To calculate your BMI, divide your weight by your height squared. The result is a single number that places you in a weight category ranging from underweight to obese. The math takes about 30 seconds with a calculator, and the formulas differ slightly depending on whether you’re using metric or imperial units.
The BMI Formulas
BMI stands for body mass index. There are two versions of the formula depending on your unit system.
If you use kilograms and meters: divide your weight in kilograms by your height in meters squared.
BMI = weight (kg) ÷ height (m) × height (m)
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 use pounds and inches: divide your weight in pounds by your height in inches squared, then multiply by 703.
BMI = [weight (lb) ÷ (height (in) × height (in))] × 703
For example, someone who weighs 155 pounds and stands 5 feet 9 inches (69 inches) would calculate: 155 ÷ (69 × 69) = 155 ÷ 4,761 = 0.03256, then 0.03256 × 703 = 22.9. The 703 is just a conversion factor that makes the imperial formula match the metric one.
What Your Number Means
For adults 20 and older, BMI falls into these standard 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 were developed primarily from data on non-Hispanic white populations, which matters if your background is different. For South Asian populations, the health risks typically associated with a BMI of 30 in white populations show up at a BMI of about 23. A 2024 meta-analysis in Circulation found that the optimal BMI threshold for predicting type 2 diabetes in South Asians was 23.3, and similar thresholds applied for high blood pressure and cholesterol problems. If you’re of South Asian, East Asian, or Southeast Asian descent, a “normal” BMI by standard charts may still carry elevated risk.
How BMI Works Differently for Kids
The formula itself is identical for children and teens, but the result is interpreted completely differently. Because body fat changes as kids grow, and boys and girls develop at different rates, a child’s BMI is plotted on age-and-sex-specific growth charts rather than compared to fixed cutoffs. The CDC uses percentile rankings:
- Underweight: below the 5th percentile
- Healthy weight: 5th to 84th percentile
- Overweight: 85th to 94th percentile
- Obesity: 95th percentile or above
This means a BMI of 22 could be perfectly healthy for a 15-year-old boy but overweight for an 8-year-old girl. You need the growth chart to interpret the number, not the adult categories. Your pediatrician’s office tracks this at regular checkups, or you can use the CDC’s online BMI calculator for children, which asks for the child’s age and sex.
Why BMI Doesn’t Tell the Whole Story
BMI is a screening tool, not a diagnosis. It measures weight relative to height, and it cannot distinguish between fat, muscle, and bone. A muscular person and an overfat person of the same height can have identical BMIs despite very different health profiles. This is why the American Medical Association adopted a policy stating that BMI “loses predictability when applied on the individual level,” even though it correlates well with body fat across large populations.
The AMA now recommends that BMI be used alongside other measures rather than as a standalone number. It specifically notes that BMI should not be the sole criterion for insurance coverage decisions.
Measurements That Add Context
If you want a more complete picture than BMI alone, waist circumference is the simplest additional measure you can take at home. Fat stored around your midsection (visceral fat) poses a greater risk for heart disease and type 2 diabetes than fat stored at your hips. The National Heart, Lung, and Blood Institute considers a waist circumference above 35 inches for women or above 40 inches for men to be a marker of increased risk, regardless of BMI.
To measure, wrap a tape measure around your bare waist just above your hip bones, at roughly the level of your belly button. Keep the tape snug but not compressing the skin, and read it after a normal exhale. Someone with a “healthy” BMI but a large waist measurement may still carry meaningful metabolic risk, while someone with an “overweight” BMI but a smaller waist and significant muscle mass may be in better shape than their number suggests.
Other clinical measures that complement BMI include body composition testing (which separates fat mass from lean mass), blood pressure, blood sugar levels, and cholesterol panels. Together, these give a far more accurate health picture than any single number.