What is BMI and How is it Calculated?
Body Mass Index (BMI) is a simple numerical measure calculated from your height and weight. It was developed by Belgian mathematician Adolphe Quetelet in the 1830s and adopted by the WHO as a population-level health screening tool.
BMI = Weight (kg) ÷ Height (m)²
Example: 70 kg, 170 cm (1.70 m)
BMI = 70 ÷ (1.70 x 1.70)
= 70 ÷ 2.89
= 24.2 → Normal weight
Imperial formula:
BMI = [Weight (lbs) ÷ Height (in)²] x 703
WHO BMI Classification
Below 18.5 → Underweight
18.5 – 24.9 → Normal weight (healthy range)
25.0 – 29.9 → Overweight
30.0 – 34.9 → Obese - Class I
35.0 – 39.9 → Obese - Class II
40.0 and above → Obese - Class III (Severe)
Asia-Pacific (Indian) BMI Guidelines
The standard WHO thresholds (25 for overweight, 30 for obese) were developed from studies of European populations. Several studies on South Asian populations - including Indians - have found that metabolic complications like insulin resistance, type 2 diabetes, and hypertension occur at lower BMI values than in Europeans.
The Asia-Pacific guidelines (used by several Indian medical bodies) suggest:
Below 18.5 → Underweight
18.5 – 22.9 → Normal weight
23.0 – 27.4 → Overweight (at risk)
27.5 and above → Obese (high risk)
Limitations of BMI
BMI does not distinguish between muscle mass and fat mass, so muscular athletes may be classified as overweight despite having low body fat. It does not account for fat distribution (abdominal fat is more risky than peripheral fat). Age, sex, and ethnicity also affect the relationship between BMI and health risk. For a more complete picture, waist circumference, waist-to-height ratio, and body fat percentage are useful additional measures.
Frequently Asked Questions
What is a healthy BMI for Indian adults?
By WHO standard criteria, a BMI of 18.5–24.9 is considered normal for adults of all ethnicities. However, several Indian health organisations and the Asia-Pacific guidelines use a lower threshold - BMI >=23 as overweight and BMI >=27.5 as obese for South Asians. This is because studies show Indians tend to carry more visceral (abdominal) fat at lower BMI values compared to Europeans, leading to higher diabetes and cardiovascular risk at the same BMI.
Does BMI differ for men and women?
The BMI formula and WHO classification ranges are the same for men and women. However, women naturally carry more body fat than men at the same BMI. A BMI of 22 in a woman typically corresponds to a higher body fat percentage than the same BMI in a man. For a sex-adjusted assessment, body fat percentage is a better metric than BMI alone.
Is BMI accurate for children?
BMI is used for children and teens aged 2–19, but it is interpreted differently - using age- and sex-specific percentiles rather than fixed ranges. For example, a BMI at the 85th percentile for a child's age and sex is classified as overweight, and the 95th percentile as obese. Our calculator uses adult WHO ranges. For children, consult a paediatrician or use a dedicated paediatric BMI chart.
What is the Devine ideal weight formula?
The Devine formula was developed in 1974 to calculate ideal body weight for medication dosing. Male: 50 kg + 2.3 kg for each inch over 5 feet. Female: 45.5 kg + 2.3 kg for each inch over 5 feet. It is a rough guide and doesn't account for body frame size or muscle mass. The healthy weight range shown in our calculator (BMI 18.5–24.9 for your height) is a broader, more practical target range.
