Body Mass Index (BMI) is a quick screening number that relates weight to height. It is not a body-fat measurement, but it is a widely used starting point for spotting whether someone is underweight, in a healthy range, overweight, or in the obese category.
How it works
BMI is weight in kilograms divided by height in meters squared. The World Health Organization categorizes the result: under 18.5 is underweight, 18.5–24.9 is healthy, 25–29.9 is overweight, and 30 or above is obese. The imperial formula multiplies pounds ÷ inches² by 703 to arrive at the same figure.
A quick example
Someone 5'9" and 170 pounds has a BMI of about 25.1 — just inside the overweight band, though a well-muscled athlete with the same numbers could still be lean. That caveat matters: BMI treats a linebacker and a couch potato identically if they weigh the same.
Tips for accurate results
Weigh yourself at the same time of day and in similar clothing to keep readings comparable. Pair BMI with a waist measurement or body-fat estimate for a fuller picture, especially if you carry a lot of muscle. And treat category boundaries as guides, not verdicts.
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