Calculated using standard mathematical and industry formulas.
Body Mass Index (BMI) measures body fat based on weight relative to height: BMI = weight (kg) / height (m)². World Health Organization guidelines classify BMI under 18.5 as underweight, 18.5–24.9 as healthy weight, 25–29.9 as overweight, and 30 or higher as obese.
How to use this calculator?
Pick your unit system — metric (cm, kg) or imperial (ft/in, lb).
Enter your height. In imperial, fill feet and inches separately.
Enter your weight — the BMI, WHO category and healthy weight range update instantly.
Compare your current weight with the healthy range shown for your exact height.
Recheck after meaningful weight changes; BMI moves with weight, not with fitness alone.
Imperial inputs are converted first: 1 ft = 30.48 cm, 1 in = 2.54 cm, 1 lb = 0.4536 kg. WHO ranges: under 18.5 underweight · 18.5–24.9 normal · 25–29.9 overweight · 30+ obese.
Step-by-step calculation example
Height 175 cm and weight 70 kg → BMI = 70 ÷ 1.75² = 22.9, which falls in the normal range (18.5–24.9).
Key insights to remember
WHO adult ranges: below 18.5 underweight, 18.5–24.9 normal, 25–29.9 overweight, 30 and above obese — identical for men and women.
For a person of average height (1.75 m), a change of about 3 kg (6.6 lb) moves BMI by one full point.
BMI cannot tell muscle from fat: many muscular athletes read 'overweight' while being lean.
Waist circumference adds information BMI misses — abdominal fat carries risk even at a 'normal' BMI.
Children and teens are assessed with age- and sex-specific BMI percentiles, not these adult cut-offs.
How to interpret your results?
Official WHO BMI Categories
Below 18.5: Underweight; 18.5 to 24.9: Normal weight; 25.0 to 29.9: Overweight; 30.0 and above: Obesity (Classes 1, 2, and 3).
Expert advice & guidance
💡 Health tip: BMI is a broad screening metric that does not distinguish muscle mass from fat tissue. Athletic individuals should combine BMI with waist circumference or body fat percentage for comprehensive assessment.
Frequently asked questions
What are the official WHO BMI categories and obesity classes?
WHO classifies BMI: Underweight (<18.5), Normal weight (18.5–24.9), Overweight (25–29.9), Class 1 Obesity (30–34.9), Class 2 Obesity (35–39.9), and Class 3 Severe Obesity (40+).
How to calculate BMI manually using height and weight?
To calculate BMI: weight in kilograms divided by height in meters squared (BMI = kg / m²). In imperial: (weight in lbs / (height in inches)²) × 703.
Is BMI accurate for bodybuilders and athletes with high muscle mass?
No, BMI does not distinguish between muscle tissue and fat mass. Bodybuilders and muscular athletes often register as overweight or obese despite low body fat percentage.
What health risks are associated with a high BMI?
BMI levels over 30 significantly increase risks of cardiovascular disease, type 2 diabetes, hypertension, sleep apnea, and osteoarthritis.
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