How to Calculate BMI
BMI is a simple ratio of weight to height squared. It's used worldwide as a quick screening tool for weight categories.
BMI Formula
Metric: BMI = weight (kg) ÷ height (m)²
Imperial: BMI = (weight (lbs) × 703) ÷ height (inches)²
Example: A person weighing 70 kg at 1.75 m tall: BMI = 70 / (1.75)² = 70 / 3.0625 = 22.9 (Normal weight)
BMI Categories (WHO)
- Underweight: BMI < 18.5 — may indicate malnutrition or other health issues
- Normal weight: BMI 18.5-24.9 — associated with lowest health risks
- Overweight: BMI 25-29.9 — increased risk for cardiovascular disease
- Obese Class I: BMI 30-34.9
- Obese Class II: BMI 35-39.9
- Obese Class III: BMI ≥ 40 — highest health risk category
Limitations of BMI
BMI is a population-level screening tool, not a diagnostic measure. It has several known limitations:
- Muscle vs fat: Athletes and bodybuilders often have "overweight" BMIs despite low body fat
- Age: Older adults tend to have more body fat at the same BMI as younger adults
- Sex: Women typically carry more body fat than men at identical BMIs
- Ethnicity: Health risks associated with BMI vary across ethnic groups. Some populations face higher risk at lower BMIs
- Fat distribution: BMI doesn't indicate where fat is stored. Visceral (abdominal) fat is more dangerous than subcutaneous fat
For a more complete picture, consider using our Body Fat Calculator or TDEE Calculator alongside BMI.
Better Metrics to Use With BMI
BMI works best as a starting point, not a final answer. Combine it with:
- Waist circumference: Men above 40 inches (102 cm) and women above 35 inches (88 cm) face higher metabolic risk, regardless of BMI
- Waist-to-hip ratio: Divides waist by hip measurement. Above 0.90 (men) or 0.85 (women) indicates higher cardiovascular risk
- Body fat percentage: Directly measures fat vs lean mass, eliminating the muscle-mass blind spot in BMI
No single number tells the full story. A combination of measurements gives the most reliable picture of health risk.
BMI for Children and Teens
Child and teen BMI is interpreted differently from adult BMI. Instead of fixed thresholds, pediatric BMI uses age- and sex-specific percentiles based on CDC growth charts. A child at the 85th percentile is considered overweight; 95th percentile is obese. This percentile approach accounts for the natural changes in body composition during growth and puberty — a BMI of 22 means something very different for a 10-year-old than for a 30-year-old.
BMI and Health Risk
Research consistently shows a U-shaped or J-shaped relationship between BMI and mortality. Both very low (<18.5) and very high (>30) BMI values are associated with increased health risks. The lowest mortality rates generally fall in the 22-25 BMI range. However, the "obesity paradox" — where moderately overweight individuals sometimes show better outcomes in certain conditions like heart failure — remains an active area of research. Context matters: a BMI of 27 in an active 35-year-old with good metabolic markers carries different risk than the same BMI in a sedentary 60-year-old with high blood pressure.
History of BMI
Belgian mathematician Adolphe Quetelet developed the BMI formula in the 1830s as a population-level statistical tool, not a clinical diagnostic. It became widely adopted by insurance companies in the mid-20th century to classify risk. The WHO formally standardized the current BMI categories in 1995. Despite its limitations, BMI remains widely used because it requires no equipment beyond a scale and tape measure, making it accessible for large-scale public health screening.