BMI, Waist Circumference, and Body Composition: Which Number to Trust

Body mass index is weight in kilograms divided by height in meters squared. It was devised in the 1830s by a Belgian statistician to describe populations, adopted by insurers in the 20th century, and by the 1990s had become the standard way to classify individual patients. The categories: under 18.5 underweight, 18.5 to 24.9 normal, 25 to 29.9 overweight, 30 and above obese.

Across a population, BMI tracks health risk reasonably well. For a given person, it can be badly wrong in both directions, and the newer guidance reflects that.

What BMI gets right

It is free, instant, and repeatable. At the population level, rates of type 2 diabetes, heart disease, sleep apnea, some cancers, and joint disease rise with BMI, steeply above 30. For most sedentary adults, BMI and body fat move together closely enough that the category is informative. As a screening tool, a starting point for a conversation, it does its job.

Where it fails

It cannot tell muscle from fat. A muscular adult can read as overweight or obese with a low body fat percentage. A sedentary adult can read as normal while carrying a high proportion of fat and little muscle, a state sometimes called normal weight obesity, which carries much of the metabolic risk of visible obesity.

It cannot tell where the fat is. Fat around the abdominal organs, visceral fat, drives insulin resistance, blood pressure, and cardiovascular risk far more than fat under the skin of the hips and thighs. Two people with the same BMI can have very different amounts of it.

It applies the same cutoffs to everyone. The thresholds were derived largely from white European populations. At the same BMI, adults of South Asian, East Asian, and some other backgrounds carry more visceral fat and develop diabetes at lower BMIs, which is why the World Health Organization and several national bodies use lower action points, around 23 and 27.5, for those groups. Older adults, meanwhile, tend to do better at slightly higher BMIs than younger adults.

In 2023 the American Medical Association advised that BMI should not be used alone to assess an individual's health. In 2025 a Lancet commission proposed that a diagnosis of obesity should require a measure of body fat or its distribution, not BMI on its own.

The tape measure

Waist circumference is the simplest measure of visceral fat and adds information that BMI misses. Measure at the top of the hip bones, roughly at the navel, after breathing out, with the tape snug but not compressing. Risk rises above about 94 centimeters (37 inches) for men and 80 centimeters (31.5 inches) for women, and is substantially raised above 102 centimeters (40 inches) and 88 centimeters (35 inches). Lower thresholds, 90 and 80 centimeters, apply to adults of Asian descent.

Waist to height ratio is better still, because it adjusts for frame size. Divide waist by height in the same units. A ratio under 0.5 is the target at any height: keep your waist less than half your height. Studies comparing measures find it predicts diabetes and cardiovascular disease more accurately than BMI or waist alone, and it works across ages and ethnic groups without separate cutoffs.

Body composition measures

Bioimpedance scales pass a tiny current through the body and estimate fat from resistance. They are convenient and imprecise; hydration, a recent meal, and exercise can move the reading by several percentage points. Used at the same time each morning, they show a trend. The absolute number should not be trusted.

DEXA scans give an accurate picture of fat, lean mass, and bone density, with a regional breakdown that shows visceral fat. They are available at clinics and some gyms for a modest cost and are the most useful single measurement for anyone serious about tracking composition rather than weight.

Skinfold calipers, in trained hands, are accurate to within a few percent. In untrained hands, they are not.

What to track

For most adults, two numbers: weight, weekly, under the same conditions, and waist, monthly, at the same spot. Weight alone is noisy and misleading during a period of strength training, when fat may fall while weight holds. Waist catches the change that matters. Write both down; Ribbon Checkup's home health log guide describes a format that keeps the trend visible.

The thing the number cannot tell you

Fitness modifies all of it. In cohort studies, adults classified as obese by BMI who are cardiorespiratorily fit have lower mortality than normal weight adults who are unfit. Fat distribution and fitness are the two variables that BMI cannot see and that matter most. See VO2 max and cardiorespiratory fitness for the second, and strength training after 40 for why the muscle side of the ratio is worth protecting.