BMI (Body Mass Index): what it means and how to use it
BMI (Body Mass Index) is a quick screening metric based on height and weight:
BMI = weight ÷ height²
It’s widely used because it’s fast and correlates with population-level risk—but it’s not a direct measure of body fat or health.
Quick BMI calculator
Adult BMI categories (common cutoffs)
Many sources use these adult cutoffs:
- Underweight: < 18.5
- Normal: 18.5–24.9
- Overweight: 25–29.9
- Obesity: ≥ 30
Cutoffs can vary slightly by guideline and population. Use BMI as a starting point, not a diagnosis.
When BMI can be misleading
BMI often over- or under-estimates risk when:
- You have high muscle mass (BMI looks “high” while body fat is low)
- Body fat distribution differs (central fat vs lower body fat)
- You’re comparing across different ages/ethnic reference ranges
That’s why pairing BMI with waist-based metrics and body fat estimates helps.
Better companion metrics than BMI alone
- Waist-to-height ratio (WHtR) (many people find it more actionable)
- Waist-to-hip ratio (WHR)
- Body fat estimator
- Measurement tracker
FAQ
Is BMI accurate?
BMI is useful at a population level. For individuals, it can be “directionally useful” but imperfect—especially if you’re very muscular or carry fat differently.
Is BMI for kids the same?
No. For children and teens, BMI is interpreted using age/sex percentiles, not adult categories.