To calculate your BMI, divide your weight by your height squared. If you’re using pounds and inches, multiply the result by 703. If you’re using kilograms and meters, the result of the division is your BMI. The number you get places you in a weight category that doctors use as a starting point for assessing health risks.
The BMI Formula
BMI stands for body mass index, and the math is the same for everyone. You just need your current weight and height.
Using pounds and inches: BMI = (weight in pounds ÷ height in inches ÷ height in inches) × 703
Using kilograms and meters: BMI = weight in kilograms ÷ (height in meters × height in meters)
If you know your height in centimeters rather than meters, you can divide your weight in kilograms by your height in centimeters, divide by your height in centimeters again, then multiply by 10,000. You’ll get the same result.
Step-by-Step Example
Say you weigh 170 pounds and stand 5 feet 8 inches tall. First, convert your height entirely to inches: 5 × 12 + 8 = 68 inches. Then square that number: 68 × 68 = 4,624. Divide your weight by that result: 170 ÷ 4,624 = 0.03677. Finally, multiply by 703: 0.03677 × 703 = 25.9. Your BMI is 25.9.
In metric, someone who weighs 77 kilograms and is 1.73 meters tall would calculate it this way: 1.73 × 1.73 = 2.99, then 77 ÷ 2.99 = 25.8. Same ballpark, same process, no conversion factor needed.
You can also skip the math entirely and use the free calculator on the CDC’s website. You plug in your height and weight and get your number instantly.
What Your BMI Number Means
For adults 20 and older, the standard categories are:
- Under 18.5: Underweight
- 18.5 to 24.9: Healthy weight
- 25 to 29.9: Overweight
- 30 to 34.9: Class 1 obesity
- 35 to 39.9: Class 2 obesity
- 40 or higher: Class 3 (severe) obesity
These ranges were set based on population-level data linking BMI to health risks like heart disease, type 2 diabetes, and high blood pressure. The higher your BMI climbs above 25, the more those risks tend to increase on average.
BMI Works Differently for Kids and Teens
The formula itself is the same for children, but the result is interpreted differently. Because kids are still growing, a raw BMI number doesn’t mean much on its own. Instead, it gets plotted on a growth chart that compares your child to other children of the same age and sex. The result is a percentile rather than a fixed category.
For children and teens ages 2 through 19, the CDC uses these percentile ranges:
- Below 5th percentile: Underweight
- 5th to 84th percentile: Healthy weight
- 85th to 94th percentile: Overweight
- 95th percentile or above: Obesity
A 10-year-old boy and a 16-year-old girl could have identical BMI numbers but fall into completely different percentile categories. The CDC’s online child and teen calculator handles the percentile lookup for you.
Lower Thresholds for Asian Populations
The standard cutoffs don’t apply equally across all ethnic groups. People of Asian descent tend to carry more abdominal fat at lower body weights, which raises the risk of type 2 diabetes and cardiovascular disease at BMI levels that would be considered “normal” by standard charts.
In 2004, the World Health Organization proposed adjusted categories for many Asian populations: a BMI of 23 to 27.5 as overweight and 27.5 or higher as obese, compared to 25 and 30 in the standard ranges. The American Diabetes Association followed suit in 2015, recommending that screening for prediabetes and diabetes begin at a BMI of 23 or higher for Asian Americans. Research has found that the BMI level associated with equivalent health risk varies meaningfully by population: roughly 23.9 for South Asian populations, 26.9 for Chinese populations, and 28.1 for Black populations, compared to the standard threshold of 30.
Where BMI Falls Short
BMI measures total body mass relative to height. It cannot distinguish between muscle, fat, bone, and water. This creates obvious blind spots.
A muscular athlete with low body fat can easily register as overweight or obese by BMI alone. On the flip side, someone with a “normal” BMI can still carry a high percentage of body fat if they have very little muscle mass. Research on athletes with physical impairments illustrates this clearly: athletes with spinal cord injuries had an average body fat percentage of 25.5%, while able-bodied athletes matched by age had just 16.7%, despite both groups having similar BMI values in the low 20s. Same BMI, very different body composition.
BMI also becomes less reliable as you age. Older adults naturally lose muscle and bone density over time, so a stable BMI can mask a gradual increase in body fat. And BMI tells you nothing about where your fat is stored, which matters because fat around the midsection carries greater health risks than fat stored in the hips and thighs.
What to Measure Alongside BMI
Waist circumference is the simplest complement to BMI. It specifically captures abdominal fat, which is more strongly linked to heart disease and metabolic problems than overall weight. The National Institutes of Health considers a waist measurement above 40 inches (102 cm) in men or above 35 inches (88 cm) in women to signal increased health risk. Those thresholds roughly correspond to a BMI of 30.
In June 2023, the American Medical Association formally recommended that doctors move beyond BMI as a standalone measure. Their policy statement noted that BMI “loses its predictability in assessing fat mass on the individual level because of body variation among different age, sex, and race groups.” The recommendation is to pair BMI with additional measures like waist circumference, body composition, or metabolic health markers for a fuller picture.
BMI remains useful as a quick screening number. It takes 10 seconds to calculate and correlates well with health outcomes across large populations. But it’s a starting point, not a diagnosis. Your waist measurement, fitness level, blood pressure, blood sugar, and family history all fill in details that a single number can’t capture.