BMI: What It Measures, How to Calculate It, and Its Limits
What BMI measures
BMI — body mass index — is a single number that compares your weight to your height. It answers one narrow question: relative to how tall you are, is your weight in a typical range? It does not measure body fat, fitness or health directly. It is a fast screening number that flags when a closer look is worth it.
The index was invented in the 1830s by Adolphe Quetelet, a Belgian statistician — not a physician — who wanted a tidy way to describe the "average man" in population studies. It became a standard health screening tool more than a century later, and that origin matters when you interpret your own number, as the last section explains.
The BMI formula
In metric units, divide your weight in kilograms by the square of your height in meters. The height gets squared because taller people are not just taller — they are also proportionally wider and deeper, so weight grows roughly with height squared, and the formula divides it back out.
In imperial units, weight in pounds over height in inches squared gives a number about 703 times too small, so the formula carries a built-in factor of 703 to match the metric result.
BMI = weight (kg) ÷ height (m)² or BMI = 703 × weight (lb) ÷ height (in)²A person weighing 70 kg at 1.75 m
Square the height: 1.75 × 1.75 = 3.0625. Divide the weight by it: 70 ÷ 3.0625 = 22.86.
BMI ≈ 22.9 — in the normal range
The same person in imperial units
70 kg is about 154.3 lb and 1.75 m is about 68.9 in. Apply the imperial formula: 703 × 154.3 = 108,472.9; then 68.9 × 68.9 = 4,747.2; and 108,472.9 ÷ 4,747.2 ≈ 22.9. Both unit systems land on the same number.
BMI ≈ 22.9 — matches the metric result
The WHO weight categories
The World Health Organization sorts adult BMI into four main bands, the same for men and women. Obesity is split into three classes because health risk climbs steeply at the higher classes:
Read the bands as screening labels, not verdicts. A BMI of 23 and a BMI of 18.6 are both "normal," but they describe quite different bodies. The number tells a doctor where to look next — waist size, blood pressure, blood sugar — not what to conclude.
| Category | BMI range (kg/m²) |
|---|---|
| Underweight | below 18.5 |
| Normal weight | 18.5 – 24.9 |
| Overweight | 25.0 – 29.9 |
| Obesity class I | 30.0 – 34.9 |
| Obesity class II | 35.0 – 39.9 |
| Obesity class III | 40.0 and above |
Finding your healthy weight range
Because BMI = weight ÷ height², the formula runs backwards: multiply the BMI boundaries by your height squared, and you get the weight range that corresponds to the normal band for your height.
weight = BMI × height²Healthy weight range at 1.75 m
Height squared: 1.75 × 1.75 = 3.0625. Lower limit: 18.5 × 3.0625 = 56.7 kg. Upper limit: 24.9 × 3.0625 = 76.3 kg.
About 56.7 – 76.3 kg
Healthy weight range at 1.60 m
Height squared: 1.60 × 1.60 = 2.56. Lower limit: 18.5 × 2.56 = 47.4 kg. Upper limit: 24.9 × 2.56 = 63.7 kg.
About 47.4 – 63.7 kg
The limits of BMI
Muscle is denser than fat, so BMI systematically overreads muscular bodies. An athlete 1.80 m tall weighing 92 kg computes as 92 ÷ 3.24 = 28.4 — squarely "overweight" — while carrying little fat. Conversely, BMI can miss "normal-weight obesity," where someone with low muscle mass sits in the normal band but carries too much fat around the organs.
Age changes the picture too. For adults over 65, research tends to find the best outcomes at slightly higher BMIs, often roughly the 23–28 band, partly because a modest reserve helps during illness and because older adults lose muscle that BMI cannot see. The standard cutoffs were calibrated on younger adults.
Children and teenagers must not use the adult bands at all. Body composition shifts constantly as they grow, so their BMI is compared against age- and sex-specific percentile charts — a 12-year-old at BMI 19 may be at a completely different percentile than a 16-year-old at the same number.
The deepest limit is the origin. Quetelet built the index to describe populations, and it still does that job well — tracking weight trends across millions of people. For one individual it is a single data point, best read next to waist circumference, blood work and activity level, never as a diagnosis on its own.