To calculate BMI, you divide your weight by your height squared. The exact formula depends on whether you’re using metric or imperial units, but either way, you only need two measurements: your height and your weight. A BMI between 18.5 and 25 is considered a healthy weight for most adults.
The BMI Formula
BMI stands for body mass index. The metric formula is straightforward: divide your weight in kilograms by your height in meters, squared.
Metric: BMI = weight (kg) ÷ [height (m)]²
So if you weigh 70 kg and you’re 1.75 m tall, you’d calculate 1.75 × 1.75 = 3.0625, then divide 70 by 3.0625 to get a BMI of 22.9.
If you measured your height in centimeters instead of meters, the formula adds a multiplier to correct the units:
Metric (cm): BMI = [weight (kg) ÷ height (cm) ÷ height (cm)] × 10,000
Imperial: BMI = [weight (lb) ÷ [height (in)]²] × 703
The 703 is a conversion factor that accounts for the difference between pounds/inches and kilograms/meters. For example, if you weigh 155 pounds and stand 5’7″ (67 inches), you’d calculate 67 × 67 = 4,489, then divide 155 by 4,489 to get 0.03453, then multiply by 703. That gives you a BMI of about 24.3.
What the Numbers Mean for Adults
The CDC groups adult BMI values into these categories for anyone 20 and older:
- 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 were developed using population-level data. They indicate where health risks like heart disease, type 2 diabetes, and high blood pressure tend to increase statistically, but they don’t tell you much about your individual health on their own.
How BMI Works Differently for Children
The formula itself is the same for kids and teens, but the result is interpreted differently. Because children are still growing, a raw BMI number doesn’t mean much without comparing it to other children of the same age and sex. This comparison produces a BMI-for-age percentile.
For children and teens ages 2 through 19, the categories are:
- 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
A 10-year-old boy and a 16-year-old girl could have the exact same BMI number but fall into completely different categories. You can’t simply look up a child’s BMI on the adult chart. The CDC’s online calculator for children will compute the percentile for you once you enter the child’s age, sex, height, and weight.
Where BMI Falls Short
BMI does not distinguish between fat, muscle, and bone mass. It only measures total weight relative to height. This creates some well-known blind spots. A muscular person can register as overweight or obese by BMI despite having low body fat. An older adult who has lost muscle mass over the years might have a “healthy” BMI while carrying excess fat around their organs.
The formula also carries historical baggage. It was originally developed in the 1830s by a Belgian statistician named Adolphe Quetelet, who was trying to describe the “average man” across populations, not diagnose individuals. It wasn’t even called “body mass index” until 1972. The American Medical Association adopted a policy in 2023 acknowledging that BMI is based primarily on data collected from non-Hispanic white populations and should not be used as a standalone measure of health. The AMA now recommends pairing BMI with other indicators like waist circumference, body composition, and metabolic markers.
BMI Thresholds Vary by Ethnicity
The standard cutoffs don’t apply equally across all populations. People of Asian descent tend to develop metabolic problems like type 2 diabetes at lower BMI values than white populations. A joint recommendation from the World Health Organization’s Western Pacific office set the overweight threshold for Asian populations at a BMI of 23 (instead of 25) and the obesity threshold at 25 (instead of 30).
Research published in Diabetes Care found that the BMI at which diabetes risk matches a BMI of 30 in white adults is roughly 24 for South Asians, 25 for Chinese populations, and 26 for Black populations. The American Diabetes Association recommends diabetes screening for all Asian American adults with a BMI of 23 or higher. Screening only at the standard cutoff of 25 would miss about 36% of Asian Americans with undiagnosed type 2 diabetes.
Better Ways to Use Your BMI
BMI is most useful as a quick screening number, not a diagnosis. If your BMI falls in the healthy range and you feel fine, it’s a reasonable signal that your weight isn’t posing obvious metabolic risk. If it falls outside that range, it’s a starting point for a more detailed picture, not the final word.
Waist circumference adds useful context because it reflects visceral fat, the kind packed around your internal organs that drives the most health risk. For most adults, a waist measurement above 35 inches for women or 40 inches for men signals elevated risk even when BMI looks normal. Body composition testing, which separates fat mass from lean mass, gives an even clearer picture but is less accessible outside clinical or fitness settings.
Tracking your BMI over time can also be more informative than any single measurement. A gradual upward trend over several years is worth paying attention to, even if each individual number still falls within the “healthy” range. The direction matters as much as the number itself.