Calculate your Body Mass Index (BMI) in metric (kg/cm) or imperial (lbs/ft) units. Includes WHO weight classification, ideal weight range for your height, BMI Prime, and a South Asian threshold comparison.
BMI formula and methodology
Metric: BMI = weight (kg) ÷ height² (m²)
Imperial: BMI = (weight (lbs) × 703) ÷ height² (inches²)
BMI Prime = BMI ÷ 25 (normal range: 0.74–1.00)
Worked example (metric):
Weight: 70 kg | Height: 170 cm = 1.70 m
BMI = 70 ÷ (1.70)² = 70 ÷ 2.89 = 24.2 → Normal weight
Ideal weight range for 170 cm:
Min = 18.5 × 2.89 = 53.5 kg
Max = 24.9 × 2.89 = 71.9 kg
Reference: World Health Organization. Obesity: preventing and managing the global epidemic. WHO Technical Report Series 894. Geneva, 2000.
WHO BMI classification table
| Category | BMI Range | Health Risk |
|---|
| Underweight | < 18.5 | Increased (nutritional deficiency) |
| Normal weight | 18.5 – 24.9 | Lowest risk |
| Overweight | 25.0 – 29.9 | Increased |
| Obese Class I | 30.0 – 34.9 | High |
| Obese Class II | 35.0 – 39.9 | Very high |
| Obese Class III | ≥ 40.0 | Extremely high |
Understanding BMI: what it measures and what it misses
What BMI measures
Body Mass Index is a numerical ratio of weight to height squared. It was developed by Belgian statistician Adolphe Quetelet in the 19th century and adopted by the WHO as a population-level screening tool for weight status. BMI correlates broadly with body fat percentage at the population level, making it useful as a first-stage health screen.
Key limitations of BMI
BMI cannot distinguish between fat mass and muscle mass. A trained athlete with 8% body fat may register BMI 27 (overweight) due to high muscle density. Conversely, an older sedentary person may have a "normal" BMI of 23 but carry significant visceral fat due to age-related muscle loss — a condition called sarcopenic obesity. BMI also does not account for fat distribution, which matters greatly for health risk.
Why South Asian BMI thresholds are different
Multiple studies published in The Lancet, JAMA, and other major journals show that South Asians develop type 2 diabetes and cardiovascular disease at lower BMI values than European populations, likely due to differences in body fat distribution and metabolic response. The Indian Council of Medical Research (ICMR) and several Indian health bodies recommend using BMI 23 as the overweight threshold for Indian adults (vs 25 in WHO standard guidelines).
Better measures to use alongside BMI
Waist circumference is a better predictor of metabolic disease risk than BMI alone. WHO elevated risk thresholds: >80 cm (women), >94 cm (men). Waist-to-hip ratio above 0.90 (men) or 0.85 (women) indicates abdominal obesity. DEXA body composition scan gives the most accurate body fat percentage measurement. Use BMI as a starting point, not a final answer.
Frequently asked questions — BMI calculator
What is a healthy BMI for Indians? ▼
The standard WHO healthy BMI range is 18.5–24.9. However, for Indian adults, the Indian Council of Medical Research and many health experts recommend 18.5–22.9 as the healthy range, with 23–24.9 classified as overweight and 25+ as obese, due to higher metabolic risk at lower BMI values in South Asian populations.
Does BMI apply to children? ▼
No. Children and teenagers use age- and sex-specific BMI percentile charts that account for normal growth patterns. A child's BMI is plotted against percentile charts to determine weight status. This calculator is for adults aged 18 and above only.
Can muscular people have a misleadingly high BMI? ▼
Yes. BMI cannot distinguish fat from muscle. Professional athletes, bodybuilders, and people who do heavy physical work often have BMI in the overweight or obese range despite low body fat percentages. For such individuals, body fat percentage measurement is a more meaningful indicator.
What is BMI Prime? ▼
BMI Prime is your BMI divided by 25 (the upper limit of normal weight). A BMI Prime of exactly 1.0 places you at the boundary of normal and overweight. Values below 1.0 indicate normal or underweight; values above 1.0 indicate overweight or obesity. It allows easy comparison across different BMI classification systems.
How often should I check my BMI? ▼
For most adults, checking every 3–6 months alongside a regular health check-up is sufficient. If you are on a weight management program, monthly checks help track progress. A significant change of ±2–3 BMI units within 3 months warrants medical evaluation.
Is the formula different for imperial measurements? ▼
Yes. For pounds and feet/inches: BMI = (weight in lbs × 703) ÷ (height in inches)². This calculator automatically converts imperial inputs to metric before calculating using the standard WHO metric formula.
What is a healthy waist circumference? ▼
WHO guidelines define elevated metabolic risk at waist circumference above 80 cm (women) and 94 cm (men) for Europeans. For South Asians, the thresholds are lower: 80 cm (women) and 90 cm (men). Measuring waist circumference alongside BMI gives a much better picture of health risk.
Should I consult a doctor about my BMI? ▼
If your BMI is below 18.5 or above 30, or if you notice significant unexplained changes in your BMI, you should speak with your doctor. BMI is a screening tool, not a diagnosis. Your doctor can order appropriate tests and recommend a personalised plan.