BMI Calculators

0 calculators tagged with “BMI”

Body Mass Index (BMI) is a weight-for-height measure used to screen for weight categories associated with health risks: BMI = weight (kg) / height² (m²). The World Health Organization classifies adults: underweight < 18.5; normal weight 18.5–24.9; overweight 25.0–29.9; obese ≥ 30. BMI is a practical population screening tool but does not directly measure body fat or its distribution. Athletes with high muscle mass, elderly individuals with muscle loss (sarcopenia), and people from different ethnic backgrounds may be misclassified by standard BMI cutoffs. Asian populations have higher metabolic risk at lower BMI — WHO recommends overweight threshold of 23 for Asian populations.

All Calculators

No calculators found for this topic.

BMI Formula

BMI = weight (kg) / height² (m²)

US units: BMI = 703 × weight (lb) / height² (in²). Example: 70 kg, 1.75 m: BMI = 70 / (1.75)² = 70 / 3.0625 = 22.9 kg/m².

WHO BMI Classification (Adults)

  • < 18.5: Underweight
  • 18.5–24.9: Normal weight
  • 25.0–29.9: Overweight
  • 30.0–34.9: Obese Class I
  • 35.0–39.9: Obese Class II
  • ≥ 40: Obese Class III (severe obesity)

Health Risks by BMI

Overweight (25–29.9): increased risk of T2DM, hypertension, cardiovascular disease. Obese (≥ 30): substantially elevated risk; sleep apnea; osteoarthritis; certain cancers. Underweight (< 18.5): increased mortality; malnutrition; osteoporosis; immunosuppression.

Limitations

  • Doesn't distinguish fat from muscle: bodybuilders may have BMI > 30 with low body fat
  • Doesn't measure fat distribution: visceral (abdominal) fat is more harmful than subcutaneous — waist circumference and waist-to-hip ratio provide additional information
  • Not valid for children (use BMI-for-age percentiles) or pregnant women
  • Different ethnic groups show different cardiometabolic risk at same BMI

Glossary

BMI (Body Mass Index)
Weight (kg) / height² (m²); WHO adult categories: < 18.5 underweight; 18.5–24.9 normal; 25–29.9 overweight; ≥ 30 obese; does not measure body fat or its distribution.
Visceral Fat
Fat stored around abdominal organs; more metabolically active and harmful than subcutaneous fat; measured by waist circumference (> 102 cm men, > 88 cm women = high risk); better predictor of T2DM and cardiovascular risk than BMI alone.
Waist Circumference
Abdominal measurement at the umbilicus; high risk: > 102 cm (40 in) men; > 88 cm (35 in) women; complements BMI by measuring fat distribution and visceral adiposity.

Frequently Asked Questions

BMI = weight (kg) / height² (m²). In US units: BMI = 703 × weight (lb) / height² (in²). Classification: < 18.5 = underweight; 18.5–24.9 = normal weight; 25.0–29.9 = overweight; ≥ 30 = obese. Example: person weighing 68 kg with height 1.70 m: BMI = 68 / (1.70)² = 68 / 2.89 = 23.5 (normal weight). BMI was developed by Adolphe Quetelet in the 1830s and is the most widely used population-level weight screening tool because it requires only weight and height measurements.

Overweight (BMI 25–29.9): increased risk of type 2 diabetes, hypertension, dyslipidemia, coronary artery disease. Obese (BMI ≥ 30): substantially higher risk of T2DM (obese individuals have 7× higher risk); cardiovascular disease; obstructive sleep apnea; weight-bearing joint osteoarthritis; certain cancers (colorectal, breast, endometrial, kidney). Severe obesity (BMI ≥ 40): markedly elevated mortality; bariatric surgery may be indicated. J-shaped relationship: both underweight and obese individuals have elevated mortality; lowest mortality at BMI 20–25. Abdominal obesity (waist circumference > 102 cm men, > 88 cm women) carries additional risk beyond BMI alone.

BMI limitations: (1) Doesn't distinguish fat from muscle: highly muscular athletes (rugby players, weightlifters) may have BMI > 30 with very low body fat percentage. (2) Doesn't measure fat distribution: visceral fat (around organs) is metabolically active and more harmful than subcutaneous fat; two people with the same BMI may have very different health risks based on fat distribution. (3) Ethnic variation: South Asian and East Asian populations develop cardiometabolic complications (T2DM, hypertension) at lower BMIs than Caucasians — WHO recommends Asian overweight threshold at BMI 23. (4) Age: elderly may have normal BMI but high fat percentage (normal-weight obesity or sarcopenic obesity). Better alternatives: DEXA scan; waist-to-height ratio; waist circumference; body fat percentage.

Adult BMI cutoffs (18.5, 25, 30) are not valid for children because BMI changes substantially with age and sex during growth. CDC and WHO use BMI-for-age percentiles: Underweight: < 5th percentile. Healthy weight: 5th–84th percentile. Overweight: 85th–94th percentile. Obese: ≥ 95th percentile (compared to CDC 2000 growth charts or WHO reference data). Age- and sex-specific: a BMI of 20 is normal for a 15-year-old but may indicate overweight in a 5-year-old. Both the CDC and WHO publish growth charts used clinically to monitor childhood BMI-for-age percentiles over time.