BMI is the world's most quoted health number — here is what it actually measures, and where it falls short.
Body Mass Index (BMI) is the world's most-quoted single health number. Doctors use it, insurers use it, fitness apps use it. But for a number that drives so many decisions, surprisingly few people actually know how it is calculated, what its categories mean, or why it is sometimes wildly misleading. This guide walks you through all three.
BMI is a ratio of weight to height — a quick way to flag whether someone might be underweight, healthy weight, overweight, or obese. It was invented in the 1830s by Belgian statistician Adolphe Quetelet, which is why you sometimes see it called the *Quetelet Index*. It was never designed for individual diagnosis; Quetelet built it to describe populations.
The metric formula is simple:
BMI = weight (kg) ÷ height (m)²
If you prefer imperial units:
BMI = (weight in lb ÷ height in inches²) × 703
Say you weigh 75 kg and are 1.75 m tall. BMI = 75 ÷ (1.75 × 1.75) = 75 ÷ 3.0625 = 24.5 — squarely inside the healthy range.
Some health bodies use lower thresholds for people of South Asian or East Asian descent because cardiometabolic risk rises at lower BMIs in those populations. A BMI of 23 may be flagged as overweight on those revised charts.
BMI knows nothing about your body composition. A 1.80 m rugby player with 95 kg of muscle is classified obese (BMI 29.3). A frail 65-year-old with poor muscle mass may sit in the healthy range while carrying dangerous visceral fat. BMI also ignores:
Two people with identical BMIs can have very different health outlooks. That is why modern clinicians pair BMI with at least one other measure — waist-to-hip ratio, body-fat percentage, or simply a tape measure around the navel.
Treat BMI as a screening tool, not a diagnosis. If yours falls outside the healthy range, the next steps are a conversation with your doctor, an honest look at your activity and nutrition, and a check on related markers like blood pressure, fasting glucose and lipids. If your BMI is normal but you carry significant abdominal fat, do not assume you are off the hook — visceral fat is a stronger predictor of metabolic risk than BMI alone.
Is BMI accurate for athletes? No. Heavily muscled people are routinely misclassified as overweight or obese. Use body-fat percentage instead.
Is BMI different for men and women? The formula is identical, but women naturally carry a higher percentage of body fat. Category thresholds are the same.
Does BMI work for children? Children have their own BMI-for-age percentile charts; the adult cutoffs do not apply.
Ready to check yours? Use our free BMI Calculator — it auto-adapts to metric or imperial units and adds a healthy-weight range based on your height.