BMI formula, limitations, and alternatives
BMI = weight(kg) / height(m)². For imperial: BMI = weight(lbs) / height(in)² × 703. The World Health Organization classifies BMI as underweight (<18.5), normal (18.5-24.9), overweight (25-29.9), and obese (≥30). These ranges were developed using predominantly European populations, and the cutoff points do not account for muscle mass, bone density, fat distribution, or ethnic differences. For example, Asian populations have higher health risks at lower BMI thresholds (23+ for overweight).
BMI is a screening tool, not a diagnostic one. A bodybuilder with 10% body fat and high muscle mass may have a BMI of 30+ (classified as obese). Conversely, an older adult with low muscle mass may have a "normal" BMI while having dangerously high body fat percentage. For more accurate health assessments, combine BMI with waist circumference, body fat percentage (DEXA or caliper), and blood biomarkers.
Implementing BMI calculation in your app
If you are building a health or fitness application, the BMI implementation is straightforward but has several UX considerations. Always validate that height > 0 and weight > 0, and provide clear error messages for nonsensical inputs (like 1200 kg or 0.5 m). Use toFixed(1) for display (BMI is conventionally shown to one decimal place). Consider adding unit toggles (metric/imperial) with automatic conversion rather than separate input fields.
For accessibility in health apps, express BMI results with appropriate color coding (green for normal, yellow for overweight, red for obese) but do not use color alone — also add text labels and icons. Never use alarmist language about weight classification; instead, frame results as "your BMI falls in the X range" and suggest consulting a healthcare provider for personalized assessment.