BMI Calculator
Calculate your Body Mass Index (BMI) using metric (kg/cm) or imperial (lbs/ft+in) measurements. See your result on a visual BMI scale and learn what your category means.
How It Works
Body Mass Index (BMI) is a screening tool developed by Belgian mathematician Adolphe Quetelet in the 1830s. It is the ratio of weight to height squared and is used as a proxy for body fatness in large population studies.
The formula:
Metric: BMI = weight (kg) / height (m)²
Imperial: BMI = 703 × weight (lbs) / height (in)²
Note: The imperial formula includes the conversion factor 703 because 1 kg/m² = 0.0703 lb/in². Some sources use 704.5 or round differently; the difference is clinically negligible.
Worked example (metric): 70 kg, 175 cm.
Height in meters = 175 / 100 = 1.75 m
BMI = 70 / 1.75² = 70 / 3.0625 = 22.86 kg/m²
Category: Normal weight.
Worked example (imperial): 165 lbs, 5 ft 10 in.
Total inches = 5 × 12 + 10 = 70 inches
BMI = 703 × 165 / 70² = 116,000 / 4,900 = 23.67 kg/m²
Category: Normal weight.
WHO BMI categories for adults:
- Below 18.5: Underweight
- 18.5 to 24.9: Normal weight
- 25.0 to 29.9: Overweight
- 30.0 and above: Obese (Class I: 30–34.9, Class II: 35–39.9, Class III: 40+)
Limitations of BMI: BMI is a population-level screening tool, not a personal health diagnosis. It does not distinguish between muscle and fat — a highly muscular athlete may have a BMI in the "overweight" range while having very low body fat. It also does not account for fat distribution (abdominal fat carries more health risk than hip/thigh fat). Age, sex, and ethnicity further affect the relationship between BMI and actual body fat percentage. Some Asian medical guidelines use lower thresholds (overweight: 23+, obese: 27.5+) because the same BMI corresponds to higher body fat percentage in Asian populations.
Use BMI as a rough guide, not a definitive health measure. Waist circumference, waist-to-hip ratio, and DEXA body composition scans provide more accurate assessments.
Frequently Asked Questions
What is a healthy BMI for adults?
The World Health Organization defines the normal healthy range as 18.5 to 24.9. However, "healthy" varies by individual. Many people with BMIs in the normal range have metabolic risk factors, and many with BMIs just above 25 are in excellent health. Consult a healthcare provider for a full assessment.
Is BMI accurate for athletes and bodybuilders?
Not reliably. BMI cannot distinguish muscle from fat. A 100 kg person at 180 cm has a BMI of 30.9 (obese), but if that weight is mostly muscle, their body fat percentage may be 12–15% — well within a healthy range. Athletes should use body fat percentage measurements instead.
How does BMI differ for children?
For children and teens (ages 2–19), BMI is plotted on age- and sex-specific growth charts as a percentile. The same numeric BMI means different things at different ages during development. Adult thresholds (18.5, 25, 30) do not apply to children.
What is the difference between overweight and obese?
Overweight (BMI 25–29.9) indicates excess body weight relative to height. Obese (BMI 30+) indicates a higher degree of excess weight associated with elevated health risks including type 2 diabetes, heart disease, and certain cancers. Obesity is further classified as Class I (30–34.9), Class II (35–39.9), and Class III (40+).
Can I be unhealthy with a normal BMI?
Yes. "Normal weight obesity" (normal BMI but high body fat percentage) is well-documented and carries similar metabolic risks to clinical obesity. Factors like waist circumference, blood pressure, cholesterol, and blood sugar provide a more complete health picture than BMI alone.