnicity.
WHO classification
| BMI range (kg/m²) | Category | Notes |
|---|---|---|
| < 18.5 | Underweight | Screen for nutritional deficiency, eating disorders |
| 18.5–24.9 | Normal weight | Lowest associated mortality risk in adults |
| 25.0–29.9 | Overweight | Increased cardiovascular and metabolic risk |
| 30.0–34.9 | Obesity class I | Substantial risk increase |
| 35.0–39.9 | Obesity class II | High risk |
| ≥ 40.0 | Obesity class III | Very high risk |
Frequently asked questions
Is BMI a reliable measure of health?
BMI is a useful population-level screening tool but limited at the individual level. It doesn’t distinguish fat from muscle, doesn’t account for fat distribution (visceral vs subcutaneous), and uses cut-offs derived primarily from European populations.
What’s a healthy BMI for older adults?
In adults over 65, a slightly higher BMI (24–29) is associated with lower mortality than the standard “normal” range. Low BMI in the elderly is a stronger predictor of poor outcomes than mild overweight.
Should I use BMI for children?
No — children and adolescents use BMI-for-age percentiles (CDC or WHO growth charts), not the adult cut-offs in this calculator.