How BMI is calculated
For adults, the World Health Organization groups BMI as: under 18.5 underweight (16 to 17 moderate thinness, under 16 severe), 18.5 to 25 normal, 25 to 30 overweight, and 30 or more obese (classes I, II and III start at 30, 35 and 40).
BMI Prime is your BMI divided by 25, the top of the normal range, so 1.00 is the upper limit. The Ponderal index divides weight by height cubed, which suits very tall or short people a little better.
What BMI can't tell you
BMI doesn't separate muscle from fat, so athletes can read as overweight. For people under 20, doctors compare BMI with age and sex percentiles instead. Use it as a rough guide, not a diagnosis.
Questions
What is a healthy BMI?
For adults, a BMI from 18.5 to 25 is in the normal range. Under 18.5 is underweight, 25 to 30 overweight, and 30 or more obese.
How do I calculate BMI in pounds and inches?
Multiply your weight in pounds by 703, then divide by your height in inches squared. 165 lb at 5'10" (70 in) is 703 × 165 ÷ 4,900 = 23.7.
Is BMI accurate for muscular people?
Not very. BMI only uses height and weight, so muscle counts the same as fat. Body fat percentage is a better measure if you lift.
Does BMI work for teenagers?
Under 20, BMI is compared against age and sex percentiles, so the adult categories are only a rough guide.