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Evidence-Based Clinical Tool
BMI & BSA Calculator
Calculates Body Mass Index and Body Surface Area from height and weight.
BMI screens for weight categories associated with health risks. BSA (Mosteller formula) is widely used for drug dosing, chemotherapy, and cardiac index calculations. Note that BMI thresholds differ for Asian populations per WHO/NIH guidelines.
Units
Formulas
BMI = weight (kg) ÷ height² (m²)
BSA = √[ height (cm) × weight (kg) ÷ 3600 ]
Height
cm
Enter height in centimetres
cm
100220 cm
Weight
kg
Enter weight in kilograms
kg
30200 kg
Enter height and weight to calculate BMI and BSA
BMI Scale
BMI Classification
| BMI (kg/m²) | Category | Health Risk |
|---|---|---|
| < 18.5 | Underweight | Malnutrition, osteoporosis, immune deficiency risk |
| 18.5 – 24.9 | Normal weight | Lowest risk; maintain through balanced diet & activity |
| 25.0 – 29.9 | Overweight | Moderate risk of CVD, T2DM, hypertension |
| 30.0 – 34.9 | Obese Class I | High risk; lifestyle intervention recommended |
| 35.0 – 39.9 | Obese Class II | Very high risk; medical management indicated |
| ≥ 40.0 | Obese Class III | Severely elevated risk; consider bariatric referral |
* For Asian populations (WHO/NIH): overweight ≥23 kg/m²; obesity ≥25 kg/m². Standard thresholds may underestimate cardiovascular risk in these individuals.
1
BMI does not distinguish fat from muscle
BMI is a weight-to-height ratio and cannot differentiate between fat mass and lean muscle. Highly muscular individuals (athletes, bodybuilders) may be classified as overweight or obese despite low body fat. Conversely, those with sarcopenic obesity may have a normal BMI while carrying excess adipose tissue. Body composition analysis (DEXA, bioimpedance) provides more accurate assessment.
2
BMI thresholds are not universal across ethnicities
WHO and NIH guidelines recommend lower BMI cut-offs for Asian populations, where cardiometabolic risk increases at BMI ≥23 kg/m² (overweight) and ≥25 kg/m² (obesity). Standard thresholds significantly underestimate risk in South Asian, East Asian, and Southeast Asian individuals. Waist circumference and waist-to-hip ratio add valuable context.
3
BMI is unreliable in children, elderly, and pregnant patients
In children and adolescents, BMI must be interpreted using age- and sex-specific percentile charts, not adult thresholds. In the elderly, BMI underestimates adiposity due to age-related muscle loss (sarcopenia). During pregnancy, pre-pregnancy BMI is the relevant metric; current weight includes fetal and fluid contributions.
4
BSA formulas carry inherent inter-formula variability
Multiple BSA formulas exist (Mosteller, DuBois & DuBois, Haycock, Boyd), each producing slightly different results. The Mosteller formula used here is widely adopted in clinical practice for its simplicity and accuracy, but differences of 5–10% across formulas can occur. For precision chemotherapy dosing, always confirm which formula your institution uses.
5
BMI does not capture fat distribution
Abdominal (visceral) adiposity carries greater cardiovascular and metabolic risk than peripheral fat, independent of total BMI. Two individuals with identical BMIs can have very different risk profiles depending on where fat is stored. Waist circumference (>102 cm in men, >88 cm in women) or waist-to-height ratio (>0.5) are complementary metrics.
6
BSA-based dosing has limitations in obesity
For obese patients, BSA-based chemotherapy dosing is debated. Some regimens use actual body weight to calculate BSA (which increases dose), while others cap BSA at 2.0 m² or use ideal/adjusted body weight. Always follow institutional or oncology-specific protocol when dosing cytotoxic drugs in patients with BMI ≥30 kg/m².
References
World Health Organization. Obesity: preventing and managing the global epidemic. Report of a WHO consultation. WHO Technical Report Series. 2000;894:1–253.
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.
National Heart, Lung, and Blood Institute. Clinical guidelines on the identification, evaluation, and treatment of overweight and obesity in adults. NIH Publication No. 98-4083. 1998.
This calculator is provided for educational and informational purposes only. BMI and BSA are screening and estimation tools with known limitations. They do not replace clinical assessment, body composition analysis, or direct consultation with a qualified healthcare professional.
