Free tool
BMI Calculator.
Body Mass Index in metric or imperial, with the WHO weight category and a plain-English read — and the two numbers a nurse actually reaches for that consumer calculators leave out: body surface area (BSA), used to dose chemo and index cardiac output, and ideal body weight (IBW), the weight many drugs (and creatinine clearance) are calculated from. Everything runs on your device. No PHI leaves the page.
Body Mass Index
Pick your units, enter height and weight. Sex is optional — it's only used for ideal body weight.
Enter a height and weight.
BMI categories (WHO, adults)
| BMI (kg/m²) | Category | Notes |
|---|---|---|
| < 18.5 | Underweight | May signal undernutrition, malabsorption, or a catabolic illness. Correlate with intake and trend. |
| 18.5–24.9 | Normal weight | The healthy reference range for most adults. |
| 25.0–29.9 | Overweight | Increased cardiometabolic risk. Body composition matters — see the caveats below. |
| 30.0–34.9 | Obesity class I | Higher risk of diabetes, hypertension, and cardiovascular disease. |
| 35.0–39.9 | Obesity class II | Substantially increased risk; consider obesity-related comorbidity screening. |
| ≥ 40.0 | Obesity class III | Highest-risk category (formerly "morbid obesity"). |
Asian populations: the WHO notes that cardiometabolic risk rises at lower BMI in many Asian groups, so some guidelines use lower cut-points — overweight at ≥ 23 and obesity at ≥ 27.5 kg/m². Use the threshold your facility or population guideline specifies.
BSA and IBW — why they're here
Body surface area (BSA) is shown using the Mosteller formula, BSA (m²) = √(heightcm × weightkg ÷ 3600). It's the denominator for chemotherapy dosing (mg/m²) and for indexed hemodynamics like cardiac index. Typical adult BSA is roughly 1.6–2.0 m².
Ideal body weight (IBW) uses the Devine formula and needs a sex and a height ≥ 5 ft. IBW (and adjusted body weight) is what many weight-based drugs — and the Cockcroft-Gault creatinine clearance — are actually calculated from, because dosing to a large actual weight can overshoot. If you entered a sex and height, IBW appears with your result; tap through to the creatinine clearance tool to use it.
References
- World Health Organization. Obesity: preventing and managing the global epidemic. WHO Technical Report Series 894. Geneva: WHO; 2000. (Adult BMI classification.)
- WHO Expert Consultation. Appropriate body-mass index for Asian populations and its implications for policy and intervention strategies. Lancet. 2004;363(9403):157–163.
- Mosteller RD. Simplified calculation of body-surface area. N Engl J Med. 1987;317(17):1098.
- Devine BJ. Gentamicin therapy. Drug Intell Clin Pharm. 1974;8:650–655. (Origin of the ideal body weight formula.)
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