How BMI is Calculated and What the Ranges Mean

What Is BMI?

Body Mass Index (BMI) is a screening measure that uses your height and weight to estimate body fatness. It is the most widely used weight-for-height indicator in clinical practice, epidemiology, and public health because it is inexpensive, quick, and requires no specialized equipment. While BMI does not measure body fat directly, research has shown that it correlates reasonably well with more direct measures of adiposity such as underwater weighing and dual-energy X-ray absorptiometry (DXA) for most adults.

BMI is not a diagnostic tool. It is a screening signal that, when combined with other assessments (waist circumference, skinfold thickness, diet, family history, and physical activity), helps clinicians determine whether further evaluation is warranted.

The BMI Formula

BMI is calculated as body mass in kilograms divided by the square of height in meters:

BMI = weight (kg) / height (m)²

For those more comfortable with imperial units, the equivalent formula is:

BMI = 703 × weight (lb) / height (in)²

The constant 703 converts the imperial unit ratio into the same numeric scale as the metric formula, so the resulting BMI value and its interpretation are identical regardless of which formula you use.

Worked Example (Metric)

A person who is 1.75 m tall and weighs 70 kg:

• 1.75² = 3.0625
• 70 / 3.0625 = 22.9

A BMI of 22.9 falls in the "normal weight" range.

Worked Example (Imperial)

A person who is 5 ft 9 in (69 in) tall and weighs 160 lb:

• 69² = 4,761
• 703 × 160 = 112,480
• 112,480 / 4,761 = 23.6

Again, this lands in the "normal weight" range.

Input Definitions

• Height: Measured without shoes, ideally to the nearest 0.5 cm or 0.25 inch. In the metric formula, height must be expressed in meters (so 175 cm becomes 1.75 m). In the imperial formula, height is expressed in total inches (so 5 ft 9 in becomes 69 in).
• Weight: Measured in light clothing without shoes, ideally on a calibrated scale. Use kilograms for the metric formula and pounds for the imperial formula. Weight fluctuates throughout the day, so for tracking purposes it is best to weigh at the same time of day under similar conditions.

Interpreting BMI Ranges

The standard adult BMI classification established by the World Health Organization (WHO) applies to adults aged 20 and older:

Category	BMI Range
Underweight	Less than 18.5
Normal weight	18.5 – 24.9
Overweight	25.0 – 29.9
Obese (Class I)	30.0 – 34.9
Obese (Class II)	35.0 – 39.9
Obese (Class III)	40.0 and above

A BMI below 18.5 suggests insufficient weight, which can be associated with nutritional deficiency, eating disorders, or underlying illness. A BMI between 18.5 and 24.9 is associated with the lowest risk of weight-related disease for most adults. A BMI of 25 to 29.9 indicates overweight, which is associated with a modestly increased risk of conditions such as type 2 diabetes and hypertension. A BMI of 30 or above defines obesity, which is associated with a significantly elevated risk of cardiovascular disease, type 2 diabetes, certain cancers, and other chronic conditions.

Important Caveats

BMI is a population-level screening tool, and its accuracy at the individual level has well-documented limitations.

Athletes and Highly Muscular Individuals

Muscle tissue is denser than fat tissue, so people with high muscle mass (athletes, bodybuilders, certain manual laborers) can have a BMI in the "overweight" or even "obese" range without having excess body fat. For these individuals, BMI overestimates adiposity and can produce misleading risk classifications.

Older Adults

With aging, adults tend to lose muscle mass and gain fat, often without a significant change in total body weight. An older adult with a "normal" BMI may carry a higher proportion of body fat than a younger adult with the same BMI, which means BMI can underestimate health risk in this population.

Children and Teens

BMI is interpreted differently for children and adolescents (ages 2–19). Instead of fixed cutoffs, pediatric BMI is expressed as a percentile relative to other children of the same age and sex. A BMI-for-age between the 5th and 85th percentile is considered healthy weight, while the 85th–95th percentile indicates overweight and the 95th percentile or above indicates obesity.

Pregnant and Lactating Individuals

BMI is not a reliable indicator of health status during pregnancy or lactation, when weight changes are expected and physiologically appropriate. Interpretation should be deferred to a prenatal care provider.

Body Composition and Fat Distribution

BMI does not distinguish between fat mass and lean mass, nor does it capture where fat is stored. Visceral (abdominal) fat carries greater metabolic risk than fat stored around the hips and thighs, so two people with identical BMI can have very different risk profiles. Waist circumference and waist-to-hip ratio are useful complementary measures.

Health Information Disclaimer

This guide is provided for general informational and educational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the guidance of a qualified healthcare provider with any questions you have regarding a medical condition, weight-management goals, or the interpretation of any health screening measure. Never disregard professional medical advice or delay seeking it because of something you have read here.

Calculate Your Own BMI

Ready to find your own BMI? Use our live BMI calculator to enter your height and weight and see your result along with the corresponding WHO category.

Calculate your BMI now →