The calculation
BMI = weight in kilograms ÷ height in meters squared. For example, 70 kg at 1.75 m gives 70 ÷ (1.75 × 1.75) = 22.9. The imperial calculation uses weight in pounds divided by height in inches squared, multiplied by approximately 703.
What the adult categories mean
The CDC categories for adults aged 20 and older are underweight below 18.5, healthy weight from 18.5 to less than 25, overweight from 25 to less than 30, and obesity at 30 or above. These categories are screening ranges, not an individual diagnosis.
What BMI cannot tell you
BMI does not distinguish muscle from fat or show where fat is distributed. It should be interpreted alongside other health information. The adult categories are not the right method for children and teenagers, who need age- and sex-specific assessment.
Source: CDC: adult BMI categories.