What Is BMI and How Is It Used?
BMI (Body Mass Index) is a measurement of a person's leanness or corpulence based on height and weight, intended to quantify tissue mass. It is widely used as a general indicator of whether a person has a healthy body weight for their height.
The value is used by the World Health Organization (WHO) and the Centers for Disease Control and Prevention (CDC) as a population-level screening tool.
BMI for Children and Teens (Ages 2–20)
For children and adolescents, the CDC uses BMI-for-age percentiles: below the 5th percentile is underweight, 5th–85th is healthy weight, 85th–95th is at risk of overweight, and above the 95th percentile is overweight.
Limitations of BMI
Athletes & bodybuilders
Muscle is denser than fat. A bodybuilder may register as "overweight" despite very low body fat.
Older adults
Older adults have more body fat relative to their BMI. A healthy BMI in an inactive older adult may still carry risks.
Ethnic variation
Several Asian countries use BMI 23 for "overweight" (vs 25) due to increased health risks at lower BMI values.
Children & adolescents
Fixed adult thresholds should never be applied to under-18s — always use age-specific percentile charts.
Frequently Asked Questions
What is a healthy BMI for adults?
According to the WHO, a healthy BMI for adults is between 18.5 and 24.9 kg/m². Below 18.5 is underweight; 25–29.9 is overweight; 30 and above is obese.
Is BMI accurate for women vs men?
BMI uses the same thresholds for both sexes, but women naturally carry more body fat than men at the same BMI value. For a more sex-specific measure, use the body fat percentage calculator.
Can I have a normal BMI but still be unhealthy?
Yes. A sedentary person with very little muscle mass can have a normal BMI with high body fat percentage and still face metabolic health risks. Waist circumference and activity levels also matter.
What is BMI Prime and why does it matter?
BMI Prime is your BMI divided by 25. It tells you how far from the upper normal limit you are: 1.0 = exactly at the boundary; below 0.74 = underweight; above 1.2 = obese.
How is the Ponderal Index different from BMI?
The Ponderal Index cubes height rather than squaring it: PI = weight (kg) ÷ height³ (m). This makes it more reliable for very tall or very short individuals. Normal PI: 11–14 kg/m³.
How much weight do I need to lose to reach a healthy BMI?
Your healthy weight range is shown in the results above (BMI 18.5–24.9 for your height). Aim for a 500 kcal/day deficit for ~0.5 kg loss per week.