Body mass index, or BMI, is a number calculated from your weight and height that places you into a weight category: underweight, normal, overweight, or obese. It’s the most widely used screening tool in medicine for assessing whether a person’s weight falls within a range associated with higher or lower health risks. A BMI between 18.5 and 24.9 is considered normal weight for adults.
How BMI Is Calculated
The formula is simple: divide your weight by your height squared. In metric units, that’s your weight in kilograms divided by your height in meters squared. If you’re working in pounds and inches, you divide your weight by your height in inches squared, then multiply by 703 to convert to the same scale.
For example, someone who weighs 150 pounds and stands 5’5″ (65 inches) would calculate: 150 ÷ (65 × 65) × 703 = 24.96. That lands right at the border between normal weight and overweight. Most doctors’ offices and online calculators handle the math for you.
Adult Weight Categories
The standard categories used by the World Health Organization and most Western health systems break down like this:
- Underweight: below 18.5
- Normal weight: 18.5 to 24.9
- Overweight: 25 to 29.9
- Obesity class 1: 30 to 34.9
- Obesity class 2: 35 to 39.9
- Extreme obesity (class 3): 40 and above
These thresholds were established based on population-level data linking higher BMI ranges to greater risk of chronic disease. They work as a quick sorting tool, but they don’t tell the whole story for every individual.
BMI for Children and Teens
The same formula applies to kids, but the results are interpreted differently. Because children’s body composition changes as they grow, BMI is plotted on age-and-sex-specific growth charts and expressed as a percentile rather than a raw number. A 10-year-old boy and a 16-year-old girl with the same BMI number could fall into completely different weight categories.
The CDC defines the categories for children and teens as follows:
- Underweight: below the 5th percentile
- Healthy weight: 5th to less than the 85th percentile
- Overweight: 85th to less than the 95th percentile
- Obesity: 95th percentile or above
- Severe obesity: 120% of the 95th percentile or a BMI of 35 or higher
Health Risks Linked to High BMI
Higher BMI ranges correlate with increased risk for a long list of conditions. The CDC identifies type 2 diabetes, heart disease, stroke, high blood pressure, high cholesterol, many types of cancer, chronic kidney disease, fatty liver disease, osteoarthritis, sleep apnea, and asthma among the most significant. Depression and other mental health conditions also appear more frequently at higher BMI levels, partly driven by the biological effects of excess body fat and partly by the social stigma associated with weight.
Being underweight carries its own risks, including weakened immune function, bone loss, and nutritional deficiencies. The “normal” range of 18.5 to 24.9 represents the window where population-level health risks are lowest, though individual circumstances vary widely.
Why BMI Doesn’t Work for Everyone
BMI measures total body mass relative to height. It cannot distinguish between fat, muscle, and bone. This creates predictable blind spots. Muscle and bone are denser than fat, so a muscular athlete with low body fat can register as overweight or even obese by BMI alone. Conversely, an older adult who has lost muscle and bone density over the years may show a “normal” BMI while carrying a relatively high percentage of body fat.
Ethnicity is another major factor. Research shows that people of Asian descent tend to develop obesity-related health problems at lower BMI levels than people of European descent. The WHO recommends a lower obesity threshold of 27.5 for Asian populations instead of the standard 30, and several Asian countries have adopted even lower cutoffs. In South Korea and Japan, for instance, a BMI of 25 qualifies as obesity class 1. For Asian Indian populations, the obesity cutoff is also 25. These differences reflect genuine biological variation in how and where the body stores fat, and how that fat affects metabolic health.
Sex and age also matter. Women naturally carry more body fat than men at the same BMI. Older adults tend to have more body fat and less muscle than younger adults at identical BMI levels. None of these distinctions show up in the number itself.
How Doctors Use BMI Today
In 2023, the American Medical Association adopted a policy recognizing that BMI is “an imperfect measure” that loses predictive accuracy when applied to individuals rather than populations. The AMA now recommends using BMI alongside other measures rather than as a standalone diagnostic tool. Those additional measures include waist circumference, body composition assessments, and metabolic markers like blood sugar and cholesterol levels.
This shift reflects what researchers have found when comparing different body measurements head to head. A large study analyzing cardiovascular disease mortality found that waist circumference and waist-to-hip ratio were better predictors of heart disease death than BMI in fully adjusted analyses. People in the highest category of waist-to-hip ratio had a 66% greater risk of dying from cardiovascular disease compared to those in the lowest category. BMI, after accounting for other risk factors, lost its association with cardiovascular death entirely. That said, when researchers tested whether swapping one measurement for another actually improved the ability to predict who would die, the practical difference was less than 1%. No single body measurement is dramatically better than another at predicting outcomes for a given person.
Alternatives Worth Knowing About
If you want a fuller picture beyond your BMI, a few additional measurements can help:
- Waist circumference: a tape measure around your midsection at the level of your navel. This captures visceral fat, the deep abdominal fat most closely tied to metabolic disease. For general population guidelines, risk rises above 35 inches for women and 40 inches for men.
- Waist-to-hip ratio: your waist measurement divided by your hip measurement. This captures body fat distribution, specifically whether you carry more fat around your middle versus your hips.
- Body composition testing: methods like DEXA scans or bioelectrical impedance scales estimate your actual percentage of fat versus lean tissue. These give a more direct answer to the question BMI is trying to approximate.
In practice, BMI remains useful as a fast, free, no-equipment screening number. It reliably identifies weight-related health risk across large groups of people. Where it falls short is in telling any one person whether their specific weight is healthy. Pairing it with a waist measurement and basic blood work gives you and your doctor a much clearer picture.