What BMI tells you
BMI estimates body fatness from height and weight. The standard categories are: under 18.5 underweight, 18.5–24.9 healthy, 25–29.9 overweight, 30+ obesity. It’s a quick screen, not a full health assessment.
What the CDC adds beyond BMI
US health bodies treat BMI as a starting point, not a diagnosis. Because it can’t tell muscle from fat or show where weight sits, a waist measurement is often used alongside it: above roughly 40 inches for men or 35 inches for women signals higher risk regardless of BMI. The obesity band itself is split into Class 1 (30–34.9), Class 2 (35–39.9) and Class 3 (40+).
BMI for children and older adults
For under-20s, BMI is read against age-and-sex percentiles rather than the adult cut-offs, so this calculator’s categories apply to adults only. In older adults a slightly higher BMI can be protective, and very muscular or very tall/short builds also shift the meaning of the number.