What BMI actually measures
Body mass index is your weight divided by your height squared. It measures size relative to height, not body fat. The CDC describes it as a screening tool that is moderately to strongly associated with more direct measures of body fat.
A quick BMI example
Someone who is 5 feet 10 inches tall (70 inches) and weighs 170 pounds has a BMI of 703 × 170 / 70², which is about 24.4. In metric units, a person who is 1.78 meters tall and weighs 77 kilograms has a BMI of about 24.3.
BMI is popular because it is free, fast and needs only a scale and a tape measure. That makes it useful for screening large groups and for spotting changes over time. Its weakness is that two people with the same height and weight get the same BMI even if their bodies are built very differently.
The CDC adult BMI categories
For adults 20 and older, the CDC uses the same BMI categories for everyone, regardless of age or sex. Children and teens are assessed differently, using percentiles for their age and sex.
| Category | BMI range |
|---|---|
| Underweight | Less than 18.5 |
| Healthy weight | 18.5 to less than 25 |
| Overweight | 25 to less than 30 |
| Obesity, class 1 | 30 to less than 35 |
| Obesity, class 2 | 35 to less than 40 |
| Obesity, class 3 (severe) | 40 or greater |
These categories are a starting point for a conversation with a healthcare professional. On their own, they do not diagnose any condition.
Why BMI can misclassify people
BMI cannot tell fat from muscle or bone, and it cannot show where fat sits on the body. That leads to predictable errors for some groups.
- Muscle. People with a lot of muscle, such as many athletes, can land in the overweight or obesity range with relatively little body fat.
- Age. Older adults tend to carry more body fat than younger adults at the same BMI. Fat around the organs can rise with age even when weight stays steady.
- Sex. At the same BMI, women generally have more body fat and less muscle than men.
- Ancestry. The relationship between BMI and body fat, and between BMI and health risk, differs across racial and ethnic groups. Some people of Asian ancestry, for example, can have metabolic problems at a BMI in the healthy range.
- Frame size. People with larger frames can weigh more because of bone, not fat.
BMI can also miss risk in the other direction. A person with a healthy BMI can still carry extra fat around the abdomen, which is linked to higher health risks. For these reasons, the CDC notes that providers should look at BMI alongside other information, such as medical history, physical exams and lab results.
Waist circumference and waist-to-height ratio
Waist measurements add something BMI lacks: a sense of how much fat you carry around your middle. Abdominal fat is closely tied to heart disease and type 2 diabetes risk.
Waist circumference. The National Heart, Lung, and Blood Institute says a waist of more than 40 inches for men or more than 35 inches for women raises the risk of heart disease and type 2 diabetes. To measure, stand up, place a tape measure around your middle just above your hip bones, and read it right after you breathe out. Keep the tape snug but not tight.
Waist-to-height ratio. This is your waist measurement divided by your height, in the same units. Researchers have proposed 0.5 as a simple boundary, meaning a waist of less than half your height. For someone who is 70 inches tall, that works out to a waist under 35 inches. For someone who is 64 inches tall, it is under 32 inches. One appeal of this ratio is that it adjusts for height, which a fixed waist cutoff does not.
Like BMI, waist measures are screening tools. They are cheap and easy, but results depend on measuring the same way each time.
Body fat tests: DEXA, bioimpedance and calipers
If you want an actual estimate of body fat, several tests are available. Each trades accuracy against cost and convenience.
| Method | How it works | Pros | Cons |
|---|---|---|---|
| DEXA (dual-energy X-ray absorptiometry) | A low-dose X-ray scan of the whole body | Precise; estimates fat, lean mass and bone density | Expensive, not widely available, involves a small radiation dose |
| Bioelectrical impedance (BIA) | A weak electrical current passes through the body, often via a scale or handheld device | Convenient, portable, relatively inexpensive | Readings shift with hydration and illness; less reliable at higher BMIs |
| Skinfold calipers | A trained person pinches and measures skin folds at several sites | Inexpensive and quick | Accuracy depends heavily on the tester's skill; calipers may not fit larger skinfolds |
| Underwater weighing and air displacement | Body volume is measured in water or in a sealed chamber | Accurate | Costly and mostly limited to research or specialty settings |
Home scales that report body fat use bioimpedance. They can be useful for watching trends if you measure under the same conditions each time, such as the same time of day. A single reading is a rough estimate, so it is best not to read too much into small changes.
When to talk to a doctor
BMI, waist size and body fat numbers are best read together, and best read with a professional. A doctor or other healthcare provider can combine them with your blood pressure, blood work, family history and habits to judge your actual health risk.
Consider making an appointment if:
- Your BMI falls in the underweight or obesity range.
- Your waist is above the NHLBI thresholds, even if your BMI looks healthy.
- Your weight has changed noticeably without a clear reason.
- You have a family history of heart disease or type 2 diabetes and want to understand your own risk.
- You are unsure what your numbers mean for you.
No single number defines your health. Treat BMI as one data point, and let a healthcare professional help you put it in context.