Body Mass Index (BMI)
Classifies weight status based on height and weight
Calculator
Why use it
Quick, universally recognized screening tool for weight-related health risks. Required for many clinical protocols, medication dosing adjustments, and surgical risk assessments.
About
The Body Mass Index (BMI) is a simple metric derived from height and weight, widely used to classify individuals into weight categories. While it does not directly measure body fat, it correlates with more direct measures of body fat and is used as a screening tool for weight-related health problems. BMI is used globally by the WHO and many national health organizations as a first-line screening tool.
When to use
- Routine health screening and wellness visits
- Assessment before surgical procedures
- Nutritional assessment and counseling
- Eligibility for bariatric surgery evaluation (BMI ≥ 40 or ≥ 35 with comorbidities)
- Medication dosing that requires weight classification
Formula
Pearls & pitfalls
- BMI does not distinguish between muscle and fat mass — athletes may have elevated BMI with low body fat.
- In elderly patients, BMI may underestimate adiposity due to loss of height and lean mass.
- Asian populations have higher metabolic risk at lower BMI cutoffs (overweight ≥ 23, obesity ≥ 27.5 per WHO Asia-Pacific).
- BMI should be complemented with waist circumference for better cardiovascular risk assessment.
- In children and adolescents, use age- and sex-specific BMI percentiles instead of fixed cutoffs.
Next steps
For BMI ≥ 25, consider waist circumference measurement, metabolic panel, and lifestyle counseling. For BMI ≥ 30, evaluate for obesity-related comorbidities and discuss treatment options. For BMI < 18.5, evaluate for underlying causes of low weight.
Evidence
BMI correlates with body fat percentage (r ≈ 0.7–0.8) and predicts all-cause mortality in a J-shaped curve. Large meta-analyses (Lancet 2016, >10 million participants) confirmed increased mortality risk at BMI < 20 and BMI > 25. WHO classification adopted in 1995 and revised in 2000.