BMI Calculator

Is BMI an accurate measure of my body fat?

BMI correlates with body fat percentage at the population level but is unreliable for individuals. Studies comparing BMI to DEXA scans consistently find that BMI misclassifies around 15 to 25 percent of adults — mostly muscular people called overweight and older sedentary adults with normal BMI but elevated visceral fat. For an individual assessment, body fat percentage measured by DEXA, hydrostatic weighing, or multi-frequency bioimpedance is far more informative, as is waist circumference which correlates more closely with metabolic disease risk. BMI remains useful as a quick, zero-cost screen but should never drive medical decisions on its own.

Why do muscular people have high BMIs?

Because muscle is denser than fat — about 1.06 g/cm3 versus 0.9 g/cm3 — so a muscular body at the same height weighs more than a fatty one. BMI only looks at weight and height, not composition. A 5'10" person at 190 pounds can be a lean fitness athlete with 8 percent body fat or a sedentary office worker with 30 percent body fat, and both show BMI 27.3 ('overweight'). This is the single most-cited limitation of BMI as a clinical tool, and is why athletes, military populations, and regular gym-goers routinely ignore the overweight label if their waist and body composition metrics are healthy.

What is a healthy BMI range for adults?

WHO and NHLBI define the normal range as 18.5 to 24.9 for adults. Below 18.5 is underweight, 25 to 29.9 is overweight, and 30 or above is obese. These cutoffs were derived from large epidemiological studies linking BMI to mortality risk, and the 'normal' band roughly corresponds to the lowest all-cause mortality rates in most Western populations. However, the relationship is a J-curve — very low BMIs also carry elevated mortality — and the optimal range shifts upward slightly for adults over 65, where BMI 25 to 27 is associated with lower mortality than BMI 22 to 24 in some studies.

Can I use this calculator for my child?

Not directly. For anyone under 20, raw BMI must be converted to a BMI-for-age percentile using the CDC growth charts, because children grow at different rates and a 'normal' BMI for a 6-year-old is very different from a 'normal' BMI for a 16-year-old. The calculator here returns the raw BMI number, which is the first step, but interpretation requires looking up the percentile on the CDC chart for that child's sex and age. Under 5th percentile is underweight, 5 to 84th is healthy, 85 to 94th is overweight, 95 and above is obese. Your pediatrician typically plots this at every well-child visit.

Does BMI account for ethnic differences in body composition?

The standard WHO cutoffs do not, but WHO released supplementary guidance in 2004 recommending lower action thresholds for Asian populations — a BMI of 23 rather than 25 for overweight and 27.5 rather than 30 for obese — because metabolic and cardiovascular risk rises at lower BMI in these populations. Clinicians in Singapore, India, Hong Kong, and parts of the UK apply these adjusted cutoffs. US clinical practice generally still uses the universal cutoffs, which some researchers argue understates risk for South and East Asian Americans. This is not tax advice or medical advice — discuss with your physician whether adjusted cutoffs should apply to you.