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BMI: What It Means and Its Limits
Body mass index (BMI) is a simple number that compares your weight to your height. It is widely used in clinics and by the public because it is fast and cheap, but it is also a rough screening tool that says very little about any single person's health. This guide explains how the number is calculated, what the standard categories mean, and where BMI genuinely falls short.
What BMI is and how it is calculated
Body mass index is a number that compares your weight to your height. It is not a measurement of body fat itself. It is a simple ratio that was developed in the 19th century as a way to describe the size of populations, and it survived as a screening tool because it is cheap, quick and reproducible: two clinicians measuring the same adult will get the same BMI. That reliability is exactly why BMI is used to define categories rather than to diagnose an individual. The National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) describes overweight and obesity in adults ages 20 and older using BMI ranges, and uses the same cut points shown below.
The arithmetic is short enough to do by hand. In metric units, divide weight in kilograms by height in metres squared: BMI = kg / m squared. In U.S. customary units, divide weight in pounds by height in inches squared, then multiply by 703: BMI = (lb / in squared) x 703. An adult who weighs 180 pounds and stands 5 feet 10 inches (70 inches) tall gets 180 divided by 4,900, which is 0.0367, times 703, which is about 25.8. The same person at 74.8 kg and 1.78 m gets 74.8 divided by 3.168, which is also 25.8. Because height is squared, BMI rises quickly as weight rises at a fixed height, and it falls as height increases at a fixed weight, which is why the same BMI can look different on a very short and a very tall person.
The standard BMI categories
For adults ages 20 and older, the categories are the same for men and women and do not change with age. NIDDK and the Centers for Disease Control and Prevention (CDC) use these ranges, and the World Health Organization (WHO) uses the same cut points in its global obesity and overweight fact sheet.
- Below 18.5: below the range considered a healthy weight for height
- 18.5 to 24.9: healthy weight range
- 25.0 to 29.9: overweight
- 30.0 and above: obesity
- 40.0 and above: severe obesity, sometimes written as class 3 obesity
What BMI cannot tell you
BMI takes two numbers, weight and height, and ignores everything else. It cannot tell muscle from fat, and it cannot say where fat sits. A person who strength trains regularly can be classified as overweight on BMI while carrying little fat around the organs, and a person with a normal BMI can carry a lot of fat viscerally, around the liver and intestines, with a waist circumference that is high for their height. Harvard T.H. Chan School of Public Health makes this point in its healthy weight coverage: body composition, fat distribution and cardiorespiratory fitness each carry information about health that a single index of weight for height cannot capture.
Using BMI responsibly
Treat BMI as one data point with a known error bar. It is useful for tracking your own direction over time, and it is useful for describing groups of people, which is what it was built for. In population data from the 2017 to 2018 National Health and Nutrition Examination Survey, reproduced by NIDDK, more than two in five U.S. adults had obesity and nearly three in four had overweight or obesity, so a large share of the population sits in the upper categories already. That is another reason the category label carries little individual information: belonging to a common category is not the same as being at individual risk.
Where BMI is weakest as an individual measure, repeating it and pairing it with other numbers is the fix. Track waist circumference alongside weight. If a BMI of 27 comes with normal blood pressure, normal blood glucose and a normal waist circumference, the number is not telling you very much on its own. If the same BMI comes with a rising waist measurement, the number is telling you something worth following up with a clinician who can look at the rest of the picture. Whatever your current BMI, the actionable information is the direction your measurements move over months, not the category the ratio puts you in this week.
Updated 2026-10-07
