Body mass index (BMI) is calculated by dividing your weight by your height squared. The result is a single number that places you into a weight category, from underweight to obese. The math takes about 30 seconds by hand, and the formula changes slightly depending on whether you’re using metric or imperial units.
The BMI Formulas
If you’re working in metric units, divide your weight in kilograms by your height in meters squared:
BMI = weight (kg) ÷ height (m)²
For example, someone who weighs 70 kg and stands 1.75 m tall would calculate: 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9.
If you only know your height in centimeters, you can use a slightly adjusted version: divide your weight in kilograms by your height in centimeters, divide by your height in centimeters again, then multiply by 10,000. The result is identical.
For pounds and inches, the formula adds a conversion factor:
BMI = [weight (lbs) ÷ height (in)²] × 703
So a person who weighs 160 pounds and is 5’8″ (68 inches) would calculate: 160 ÷ (68 × 68) = 160 ÷ 4,624 = 0.0346, then 0.0346 × 703 = 24.3.
What Your BMI Number Means
For adults 20 and older, the CDC uses these categories:
- 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 cutoffs apply broadly, but there’s one notable exception. Clinical guidelines recommend using a lower threshold of 23 or above (rather than 25) to screen for excess body fat in Asian adults, because health risks tend to rise at lower BMI levels in this population.
BMI for Children and Teens
The formula itself is the same for kids, but the number means something different. Because children’s body composition shifts as they grow, a raw BMI score isn’t compared to a fixed scale. Instead, it’s plotted against CDC growth charts that account for the child’s age and sex. The result is a percentile showing how your child compares to other children of the same age and sex.
- Underweight: below the 5th percentile
- Healthy weight: 5th to 84th percentile
- Overweight: 85th to 94th percentile
- Obesity: 95th percentile or higher
- Severe obesity: 120% of the 95th percentile or higher, or a BMI of 35 or above
A 10-year-old boy and a 16-year-old boy with the same BMI number could fall into entirely different categories. That’s why pediatric BMI always requires the percentile step, and why online calculators for children ask for a date of birth.
Where BMI Falls Short
BMI treats all weight the same. It cannot distinguish between fat, muscle, and bone. A lean, muscular person can easily register as overweight or obese by BMI despite carrying very little body fat. On the other end, an older adult who has lost significant muscle mass (a condition called sarcopenia) may have a “healthy” BMI while carrying a higher proportion of fat than that number suggests.
BMI also says nothing about where your body stores fat. Two people with the same BMI can have very different health risk profiles if one carries fat around the organs in the abdomen and the other carries it in the hips and thighs. Visceral fat, the deep abdominal kind, is far more strongly linked to heart disease, type 2 diabetes, and other metabolic problems.
The American Medical Association formally acknowledged these limitations in 2023, noting that BMI is based primarily on data collected from non-Hispanic white populations and loses predictive accuracy when applied to individuals. The AMA now recommends that BMI not be used as a sole measure and should be paired with other indicators of metabolic health.
Measurements That Add Context
If you want a fuller picture than BMI alone provides, waist circumference is the simplest addition. Wrap a measuring tape around your midsection at the level of your navel while standing. For most men, a waist measurement above 40 inches signals increased risk; for most women, the threshold is 35 inches.
Waist-to-hip ratio goes a step further. Measure your waist, then measure the widest point around your hips, and divide the first number by the second. A large study of nearly 388,000 people published in JAMA Network Open in 2023 found that waist-to-hip ratio was a better predictor of future health problems than BMI, likely because it more directly reflects levels of visceral fat. A healthy ratio for most men is below 0.95.
Neither of these measurements requires special equipment, and both capture something BMI misses: how your body distributes its weight. Used together with BMI, they give you a more complete snapshot of where you stand. Healthcare providers may also look at blood pressure, blood sugar, cholesterol levels, and family history before drawing any conclusions about weight-related health risk.