BMI: What the Number Shows—and What It Doesn’t

A simple calculation, designed for populations

Body mass index (BMI) is your weight in kilograms divided by your height in meters squared. A person who weighs 70 kg and is 1.75 m tall has a BMI of 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 ≈ 22.9 kg/m². Calculate yours in seconds with the BMI Calculator.

The formula was proposed in the 19th century by Belgian statistician Adolphe Quetelet to describe populations, not individuals. It remains useful because it’s quick, inexpensive, and on average is associated with the risk of several diseases. But when you look at one person, it can be misleading.

BMI ranges used by the WHO for adults

These ranges apply to adults ages 20 to 59. BMI is interpreted differently for other age groups.

What BMI doesn’t measure

• Muscle vs. fat: BMI only considers total body weight. A muscular athlete who is 1.80 m tall and weighs 90 kg has a BMI of 27.8, classified as “overweight” on the chart, even with little body fat. The reverse can also happen: someone with a normal BMI may have little muscle and more body fat than is ideal.
• Where fat is stored: Fat stored around the abdomen (visceral fat) is more strongly linked to cardiovascular disease and diabetes than fat around the hips and thighs. Two people with the same BMI can have very different health risks.
• Older adults: As we age, we lose muscle mass and height. BMI may need to be interpreted differently for older adults, taking age-related changes and overall health into account.
• Children and teens: The adult chart isn’t used. Assessment relies on the WHO’s growth charts, which compare BMI with age and sex.
• Pregnant people: Expected weight gain depends on BMI before pregnancy and is monitored during prenatal care.
• Differences between populations: The WHO recognizes that the health risks associated with each BMI range can vary across ethnic groups.

Examples: same BMI, different situations

Here’s how the same number can tell different stories:

• Ana, 35, 1.62 m, 66 kg: BMI of 66 ÷ 2.6244 ≈ 25.1, just into the overweight range. She strength trains three times a week and has a 74 cm waist. The chart alone may overstate her risk.
• Carlos, 52, 1.75 m, 75 kg: BMI of 24.5, in the “healthy weight” range. But he’s inactive and has a 99 cm waist, which is above the reference point for increased risk in men. Here, BMI may understate the concern.
• Maria, 74, 1.55 m, 52 kg: BMI of 21.6. The adult chart classifies this as “healthy weight,” but some references for older adults use 22 as the lower cutoff. A lower BMI may warrant attention to nutrition and muscle loss.

That’s why healthcare professionals use BMI as a screening tool: it’s a starting point for asking more questions, not a conclusion.

Waist measurement: a useful addition to BMI

Measure your waist with a tape measure while standing, at the midpoint between your lowest rib and the top of your hip bone, after a normal exhale. WHO reference points for adults indicate:

Another simple measure used by healthcare professionals is waist-to-height ratio: values below 0.5 are generally considered healthy. None of these measurements can diagnose a condition on its own; they can help determine whether further assessment may be useful.

How to use your BMI

1. Calculate your BMI and measure your waist on the same day, preferably in the morning.
2. Write down the results and repeat every few months: the trend matters more than a single reading.
3. If you want to change your weight, first learn about your daily energy needs with the Calorie Calculator, then bring the results to your appointment.
4. Consider other factors too: blood pressure, lab results, fitness, sleep, and eating habits.

Sources: World Health Organization — obesity and overweight. Updated October 2026. For informational purposes only; this content is not a substitute for advice from a doctor, dietitian, or other healthcare professional.

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