BMI, or Body Mass Index, is a number calculated from your height and weight that is used to estimate whether you fall into an underweight, normal, overweight, or obese weight category. It was developed in the mid-1800s by Belgian mathematician Adolphe Quetelet and has been widely used in public health and clinical practice for decades.
The BMI Formula
BMI = weight (kg) ÷ height (m)²
In imperial units:
BMI = (weight (lbs) × 703) ÷ height (inches)²
For example, someone who is 5'9" (175 cm) and weighs 170 lbs (77 kg):
BMI = (170 × 703) ÷ (69 × 69)
BMI = 119,510 ÷ 4,761
BMI = 25.1
What the Ranges Mean
The standard BMI categories used by the World Health Organization (WHO) and the CDC are:
Under 18.5: Underweight
18.5 – 24.9: Normal weight
25.0 – 29.9: Overweight
30.0 – 34.9: Obese (Class I)
35.0 – 39.9: Obese (Class II)
40.0 and above: Obese (Class III, severe)
These thresholds are population-level guidelines, not clinical diagnoses. Falling outside the "normal" range is not a diagnosis of anything — it's a signal for further evaluation.
Where BMI Falls Short
BMI is a useful population-level screening tool, but it has well-documented limitations when applied to individuals:
It doesn't distinguish fat from muscle. Muscle is denser than fat, so an athlete or bodybuilder may have a high BMI despite having very low body fat. Conversely, someone with low muscle mass and relatively high body fat ("skinny fat") can have a normal BMI while carrying significant health risk.
It doesn't account for where fat is distributed. Abdominal fat (visceral fat, around the organs) carries substantially higher cardiovascular risk than fat stored in the hips and thighs. Two people with identical BMIs can have very different health profiles depending on fat distribution.
It doesn't account for age. Older adults tend to lose muscle mass and gain fat even without changing weight, which can make BMI less informative as an indicator of metabolic health in aging populations.
It was calibrated on European populations. Research has shown that people of Asian descent may face elevated health risks at lower BMI thresholds. The WHO acknowledges that a BMI of 23+ may represent overweight and 27.5+ may represent obesity for these populations.
Better Measures of Health
For a more complete picture than BMI alone, clinicians often consider:
Waist circumference: A waist measurement above 35 inches (women) or 40 inches (men) is associated with elevated cardiovascular risk, regardless of BMI.
Waist-to-height ratio: Dividing waist circumference by height — a ratio below 0.5 is generally considered healthy. This measure accounts for body size better than a fixed cutoff.
Body fat percentage: Measured by methods like DEXA scan, hydrostatic weighing, or BIA (bioelectrical impedance), this directly measures what BMI is trying to approximate.
Metabolic markers: Blood pressure, fasting blood glucose, cholesterol levels, and triglycerides paint a more accurate picture of metabolic health than body composition alone.
Why BMI Is Still Used
Despite its limitations, BMI remains widely used because it's free, instantaneous, and requires no equipment beyond a scale and measuring tape. In epidemiology and public health, it's useful for tracking weight-related trends across large populations over time.
Clinically, BMI is best understood as a starting point — a low-cost screening tool that flags individuals for closer examination, not a definitive assessment on its own.
To calculate your BMI using metric or imperial units and see your weight category and healthy weight range, use the BMI Calculator. To estimate your daily calorie needs based on your height, weight, age, and activity level, use the Calorie Calculator.