BMI Calculator guide
BMI Formula for Adults
BMI, or body mass index, compares a person's weight with their height using a mathematical ratio. This is general information for adults age 20 or older, not a medical diagnosis.
The metric BMI formula
BMI = weight (kg) ÷ [height (m)]². When height is measured in centimeters instead of meters, use BMI = weight (kg) × 10,000 ÷ [height (cm)]². A result is expressed in kg/m².
Metric worked example
At 70 kg and 175 cm: BMI = 70 × 10,000 ÷ (175 × 175) = 22.857142857… The one-decimal display is 22.9, and the CDC primary adult screening category is Healthy weight. This is an arithmetic example, not a suggested target weight.
U.S. customary formula and example
Convert each pound to 0.45359237 kilograms and each inch to 0.0254 meters. For 150 lb and 5 ft 6 in (66 inches), BMI = (150 × 0.45359237) ÷ [(66 × 0.0254)²] = 24.2103849738…, displayed as 24.2. The rounded shorthand factor 703 is useful for manual approximation but differs slightly from exact unit conversion.
Adult screening-category boundaries
| Category | BMI (kg/m²) |
|---|---|
| Underweight | Less than 18.5 |
| Healthy weight | 18.5 to less than 25 |
| Overweight | 25 to less than 30 |
| Obesity | 30 or greater |
The category is decided from the unrounded BMI. For example, 24.975 rounds to a visible 25.0 but remains below the category threshold of 25.
Why BMI is not a diagnosis
BMI does not directly measure body fat, muscle mass, bone density or fat distribution. It is one screening measure among others, not a stand-alone individual health verdict, clinical recommendation, or disease diagnosis. The same adult category thresholds apply across sexes, but personal health assessment involves additional factors.
Different interpretation for children and pregnancy
For ages 2–19, BMI-for-age percentiles require age and sex, not fixed adult categories.
During pregnancy, this calculator is not a tool for assessing pregnancy weight gain. CDC pregnancy guidance refers to BMI before pregnancy. This app does not provide pregnancy-specific assessments.
Official references
CDC: About BMI; CDC: Adult BMI Categories; CDC: Child and Teen BMI Calculator; CDC: Pregnancy Weight Guidance; NIST: Exact inch conversion; NIST Handbook 44 conversion factors. Official links are sources, not endorsements of Mycelgrid.