The formula, and why two unit systems don’t match exactly
BMI is defined as weight divided by height squared, using kilograms and meters: BMI = kg ÷ m². A 70 kg adult who is 1.75 m tall has a BMI of 70 ÷ 1.75² = 22.9.
CDC’s US-units shortcut is BMI = 703 × lb ÷ in², where 703 is a rounded conversion factor (the exact value is closer to 703.07). Run the same person through both formulas and the results differ by a few thousandths of a point. That gap is the rounding at work, and it never moves anyone across a category boundary.
The formula does not change for sex or age past 20; CDC states plainly that adults use "standard BMI categories regardless of age, sex, or race." What differs between a BMI calculator "for men" and "for women" online is almost always just which example numbers the page uses, not the math underneath.
What the categories mean
CDC and WHO split adult BMI into four bands: underweight below 18.5, healthy weight 18.5–24.9, overweight 25–29.9, and obesity at 30 and above. Obesity is further split into Class 1 (30–34.9), Class 2 (35–39.9), and Class 3 or "severe obesity" (40 and above), which is the breakdown clinicians use to decide how urgently weight-related risk needs addressing.
These bands come from population studies linking BMI to the risk of conditions like type 2 diabetes and heart disease, not from any single ideal number. CDC is explicit that BMI is "a screening measure" and "not intended to diagnose disease." It flags who might benefit from a closer look, without passing judgment on any one person’s health.
Worked examples
5'9", 160 lb: total height is 69 in, so BMI = 703 × 160 ÷ 69² = 703 × 160 ÷ 4,761 ≈ 23.6, healthy weight and comfortably inside the 18.5–24.9 band.
175 cm, 95 kg: BMI = 95 ÷ 1.75² = 95 ÷ 3.0625 ≈ 31.0, Class 1 obesity, just past the 30 threshold. The healthy-weight range for that same height is about 56.7–76.3 kg, so getting back into that range would take a loss of roughly 19 kg at minimum, more if the target is the middle of the band rather than its upper edge.
5'0", 95 lb: 60 in tall, BMI = 703 × 95 ÷ 3,600 ≈ 18.6, just inside healthy weight and only a tenth of a point above the underweight cutoff.
Where BMI gets the wrong answer
BMI cannot tell fat from muscle or bone, because it never measures either; it only weighs total body mass against height. CDC’s own FAQ says this outright: BMI "cannot distinguish fat mass from lean body mass." A muscular adult with low body fat can be flagged as heavier than healthy under BMI’s overweight or obesity thresholds while remaining metabolically healthy, and the reverse also happens: a sedentary adult with normal BMI but high visceral fat ("normal weight obesity") can carry real metabolic risk that BMI misses entirely.
BMI also has nothing to say about fat distribution across the body, and abdominal fat raises heart-disease risk more than fat stored elsewhere at the same total BMI. Waist circumference or waist-to-height ratio catch this distinction; a BMI number by itself does not. For adults under 20, pregnant women, and older adults with age-related muscle loss, CDC directs people to age-specific tools instead of the adult calculator.
Asian population cutoffs: a real distinction, backed by a specific study
Reviewing the evidence in 2004, a WHO expert panel set lower BMI thresholds for people of Asian descent, since their diabetes and heart-disease risk climbs at BMI values the original categories did not flag. At a given BMI, Asian adults tend to carry more body fat than European adults, which is the underlying reason the cutoffs shift. Published in The Lancet (2004;363:157–63), the consultation set out additional "public health action points" at BMI 23.0 and 27.5, plus 32.5 and 37.5 further along the same continuum, while keeping the original WHO/CDC cutoffs as the international default.
This calculator defaults to the standard WHO/CDC bands and offers the Asian cutoffs as an explicit toggle instead of picking one silently. Which standard fits best depends on the population being assessed, and several national health agencies, including Singapore’s and Japan’s, have adopted the lower thresholds for exactly that reason.