BMI (Body Mass Index) Calculator

BMI was designed by Adolphe Quetelet in 1832 as a population-level screening tool, not an individual diagnostic. The formula (weight ÷ height²) cannot distinguish between muscle, fat, or bone mass. Research shows that muscular individuals may be classified as "overweight" despite low body fat. For athletes or those with significant muscle mass, body composition methods like DEXA scans or skinfold measurements provide more accurate assessments. Studies in the Journal of Sports Sciences confirm that up to 50% of athletes are misclassified by BMI.

WHO recommends lower BMI cutoffs for Asian populations because research shows they develop obesity-related health risks (diabetes, cardiovascular disease) at lower BMI values. A 2004 WHO expert consultation found that Asians have higher body fat percentage at the same BMI compared to Caucasians. The suggested Asian-specific cutoffs are: overweight ≥23, obese ≥27.5 (versus standard 25 and 30). This reflects differences in body composition and fat distribution patterns across ethnic groups.

Meta-analyses of large cohort studies (including the Global BMI Mortality Collaboration study of 10.6 million participants) found that all-cause mortality is lowest in the BMI range of 20-25 for non-smokers. However, the relationship is J-shaped: both very low (<18.5) and high (>30) BMI are associated with increased mortality. Interestingly, the "overweight" category (25-30) shows minimal increased risk in many studies, leading to the "obesity paradox" debate in medical literature.

BMI becomes less reliable with age due to age-related changes in body composition. Older adults typically lose muscle mass (sarcopenia) and bone density while potentially gaining fat. Research suggests that slightly higher BMI values (25-27) may actually be protective in adults over 65, associated with better outcomes in illness and lower mortality. Additionally, height loss due to spinal compression can artificially inflate BMI calculations. Waist circumference may be a more useful metric for older adults.

The current WHO BMI thresholds (18.5, 25, 30) were established through epidemiological studies examining the relationship between BMI and health outcomes. The 1995 WHO Expert Committee reviewed mortality and morbidity data showing that health risks increase progressively above BMI 25, with sharp increases above 30. The underweight cutoff of 18.5 was based on studies showing increased mortality and health problems below this level. These thresholds represent points where population-level disease risk notably changes.

Body fat percentage provides more insight into body composition and health risk, but BMI remains useful as an accessible screening tool. Research in Obesity Reviews shows that body fat percentage better predicts metabolic health outcomes. However, accurate body fat measurement requires specialized equipment (DEXA, BIA, hydrostatic weighing). For most people, using BMI alongside waist circumference provides a practical compromise—central obesity (high waist circumference) combined with elevated BMI indicates higher health risk than either metric alone.