BMI calculator (educational only)

Body mass index is a single ratio: weight in kilograms divided by the square of height in meters (kg/m²). In US units the tool multiplies by 703 to get the same figure from pounds and inches. The World Health Organization uses it to classify adults: under 18.5 is underweight, 18.5–24.9 is the normal range, 25–29.9 is overweight, and 30 or above is obesity, with further subdivisions (class I–III) above 30. These cutoffs were chosen because population-level disease risk climbs as BMI rises — they were never designed as an individual diagnostic.

The well-documented limitations

BMI's central weakness is that it cannot tell fat from lean mass. A critical review by Frank Nuttall (Nutrition Today, 2015) notes that "BMI does not differentiate between body lean mass and body fat mass" and that in NHANES III data, adults at an identical BMI of 25 spanned roughly 14–35% body fat in men and 26–43% in women — the same number describing lean athletes and people with clinically high adiposity. The correlation between BMI and measured body fat is weakest in men, and the formula also ignores where fat is carried: visceral (abdominal) fat carries different metabolic risk than subcutaneous fat at the same BMI, which is why the WHO calls BMI a "surrogate marker of fatness" and points to waist circumference as a complementary measurement. Age and ethnicity shift the picture further — some health bodies apply lower cutoffs for populations of Asian descent.

Why it still appears everywhere in peptide research

Despite the caveats, BMI persists because it is cheap, repeatable, and standardized across decades of studies. Obesity-related peptide trials — semaglutide's STEP program, tirzepatide's SURMOUNT program — recruit participants by BMI threshold and report outcomes by BMI category. Reading those papers requires knowing which category a given height and weight falls into, which is the narrow job this calculator does. It performs the division; interpreting the result for any individual is a clinical task — see the medical disclaimer.

How is BMI calculated in US units? The metric formula is weight in kilograms divided by height in meters squared. With pounds and inches, the same ratio is approximated by multiplying by a conversion constant: BMI = 703 × weight (lb) ÷ height (in)². The 703 factor simply converts pounds and inches into the metric result; both formulas give the same number.

Why can a muscular person get an 'overweight' BMI? BMI only sees total weight, not what the weight is made of. Muscle is denser than fat, so lean, muscular people can cross the 25 kg/m² threshold with low body fat, while people with low muscle mass can sit in the 'normal' range with high body fat. Nuttall's 2015 review reports NHANES data in which people at the same BMI of 25 ranged from roughly 14% to 35% body fat among men.

Is BMI a diagnosis? No. It is a screening and classification metric used to sort populations into research categories. The WHO describes BMI as a surrogate marker of fatness, and clinical assessment adds measurements such as waist circumference, body composition, and metabolic labs before drawing conclusions about an individual.

Why does a peptide education site have a BMI calculator? Because obesity-related peptide trials define their populations by BMI. Trial inclusion criteria for GLP-1 receptor agonists such as semaglutide and tirzepatide are typically written as BMI thresholds (for example, ≥30, or ≥27 with a weight-related condition). Knowing your BMI category tells you which trial populations a published result actually describes.