How Adult BMI Is Calculated and Interpreted

What BMI is and what it measures

Body mass index (BMI) is a calculated measure of weight relative to height. It is used as a quick screening number to estimate whether an adult's weight falls into a category that public health agencies associate with health risk. The calculation itself is simple arithmetic, which is why BMI is widely used in clinical and public health settings.

BMI does not measure body fat directly. It is a surrogate indicator that correlates with body fat for many people, but it cannot distinguish between muscle, bone, fat, or water weight. As the CDC explains, BMI is a screening measure and is not intended to diagnose disease or illness. It is one potential health indicator that should be considered with other factors such as medical history, health behaviors, physical exam findings, and laboratory findings.

• BMI is a calculated ratio of weight to height, not a direct measure of body fat.
• For adults 20 and older, BMI is interpreted using standard categories regardless of age, sex, or race.
• Children and teens aged 2 through 19 use a separate CDC BMI calculation based on age- and sex-specific percentiles.

Sources: Centers for Disease Control and Prevention (CDC), American Cancer Society

Metric formula with a worked example

The metric BMI formula uses weight in kilograms divided by height in meters squared: BMI = weight (kg) ÷ [height (m)]². If you have your height in centimeters, divide by 100 to convert it to meters before squaring.

For example, consider a person who weighs 65 kilograms and is 1.83 meters tall. First square the height: 1.83 × 1.83 = 3.3489. Then divide the weight by that number: 65 ÷ 3.3489 ≈ 19.4. The resulting BMI is approximately 19.4, which falls within the healthy weight category used by the CDC. Intermediate values are rounded here for simplicity; exact results may vary slightly depending on rounding at each step.

• Weight unit: kilograms (kg)
• Height unit: meters (m)
• Formula: BMI = kg ÷ m²

Sources: American Cancer Society

Imperial formula with a worked example

For imperial units, the formula is BMI = (weight in pounds × 703) ÷ (height in inches × height in inches). The number 703 is a conversion factor that adjusts the result so it matches the metric formula.

The American Cancer Society provides a worked example: a person who weighs 120 pounds and is 5 feet 3 inches (63 inches) tall. Square the height: 63 × 63 = 3,969. Multiply weight by 703: 120 × 703 = 84,360. Then divide: 84,360 ÷ 3,969 = 21.3. That BMI falls within the healthy weight range. A second example: a person who weighs 154 pounds and is 71 inches tall has BMI = (154 × 703) ÷ (71 × 71) = 108,262 ÷ 5,041 ≈ 21.5.

• Weight unit: pounds (lb)
• Height unit: inches (in)
• Formula: BMI = (lb × 703) ÷ in²

Sources: American Cancer Society, Centers for Disease Control and Prevention (CDC)

Reading your result against adult BMI categories

Once you have calculated your BMI, you can compare it with the standard categories published by the CDC for adults aged 20 and older. The CDC categorizes adult BMI into underweight, healthy weight, overweight, and obesity, with obesity further divided into three classes. The same categories are used regardless of age, sex, or race for adults.

These categories are a general guide to weight status, not a diagnosis. Your healthcare provider can help you interpret your BMI in the context of your overall health.

Sources: Centers for Disease Control and Prevention (CDC)

Why BMI alone is not a diagnosis

BMI is a screening measure and is not intended to diagnose disease or illness. The CDC states that BMI should be considered along with other factors when assessing an individual's health, which may include medical history, health behaviors, physical exam findings, and laboratory findings. A high or low BMI does not by itself confirm or rule out a health condition.

For most people, the CDC notes that BMI is a good indicator of whether they have too much or too little body fat. However, BMI cannot distinguish fat mass from lean body mass such as muscle and bone, and it cannot indicate where fat is located in the body. That means two people with the same BMI can have very different body compositions and health profiles.

Because of these limitations, BMI is best understood as a starting point for a conversation with a healthcare provider rather than a standalone verdict on your health.

• BMI does not distinguish between muscle, bone, fat, or water weight.
• It does not reflect where fat is stored in the body, which matters for health risk.
• It may be less informative for athletes, older adults who have lost muscle mass, and others with atypical body composition.

Sources: Centers for Disease Control and Prevention (CDC), American Cancer Society, National Heart, Lung, and Blood Institute (NHLBI), National Institutes of Health

Key limitations of BMI as a screening number

BMI has well-known limitations when applied to individuals. It was developed as a population-level tool, and when used for a single person it can misclassify health status. For example, the CDC explains that people with a lot of muscle might have a BMI in the overweight category (25.0 to less than 30.0) or obesity category (30.0 or more) because BMI cannot distinguish between fat, muscle, and bone mass.

The American Cancer Society notes that BMI may not be accurate for people with greater muscle mass, such as athletes, or in older people and others who have lost muscle mass. NHLBI adds that BMI does not take into account muscle mass, bone density, or body composition. Stanford Health Care similarly notes that some muscular people may have a BMI in the overweight range but are not considered overweight because muscle tissue weighs more than fat tissue.

BMI cutoffs are also not the same for children and teens. For individuals aged 2 to 19, BMI is calculated and then interpreted using age- and sex-specific percentiles, which is different from the fixed categories used for adults.

• Designed for populations, not individuals.
• Does not measure body fat or its distribution.
• May overestimate risk in muscular individuals and underestimate risk in those with low muscle mass.
• Uses the same fixed categories for all adults 20 and older, which may not fit every individual equally well.

Sources: Centers for Disease Control and Prevention (CDC), American Cancer Society, National Heart, Lung, and Blood Institute (NHLBI), National Institutes of Health, Stanford Health Care

When to discuss results with a healthcare provider

The CDC advises discussing your BMI or BMI category with your healthcare provider because BMI may relate to your overall health and well-being. The CDC also states that people may consider seeking advice from their healthcare providers about their BMI, and that a BMI calculator is not a substitute for professional medical advice.

A healthcare provider can consider other factors such as skinfold thickness, waist size, diet, family health problems, blood pressure, cholesterol, and blood sugar to assess whether your weight might pose health risks. The American Cancer Society notes that a person with a high BMI should be evaluated by a healthcare provider, who might use these additional factors to form a more complete picture.

Remember that BMI is just one number. Your provider can help you understand what it means for you personally and whether any further evaluation or changes are appropriate.

Sources: Centers for Disease Control and Prevention (CDC), American Cancer Society