Body Mass Index (BMI)
Calculate body mass index from height and weight using the standard WHO formula (weight in kg divided by height in metres squared). BMI is the most widely used screening measure for weight classification in adults.
inputs
✓ when to use
Use for adults aged 18 and over as a first-line population screening tool for weight status. Appropriate in primary care, preventive health checks, pre-operative assessment, and epidemiological surveillance. Can be used at any clinical encounter where weight status is relevant.
✗ when not to use
BMI does not distinguish fat mass from lean mass. Use with caution in athletes and individuals with high muscle mass (may overestimate adiposity), older adults with sarcopenia (may underestimate), pregnancy, oedematous states, and amputees (use adjusted formulae). In Asian populations, consider lower thresholds for overweight (≥23) and obesity (≥27.5) as recommended by the WHO Asia-Pacific guidelines. BMI alone should not be used to diagnose obesity — clinical assessment including waist circumference and comorbidity evaluation is essential.
clinical pearls
- BMI is a screening tool, not a diagnostic test. Two patients with identical BMIs can have vastly different body compositions, metabolic risk profiles, and health outcomes.
- In South Asian, Chinese, and other Asian populations, cardiometabolic risk increases at lower BMI thresholds. The WHO Asia-Pacific guidelines suggest overweight at ≥23 and obesity at ≥27.5 — consider using these thresholds for patients of Asian heritage.
- The 'obesity paradox' — where overweight or mildly obese patients sometimes show better outcomes in certain acute conditions (heart failure, post-MI, dialysis) — is a well-recognised epidemiological finding. It does not mean excess adiposity is protective; it likely reflects confounders including reverse causation and selection bias.
- In elderly patients (>65), a slightly higher BMI (25–27) may be associated with lower mortality than the 'normal' range, likely due to the protective effect of nutritional reserve against sarcopenia and frailty.
- Always measure waist circumference alongside BMI. Central adiposity (waist ≥94cm men, ≥80cm women in European populations) is a stronger predictor of cardiometabolic risk than BMI alone.