BMI Calculator

Body Mass Index (BMI) = weight (kg) ÷ height² (m²). Imperial: BMI = 703 × weight (lb) ÷ height² (in²). Example: 70 kg at 1.75 m → BMI = 70 / (1.75²) = 70 / 3.0625 = 22.9. Categories (WHO): Underweight: < 18.5. Normal weight: 18.5–24.9. Overweight: 25–29.9. Obese Class I: 30–34.9. Obese Class II: 35–39.9. Obese Class III: ≥ 40. BMI was developed by Adolphe Quetelet in the 1830s and is widely used in clinical settings despite its limitations.

BMI is a simple screening tool but has significant limitations: It doesn't measure body fat percentage or distribution. Muscular individuals (athletes) may have high BMI but low body fat. Older adults lose muscle mass — BMI may underestimate body fat. Different ethnic groups have different BMI thresholds for health risk (Asians at higher risk at lower BMI). BMI doesn't account for bone density. Better measures: waist circumference, waist-to-hip ratio, DEXA scan body fat %, or waist-to-height ratio. A person's overall health picture is far more than one number.

Healthy weight range = BMI 18.5 to 24.9, applied to your height. Formula: min weight = 18.5 × height² (m²), max weight = 24.9 × height² (m²). Example: 1.75 m tall → healthy range = 18.5 × 3.0625 to 24.9 × 3.0625 = 56.7 kg to 76.1 kg (125 lb to 168 lb). Note: these are statistical ranges, not personal prescriptions. Age, gender, ethnicity, muscle mass, and medical conditions all influence what's healthy for a specific individual.

Adults (20+): The standard BMI categories apply. Children (2–19): Compared to age and sex-specific growth charts — percentiles, not categories. Same BMI can be normal for one child and overweight for another of different age/sex. Women: Typically have more body fat than men at the same BMI due to physiological differences (reproductive organs, hormones). Elderly: May need lower BMI cutoffs — some studies show slightly higher BMI (25–27) is associated with lower mortality in over-65 age group (obesity paradox).

BMI: Calculated from height and weight — easy, free, non-invasive. Does not directly measure fat. Body fat %: Directly measures what proportion of body weight is fat. Methods: DEXA scan (most accurate), hydrostatic weighing, skinfold calipers, bioelectrical impedance (consumer scales). Healthy body fat %: Men: 10–20% (athletic: 6–13%). Women: 20–30% (athletic: 14–20%). Two people with identical BMI can have very different body fat % — the athletic person has more muscle and less fat than the sedentary person.