Body mass index compares weight against height to give a single screening number used by clinicians and health services worldwide. It is quick and useful at population level, but it says nothing about body composition — which is why the result here comes with the category boundaries and their limits clearly stated.
Below 18.5 is underweight, 18.5 to 24.9 is the healthy range, 25 to 29.9 is overweight, and 30 or above falls in the obesity range. These are the World Health Organization thresholds for adults.
Muscle is denser than fat, so a heavily built athlete can post a BMI above 30 with very low body fat. BMI measures mass, not what that mass is made of.
Not with these thresholds. Children and teenagers are assessed against age and sex-specific percentile charts, because healthy body composition changes throughout growth.
Several health bodies use lower thresholds for people of South Asian, Chinese, and some other backgrounds, where cardiometabolic risk rises at a lower BMI. Ask a clinician what applies to you.
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