BMI (Body Mass Index) is one of the most used — and most misunderstood — health measurements. It’s a quick screening tool, not a diagnosis. Here’s exactly how to use it, what the numbers mean, and when to look beyond it.
How to Calculate BMI
Use our free BMI calculator — just enter your weight and height. But if you want to understand the math:
- Metric: BMI = weight(kg) ÷ height(m)²
- Imperial: BMI = 703 × weight(lbs) ÷ height(in)²
What Your BMI Number Means
| BMI Range | Category | Health Implications |
|---|---|---|
| Below 18.5 | Underweight | May indicate malnutrition, increased risk of bone loss, immune issues |
| 18.5 – 24.9 | Normal weight | Associated with lowest health risk for most people |
| 25.0 – 29.9 | Overweight | Elevated risk of cardiovascular disease, type 2 diabetes |
| 30.0 – 34.9 | Obese (Class 1) | Significantly elevated health risks |
| 35.0 – 39.9 | Obese (Class 2) | High health risk, medical intervention often recommended |
| 40.0+ | Severely Obese (Class 3) | Very high health risk |
The Real Limitations of BMI
BMI has been widely criticized because it doesn’t distinguish between fat mass and muscle mass. Practical examples:
- A muscular athlete may have a BMI of 28 (classified as overweight) with very low body fat
- An older person may have a BMI of 23 (normal) but high visceral fat and low muscle mass — a condition called “normal-weight obesity”
- BMI was developed from data on European men in the 18th century and may not apply equally to all ethnicities
Better Metrics to Use Alongside BMI
For a more complete picture of health-related body composition, combine BMI with:
- Waist circumference — men: under 40 inches is low risk; women: under 35 inches is low risk. Use our waist-to-hip ratio calculator for a more detailed assessment.
- Body fat percentage — DEXA scan or bioelectrical impedance gives a direct measure of fat vs lean mass
- Waist-to-height ratio — keep your waist circumference below half your height for reduced cardiovascular risk
BMI for Children and Teens
Adult BMI categories don’t apply to children. For ages 2-18, BMI is interpreted using age- and sex-specific percentile charts because body composition changes significantly during growth. A healthcare provider should interpret BMI for children.
Women naturally have higher body fat than men at the same BMI due to hormonal differences. The same BMI may represent different health risks for men and women, which is one reason BMI has limitations as a sole health measure.
Yes — this is called “TOFI” (Thin Outside, Fat Inside). People with normal BMI can have dangerous levels of visceral fat (fat around organs) with a relatively low body mass. Waist circumference and metabolic markers are better predictors in these cases.