BMI Formula Explained: How to Calculate, Interpret & Understand Its Limits
BMI = weight(kg) / height(m)². This guide explains the full formula, CDC categories, real examples, and why BMI alone is an incomplete health measure.
The BMI Formula — Metric and US Units
The original BMI formula, developed by Belgian mathematician Adolphe Quetelet in the 1830s, uses metric units: divide weight in kilograms by height in meters squared. For US customary units, multiply by the conversion factor 703. Metric example: 70 kg person, 1.75 m tall → BMI = 70 / (1.75)² = 70 / 3.0625 = 22.9 US example: 154 lb person, 5'9" (69
CDC BMI Categories for Adults
The CDC defines four BMI weight categories for adults aged 20 and older. These thresholds apply regardless of age or sex for adults, although the CDC now recommends that healthcare providers consider race and ethnicity when interpreting results — particularly for Asian-American patients, who face elevated metabolic risk at BMI levels below the stan
Where BMI Falls Short
BMI cannot distinguish between fat mass and muscle mass. Athletes with high muscle mass routinely score in the 'overweight' range (BMI 26–30) despite excellent metabolic health and low body fat percentage. Conversely, people with low muscle mass — particularly older adults — may score 'normal' while carrying metabolically risky body fat levels. Thi
BMI for Children and Teenagers (Ages 2–19)
For children aged 2–19, BMI is calculated the same way but interpreted using age- and sex-specific CDC growth chart percentiles — not the fixed adult thresholds. A child's BMI percentile shows how their measurement compares to peers of the same age and sex: below the 5th percentile is underweight; 5th–84th is healthy weight; 85th–94th is overweight
Frequently Asked Questions
Can you be healthy with a BMI over 25?
Yes. Athletes and people with high muscle mass routinely have BMI values of 26–30 despite excellent cardiovascular fitness and metabolic health. A full assessment — blood panels, blood pressure, body composition — provides a far more accurate health picture than BMI alone.
What BMI is considered obese?
A BMI of 30.0 or higher is obese by CDC standards. Class I is 30–34.9, Class II is 35–39.9, and Class III (severe obesity) is 40 and above. At each class, associated health risks — including type 2 diabetes, heart disease, and sleep apnea — increase substantially.
Does BMI change with age for adults?
The formula doesn't change, but body composition shifts with age. Older adults tend to carry more fat and less muscle at the same BMI as younger adults, meaning metabolic risks at a given BMI increase with age. Adult BMI categories remain the same regardless of age.
Is BMI accurate for Asian or Black Americans?
Standard thresholds may underestimate metabolic risk in Asian Americans (elevated risk may begin at BMI 23+) and may overestimate risk in some Black Americans who tend to have higher bone density and muscle mass. The CDC recommends clinicians use ethnicity-specific interpretation
What is a healthy BMI range to maintain long-term?
For most adults, a BMI of 20.0–24.9 is associated with the lowest overall mortality in large epidemiological studies. For adults over 65, research suggests a slightly higher range (22–27) may be protective. Your physician can advise on the right target for your specific age, sex,