How BMI Is Calculated
Body Mass Index is a simple ratio of weight to height squared:
The result places you in one of four categories defined by the World Health Organization: underweight (below 18.5), normal weight (18.5-24.9), overweight (25-29.9), or obese (30+).
Why BMI Is Limited
BMI was designed as a population-level screening tool, not an individual health metric. It has significant blind spots:
It doesn't distinguish muscle from fat. A 200-lb person at 12% body fat and a 200-lb person at 35% body fat get the same BMI. Most serious lifters will register as "overweight" or "obese" by BMI despite being lean.
It ignores body composition entirely. Two people with identical BMIs can have vastly different health profiles depending on where they carry their weight, how much is muscle, and their metabolic markers.
It doesn't account for age, sex, or ethnicity. Health risks associated with the same BMI differ across demographics.
For a more meaningful measure of body composition, use the Body Fat Calculator instead.
When BMI Is Useful
Despite its limitations, BMI is a reasonable quick check for people who don't do resistance training and are within a normal body composition range. It's also useful for tracking large-scale weight changes over time.