To determine your BMI, divide your weight by your height squared. If you’re using pounds and inches, multiply the result by 703. The number you get falls into one of several weight categories, with 18.5 to 24.9 considered a healthy range for most adults. Here’s exactly how to do the math and what your result actually tells you.
The BMI Formula
BMI stands for body mass index, and the calculation is the same for everyone: weight divided by height squared. The specific formula depends on whether you’re working in metric or U.S. units.
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)
For example, if you weigh 160 pounds and stand 5’7″ (67 inches), the math looks like this: 160 ÷ 67 ÷ 67 = 0.03564, then multiply by 703. Your BMI would be about 25.1. If you’d rather skip the arithmetic, the CDC and NIH both offer free online calculators where you just plug in your height and weight.
Getting an Accurate Measurement
Your result is only as good as the numbers you start with. A digital scale on a hard floor (tile or wood, not carpet) gives the most reliable weight. Weigh yourself without shoes and heavy clothing, standing with both feet centered on the scale. Record the number to the nearest decimal, like 155.5 pounds.
Height is trickier to measure alone, but you can get close. Stand barefoot against a flat wall on a hard floor with your feet together, legs straight, and shoulders level. Look straight ahead. Place a rigid, flat object (a hardcover book works well) on top of your head so it forms a right angle with the wall, then mark where the bottom of the book meets the wall. Measure from the floor to that mark with a tape measure and record to the nearest eighth of an inch.
What the BMI Categories Mean
The CDC defines these weight categories for adults 20 and older:
- Underweight: below 18.5
- Healthy weight: 18.5 to 24.9
- Overweight: 25 to 29.9
- Obesity (Class 1): 30 to 34.9
- Obesity (Class 2): 35 to 39.9
- Obesity (Class 3, severe): 40 or higher
These thresholds were developed from population-level data linking BMI ranges to rates of heart disease, type 2 diabetes, and other chronic conditions. A higher class of obesity generally corresponds to higher health risk. Class 1 is sometimes called low-risk obesity, Class 2 moderate-risk, and Class 3 high-risk.
BMI Works Differently for Kids and Teens
If you’re calculating BMI for someone under 20, the formula is the same, but the result is interpreted completely differently. Because children and teens are still growing, a raw BMI number doesn’t mean much on its own. Instead, the result is compared to other kids of the same age and sex using percentile charts.
A child at the 60th percentile, for instance, has a BMI higher than 60% of kids their age and sex. The categories shift accordingly: below the 5th percentile is underweight, 5th to 84th is healthy weight, 85th to 94th is overweight, and the 95th percentile or above is classified as obesity. The CDC’s online calculator for children handles this percentile conversion automatically once you enter the child’s age and sex along with height and weight.
Where BMI Falls Short
BMI is a screening tool, not a diagnosis. It measures weight relative to height, but it cannot distinguish between fat, muscle, and bone. Someone who carries a lot of muscle mass (a competitive athlete, for example) can easily land in the “overweight” or even “obese” range while having a low body fat percentage. On the other end, an older adult who has lost muscle over the years might have a normal BMI but carry a disproportionate amount of body fat.
In 2023, the American Medical Association adopted a policy acknowledging these limitations. The AMA noted that while BMI correlates well with body fat across large populations, it “loses predictability when applied on the individual level.” Their recommendation: use BMI alongside other measures rather than treating it as a standalone verdict on someone’s health.
Ethnicity also matters. Research from a WHO expert consultation found that people of Asian descent tend to carry a higher percentage of body fat at the same BMI compared to white or European populations, and face elevated risk of type 2 diabetes and cardiovascular disease at BMIs below 25. Several Asian countries now use lower action points, with increased risk beginning around a BMI of 23 rather than 25.
Other Measurements Worth Knowing
Waist circumference is one of the simplest additions to BMI because it reflects visceral fat, the type stored around your internal organs that drives metabolic risk. According to the Heart Foundation, health risk increases when waist size exceeds about 37 inches (94 cm) for men or about 31.5 inches (80 cm) for women. You can have a BMI in the healthy range and still carry excess visceral fat, or have an elevated BMI with a healthy waist measurement.
To measure, wrap a flexible tape measure around your bare waist at the level of your belly button. Keep the tape snug but not compressing the skin, and take the reading at the end of a normal exhale. Combining this number with your BMI gives you a more practical snapshot of metabolic risk than either measurement alone. Your doctor may also look at blood pressure, blood sugar, cholesterol, and body composition to round out the picture.