Body mass index (BMI) is calculated by dividing your weight by your height squared. If you’re using pounds and inches, you multiply the result by 703 to convert to the standard scale. The math takes about 30 seconds with a calculator, and the result places you in one of several weight categories used in medical screening.
The Formula Using Pounds and Inches
Most people in the U.S. know their weight in pounds and height in feet and inches, so start here. First, convert your full height to inches. If you’re 5 feet 6 inches tall, that’s 66 inches (5 × 12 + 6).
Then follow these three steps:
- Square your height in inches. Multiply your height by itself. For 66 inches: 66 × 66 = 4,356.
- Divide your weight by that number. If you weigh 155 pounds: 155 ÷ 4,356 = 0.0356.
- Multiply by 703. That conversion factor bridges the gap between imperial and metric units. 0.0356 × 703 = 25.0.
So a person who is 5’6″ and 155 pounds has a BMI of 25.0.
The Formula Using Kilograms and Meters
If you know your weight in kilograms and height in meters, the calculation is simpler because no conversion factor is needed. Just divide your weight in kilograms by your height in meters squared. A person who weighs 70 kg and stands 1.75 meters tall would calculate: 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9.
What Your Number Means
Once you have your BMI, it falls into one of four main categories established by the CDC:
- Below 18.5: Underweight
- 18.5 to 24.9: Healthy weight
- 25.0 to 29.9: Overweight
- 30.0 or higher: Obesity
Obesity is further divided into three classes. Class 1 ranges from 30.0 to 34.9, Class 2 from 35.0 to 39.9, and Class 3 (sometimes called severe obesity) is 40.0 and above. These classes matter because health risks increase at each threshold. A BMI in the obesity range is associated with higher rates of high blood pressure, type 2 diabetes, heart disease, stroke, many types of cancer, chronic kidney disease, fatty liver disease, sleep apnea, and joint problems like osteoarthritis.
Being in the “healthy weight” range doesn’t guarantee good health, and landing in the “overweight” range doesn’t guarantee problems. These categories are screening tools, not diagnoses.
Why BMI Doesn’t Tell the Whole Story
BMI uses only two inputs: weight and height. It cannot distinguish between fat, muscle, and bone mass. A muscular person and a person carrying excess body fat at the same height and weight will get the identical BMI, even though their health profiles may look very different. The CDC acknowledges that while BMI correlates well with body fat across large populations, it loses predictability when applied to any single individual.
The American Medical Association has formally recognized these limitations. In a 2023 policy statement, the AMA noted that BMI cutoffs for underweight, normal, overweight, and obesity were developed primarily from data on non-Hispanic White populations. Body shape and composition vary meaningfully across racial and ethnic groups, sexes, and age ranges. The AMA now recommends that BMI be used alongside other measures rather than as a standalone number.
BMI Thresholds Differ for Some Populations
The standard cutoffs don’t apply equally to everyone. The World Health Organization has proposed lowering the overweight threshold for Asian populations from 25 to 23, because people of Asian descent tend to develop conditions like type 2 diabetes and heart disease at lower BMI values. If you’re of Asian descent, a BMI of 23 or above may already carry elevated risk, even though it falls within the “healthy” range on the standard scale.
Research from the American Heart Association also shows that where your body stores fat matters more than total weight. Fat around the waist is more metabolically active than fat around the hips and is more closely tied to insulin resistance, heart disease, and diabetes. A large study published in the Journal of the American Heart Association found that waist-to-hip ratio predicted heart attack risk better than BMI for both men and women. Measuring your waist circumference with a tape measure at the level of your navel can give you a useful data point that BMI misses entirely.
How BMI Works Differently for Children
For children and teens aged 2 through 19, the same formula produces the BMI number, but interpretation changes completely. Instead of fixed cutoffs, a child’s BMI is plotted on age-and-sex-specific growth charts maintained by the CDC. The result is expressed as a percentile, which shows how a child’s BMI compares to other children of the same age and sex. A 10-year-old girl at the 75th percentile has a BMI equal to or higher than 75% of 10-year-old girls in the reference population.
For children, the 85th percentile marks the threshold for overweight and the 95th percentile marks the threshold for obesity. These percentile-based cutoffs exist because body fat naturally shifts during childhood. A BMI of 18 means something very different for a 6-year-old than for a 16-year-old, so the fixed adult categories simply don’t apply.
Measures Worth Tracking Alongside BMI
Because BMI is a rough screening tool, pairing it with other measurements gives you a more accurate picture. The AMA suggests considering waist circumference, body composition (the ratio of fat to lean tissue), and metabolic factors like blood sugar and cholesterol levels.
Waist circumference is the simplest to check at home. For most adults, a waist measurement above 40 inches for men or 35 inches for women signals higher risk for heart disease and type 2 diabetes, regardless of BMI. Waist-to-hip ratio goes a step further by comparing waist size to hip size, capturing how centrally your body stores fat. These numbers are especially useful if your BMI falls in a borderline range and you want a clearer sense of where you stand.