Body mass index, or BMI, is a number calculated from your weight and height that serves as a quick screening tool for weight categories. It doesn’t measure body fat directly, but it correlates well enough with body fat at the population level that doctors use it as a starting point for assessing weight-related health risks. The formula is simple, and you can calculate it yourself in under a minute.
How BMI Is Calculated
BMI uses the same basic formula regardless of which units you prefer: divide your weight by the square of your height. If you’re working in metric units, divide your weight in kilograms by your height in meters squared. A person who weighs 70 kg and stands 1.75 m tall would calculate 70 ÷ (1.75 × 1.75) = 22.9.
If you’re using pounds and inches, the math needs one extra step. Divide your weight in pounds by your height in inches squared, then multiply by 703. So a person who weighs 160 pounds and is 67 inches tall would calculate 160 ÷ (67 × 67) × 703 = 25.1.
The conversion factor of 703 simply accounts for the difference between metric and imperial units. Both formulas produce the same BMI number for the same person.
What the Numbers Mean for Adults
For adults 20 and older, BMI falls into standard weight categories set by the CDC:
- Underweight: below 18.5
- Healthy weight: 18.5 to 24.9
- Overweight: 25 to 29.9
- Class 1 obesity: 30 to 34.9
- Class 2 obesity: 35 to 39.9
- Class 3 (severe) obesity: 40 or higher
These thresholds are the same for men and women. They were established based on large population studies linking BMI ranges to disease risk, but they aren’t universal. The World Health Organization has proposed lower cutoffs for Asian populations, where health risks tend to appear at lower BMI levels. Under those guidelines, the “normal” range tops out at 23 instead of 25, and overweight starts at 23 rather than 25. This matters if you’re of South Asian, East Asian, or Southeast Asian descent, as your risk for conditions like type 2 diabetes may be elevated at a BMI that would be classified as “healthy” under standard cutoffs.
How BMI Works for Children and Teens
BMI is interpreted differently for anyone between ages 2 and 19. Because children and teens are still growing, a raw BMI number doesn’t mean much on its own. Instead, the number is plotted against age- and sex-specific growth charts to produce a percentile. That percentile shows how a child compares to other kids of the same age and sex. A child at the 75th percentile, for example, has a BMI higher than 75% of peers in the same group.
The pediatric categories are:
- Underweight: below the 5th percentile
- Healthy weight: 5th to 84th percentile
- Overweight: 85th to 94th percentile
- Obesity: 95th percentile or above
- Severe obesity: 120% of the 95th percentile or above, or a BMI of 35 or higher
A 10-year-old boy and a 16-year-old girl could have identical BMI numbers but land in completely different percentile categories. This is why pediatricians track BMI over time on growth charts rather than relying on a single reading.
Health Risks Linked to High BMI
BMI matters clinically because higher values correlate with a range of serious health conditions. The relationship is strongest for type 2 diabetes: nearly 9 in 10 people with type 2 diabetes have overweight or obesity. Cardiovascular risks also climb, including high blood pressure, heart disease, heart attack, heart failure, and stroke.
Elevated BMI is associated with fatty liver disease, which can progress to severe liver damage or cirrhosis. It raises the likelihood of certain cancers, including colon, rectal, and prostate cancers in men, and breast, uterine, and gallbladder cancers in women. Breathing problems like sleep apnea and asthma become more common, and excess weight is a leading contributor to osteoarthritis in weight-bearing joints like knees, hips, and ankles.
The metabolic effects extend further: gallstones, pancreatitis, chronic kidney disease, pregnancy complications including gestational diabetes and preeclampsia, and fertility issues. Mental health can be affected too, with higher rates of depression, chronic stress, and body image challenges. None of these outcomes are guaranteed at any particular BMI, but the statistical risk increases as BMI rises above the healthy range.
Where BMI Falls Short
BMI has a fundamental limitation: it cannot tell the difference between fat, muscle, and bone. All three contribute to your weight, but they carry very different health implications. A muscular athlete and a sedentary person of the same height and weight will have identical BMI numbers despite dramatically different body compositions. This is why some professional athletes register as “overweight” or even “obese” by BMI standards while carrying very little body fat.
BMI also says nothing about where fat is stored. Fat carried around the midsection (visceral fat) is more metabolically dangerous than fat stored in the hips and thighs. Two people with the same BMI could face quite different health risks depending on their fat distribution. And BMI doesn’t account for age-related changes in body composition. Older adults tend to lose muscle and gain fat over time, so a “healthy” BMI in someone over 65 could still mask an unhealthy amount of body fat.
Research published in the Annals of Family Medicine found that BMI and direct body fat measurements agreed on whether a person was at a healthy or unhealthy weight only about 60% of the time. That 40% disagreement rate is significant. More precise methods exist, like DEXA scans, which use low-dose X-rays to distinguish fat from lean tissue and bone. But they’re expensive and impractical for routine checkups, which is why BMI persists as the default screening tool.
What BMI Can and Cannot Tell You
BMI is best understood as a first pass, not a diagnosis. It’s cheap, fast, and requires nothing more than a scale and a tape measure. At the population level, it’s remarkably useful for tracking trends in weight-related disease. At the individual level, it’s a rough estimate that needs context.
If your BMI falls in the healthy range and you have no other risk factors, that’s generally reassuring. If it falls outside that range, it’s worth looking deeper. Your doctor might consider waist circumference, blood sugar, cholesterol levels, blood pressure, family history, and how physically active you are before drawing any conclusions. BMI opens the conversation. It doesn’t end it.