How BMI is calculated
BMI is calculated as weight in kilograms divided by height in meters squared: BMI = weight (kg) ÷ height² (m). Using imperial units, the equivalent formula is BMI = 703 × weight (lb) ÷ height² (in).
The result is a single number, unrelated to age or sex in the basic adult formula, which is then compared against standard reference ranges to place a person into a general category.
The standard BMI categories
For adults, commonly used reference ranges are: below 18.5 as underweight, 18.5 to 24.9 as normal weight, 25 to 29.9 as overweight, and 30 or above as obese. These thresholds come from population-level health research and are intended as a general screening tool, not an individual diagnosis.
Because BMI uses only height and weight, two people with the same BMI can have very different body compositions — one might carry more muscle, the other more fat — while landing in the same category.
Why BMI has limitations
BMI doesn't distinguish between muscle and fat, so athletes and very muscular individuals are sometimes classified as overweight or obese despite having low body fat. It also doesn't account for where fat is distributed on the body, which research links to different health risk profiles even at the same total body fat percentage.
BMI reference ranges were developed primarily from studies of adult populations and may be less accurate for children, older adults, pregnant women, and some ethnic groups, for whom the relationship between BMI and health risk can differ from the general adult population.
Using BMI alongside other measures
Because of these limitations, many health professionals treat BMI as one screening input among several, rather than a stand-alone measure of health. Waist circumference, waist-to-height ratio, and body-fat percentage estimates can provide additional context that BMI alone doesn't capture.
This guide is educational and not medical advice. If you have concerns about your weight, body composition, or overall health, a licensed healthcare provider can assess your situation using a fuller range of measurements than BMI alone provides.
Frequently asked questions
Is BMI accurate for everyone?
No single number is. BMI is a reasonable population-level screening tool but is known to be less reliable for very muscular individuals, children, older adults, and some population groups, since it can't distinguish fat from muscle.
What's a healthier alternative to BMI alone?
There isn't one single replacement, but combining BMI with waist circumference, waist-to-height ratio, or a body-fat percentage estimate generally gives a fuller picture than BMI alone.
Can BMI be high even if someone is healthy?
Yes. Because BMI can't tell muscle from fat, a muscular, low-body-fat individual can have a BMI in the 'overweight' or 'obese' range while being in good metabolic health.
Why does BMI use height squared instead of just height?
Weight tends to scale with the square of height across a population, so dividing by height squared, rather than height alone, produces a number that is more comparable across people of different heights.