BMI: What It Measures and When It Lies

By Hutu Laurentiu

5 min read · Updated October 2026

Body Mass Index is one number, one formula, and a century of arguments. It's genuinely useful for populations and genuinely misleading for many individuals — knowing both sides is the point of this guide.

The formula (with correct units)

BMI = weight (kg) ÷ height² (m²)

Worked example: 70 kg, 1.75 m → 70 ÷ (1.75×1.75) = 70 ÷ 3.0625 = 22.9. Metric first, always. The imperial shortcut: BMI = 703 × pounds ÷ inches². 155 lb, 69 in → 703×155÷4761 = 22.9 — same person, same answer. If you get a BMI of 2 or 200, you mixed the units.

The categories (WHO standard)

• Below 18.5 — underweight
• 18.5-24.9 — healthy range
• 25-29.9 — overweight
• 30 and above — obesity (class I 30-34.9, II 35-39.9, III 40+)

Our example's 22.9 sits mid-range. The scale is deliberately simple; its inventor (Lambert Adolphe Quetelet, 1830s) was an astronomer building population statistics, not a clinician diagnosing individuals — remember that origin and the edge cases make sense.

Where BMI lies: the documented cases

• Muscular people. Muscle is denser than fat. Many athletes cross BMI 25-30 at healthy body-fat percentages — by BMI alone, a rugby prop is "obese". The number can't tell tissue types apart.
• Height extremes. The formula scales weight with height², but body shape doesn't. Tall people get under-called; short people over-called. Corrections exist (new BMI divides by height^2.5) but aren't standard.
• Elderly. Adults over 65 may have healthiest outcomes at BMI 23-29 — slightly above the standard "healthy" band — partly because some reserves help through illness.
• Ethnic differences. At the same BMI, body-fat and metabolic risk differ across populations. WHO notes several Asian populations face elevated risk at BMI ≥23, which is why some countries use adjusted thresholds.
• Fat distribution. Waist circumference and waist-to-height ratio (waist ÷ height, keep under ~0.5) predict metabolic risk better than BMI — a "pear" and an "apple" can share a BMI with very different risk.

What to use it for anyway

BMI is free, instant, and directionally useful: tracking your own number over months shows real trend. For populations, it's excellent — public health depends on it. Just don't let a single label define health: pair it with waist measure, blood pressure, activity, and for anything medical, a professional who can see you, not just your number.

Common questions

Does BMI ignore age and sex? The formula ignores both; adult thresholds are the same for men and women. Children use age-and-sex percentile charts instead (BMI-for-age), never the adult table.

Why 703 in the imperial formula? It's the unit-conversion constant (kg/m² ↔ lb/in²) rounded to an integer — pure algebra, no magic.