BMI introduction
BMI estimates leanness or corpulence from height and weight. It is a widely used screen for whether body weight is in a healthy range for height — underweight, normal, overweight, or obese — with cut-offs that can vary by region and age. It is imperfect alone, but useful for deciding whether further assessment is warranted.
BMI table for adults
World Health Organization (WHO) adult categories for people age 20+, used for both men and women in this calculator.
| Classification | BMI range — kg/m² |
|---|---|
| Severe Thinness | < 16 |
| Moderate Thinness | 16 – 17 |
| Mild Thinness | 17 – 18.5 |
| Normal | 18.5 – 25 |
| Overweight | 25 – 30 |
| Obese Class I | 30 – 35 |
| Obese Class II | 35 – 40 |
| Obese Class III | > 40 |
BMI for children and teens (ages 2–20)
The CDC classifies children and teens by BMI-for-age percentiles, not the adult kg/m² bands above. This tool still computes BMI for ages 2–19 and shows adult categories for reference only — ask a clinician for percentile-based interpretation.
| Category | Percentile range |
|---|---|
| Underweight | < 5% |
| Healthy weight | 5% – 85% |
| At risk of overweight | 85% – 95% |
| Overweight | > 95% |
Risks associated with being overweight
According to the CDC, excess weight is linked to higher risk of high blood pressure, dyslipidemia, type 2 diabetes, coronary heart disease, stroke, gallbladder disease, osteoarthritis, sleep apnea, certain cancers, reduced quality of life, and higher mortality versus a healthy BMI. Aim to discuss lifestyle changes with a doctor rather than relying on BMI alone.
Risks associated with being underweight
Underweight status can involve malnutrition, anemia, osteoporosis, weaker immunity, growth issues in youth, reproductive hormone disruption, surgical complications, and higher mortality. Unexplained low weight warrants medical evaluation — it can signal underlying illness or disordered eating.
Limitations of BMI
BMI estimates excess weight, not body fat. Age, sex, ethnicity, muscle mass, and activity change how the same BMI should be read. Older adults and women often carry more fat at a given BMI; muscular athletes may score “overweight” while remaining metabolically healthy. Use BMI with waist measures, labs, and clinical judgment — especially for children, where maturation and height matter.
BMI formula
Metric: BMI = mass (kg) ÷ height² (m). US: BMI = 703 × mass (lbs) ÷ height² (in).
BMI Prime
BMI Prime is measured BMI divided by 25 kg/m² (the usual upper “normal” limit). Values below ~0.74 map to underweight bands; 0.74–1 to normal; above 1 to overweight; above ~1.2 toward obesity classes. It is dimensionless and useful for comparing how far someone sits from the normal upper bound.
Ponderal Index
Ponderal Index uses height cubed (metric: kg / m³) instead of squared. It can be steadier than BMI for very tall or short adults, though it shares many of BMI's body-composition limits.
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Disclaimer: This calculator is educational, not a diagnosis. It does not replace medical advice. If you have health concerns, consult a qualified clinician.
