Medical Calculators › General › BMI / BSA / IBW

BMI / BSA / IBW

Comprehensive Body Anthropometric Calculator

Calculates BMI (with WHO classification), Body Surface Area (Mosteller and Du Bois methods), Ideal Body Weight (Devine formula), Adjusted Body Weight (for obesity), and Dosing Weight in a single tool. Essential for drug dosing, nutritional assessment, and chemotherapy.

Drug DosingNutritionOncology
Anthropometric Calculator

Formulas Used

BMIWeight (kg) ÷ Height² (m²)
BSA (Mosteller)√(Height (cm) × Weight (kg) / 3600) — preferred for clinical use
BSA (Du Bois)0.007184 × Height (cm)⁰·⁷²⁵ × Weight (kg)⁰·⁴²⁵
IBW (Devine)Male: 50 + 0.905 × (Ht cm − 152.4)  ·  Female: 45.5 + 0.905 × (Ht cm − 152.4)
Adjusted BWIBW + 0.4 × (Actual BW − IBW) — used when actual weight >IBW by >30%
Dosing WeightThe weight to use for dosing — IBW if actual ≤ IBW; adjusted BW if obese (>30% over IBW) for most drugs

BMI Classification (WHO)

BMICategoryClinical Notes
< 18.5UnderweightNutritional assessment; screen for malabsorption, eating disorder, malignancy
18.5–24.9NormalHealthy weight range
25.0–29.9OverweightLifestyle modification; cardiovascular risk assessment
30.0–34.9Obese (Class I)Structured weight management; screen for T2DM, OSA, NAFLD
35.0–39.9Obese (Class II)Bariatric evaluation; intensive lifestyle intervention
≥ 40.0Obese (Class III)Bariatric surgery consideration; multidisciplinary metabolic clinic

For Asian populations (including Indian patients), lower thresholds apply: BMI ≥ 23 = overweight, ≥ 27.5 = obese — at these levels, cardiometabolic risk is equivalent to Western thresholds of 25 and 30 respectively.

Clinical Application

  • Dosing weight: Many drugs — including aminoglycosides, vancomycin, chemotherapy — should NOT be dosed to actual body weight in obesity. Use IBW if patient is not obese; if obese (actual > 130% IBW), use adjusted BW (IBW + 0.4 × excess). Specific drugs have specific rules — always check individual drug dosing guidelines.
  • BSA for chemotherapy: Most cytotoxic agents are dosed per m² BSA. Mosteller's formula is the most commonly used. Note that at extremes of body weight (very low or very high BSA), BSA-based dosing may be inappropriate — discuss with medical oncology.
  • Perioperative nutrition: BMI alone does not diagnose malnutrition. Use formal assessment tools (MUST, NRS-2002) and check functional markers (grip strength, recent weight loss, clinical frailty).
  • Bariatric surgery criteria (NICE/ASMBS): BMI ≥ 40, OR ≥ 35 with obesity-related comorbidities (T2DM, OSA, HTN, NASH), after failure of structured conservative management for at least 6 months.
  • Asian-specific thresholds (BMI ≥23 overweight, ≥27.5 obese) are recommended by WHO and Indian guidelines for risk stratification in South Asian patients.

References

  1. WHO. Physical status: the use and interpretation of anthropometry. WHO Technical Report Series No. 854. Geneva, 1995.
  2. Mosteller RD. Simplified calculation of body surface area. N Engl J Med. 1987;317(17):1098.
  3. Devine BJ. Gentamicin therapy. Drug Intelligence and Clinical Pharmacy. 1974;8:650–655.
  4. WHO Expert Consultation. Appropriate BMI for Asian populations. Lancet. 2004;363(9403):157–163.

Related Calculators

eGFR Caprini VTE Risk Charlson CCI