Body Mass Index is calculated by dividing your weight in kilograms by the square of your height in meters. It takes about 10 seconds to compute and is used by doctors, insurers, researchers, and governments worldwide. It is also frequently misused, misunderstood, and applied to people it was never designed for.
How BMI Is Calculated
The formula: BMI = weight(kg) / height(m)² — or in US units: BMI = (weight(lbs) × 703) / height(in)². The standard categories are: Under 18.5 = Underweight, 18.5–24.9 = Normal weight, 25–29.9 = Overweight, 30+ = Obese.
What BMI Gets Right
At the population level, BMI correlates reasonably well with body fat percentage and health risks. It's free, instant, and requires no equipment. Studies consistently show that people with BMI over 30 have higher rates of type 2 diabetes, cardiovascular disease, and certain cancers. As a screening tool — a first-pass flag — it does its job.
What BMI Gets Wrong
BMI cannot distinguish between fat and muscle. An elite athlete with very low body fat can register as "overweight" because muscle is dense. Conversely, someone with a "normal" BMI can carry dangerous levels of visceral fat. The formula also doesn't account for age, sex, or ethnicity — a 25-year-old and a 65-year-old with identical BMIs have very different health profiles.
💡 A better picture: Waist circumference combined with BMI is more predictive of metabolic health than either measure alone. Waist over 35" (women) or 40" (men) is a stronger risk signal than BMI category.
The BMI Categories in Context
- Under 18.5 (Underweight) — associated with nutritional deficiencies, bone density loss, and immune issues
- 18.5–24.9 (Normal) — lowest all-cause mortality in most studies
- 25–29.9 (Overweight) — modestly elevated risk; some studies show similar mortality to "normal" in older adults
- 30+ (Obese) — meaningfully elevated risk for multiple conditions; higher subcategories (35+, 40+) carry greater risk
How to Use Your BMI Number
Think of it as one data point, not a verdict. If your BMI puts you in a concerning category, it's worth a conversation with a doctor — especially combined with waist measurement, blood pressure, blood glucose, and cholesterol. If your BMI is "normal" but you feel unwell or have other risk factors, take those seriously too.