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BMI

General

Body Mass Index

A simple numerical measure of body fat based on height and weight, used to classify individuals as underweight, normal weight, overweight, or obese.

Definition

BMI (Body Mass Index) is a screening index that estimates body fat based on a person's weight and height. Developed by Belgian statistician Adolphe Quetelet in the 19th century, BMI provides a simple, non-invasive way to classify individuals into weight categories associated with various health risks.

BMI = Weight (kg) / Height (m)²

BMI is widely used by healthcare providers and epidemiologists to identify population-level obesity trends and screen individuals for weight-related health risks. However, BMI is a population screening tool — it provides a signal, not a diagnosis. It cannot distinguish muscle from fat, nor does it measure fat distribution, which is increasingly recognised as critical for metabolic health.

For Indian and South Asian populations specifically, research shows that health risks (type 2 diabetes, cardiovascular disease, metabolic syndrome) begin at lower BMI values than traditional WHO cut-offs — making adjusted BMI thresholds important for this population.

Formula

BMI = Weight (kg) / [Height (m)]²

Or using centimetres:

BMI = Weight (kg) × 10,000 / [Height (cm)]²

WHO BMI Categories (adjusted for South Asians):

BMI Range Global Category Indian/South Asian Category
< 18.5 Underweight Underweight
18.5–22.9 Normal Normal
23–24.9 Normal (global) Overweight
25–29.9 Overweight Obese Class I
≥ 30 Obese Obese Class II+

Worked Example

Rohit: Weight 78 kg, Height 1.72 m (172 cm)

BMI = 78 / (1.72)² = 78 / 2.9584 = 26.4

Global WHO classification: Overweight (25–29.9) Indian-adjusted classification: Obese Class I (>25 triggers higher risk category)

Sunita: Weight 55 kg, Height 1.60 m (160 cm)

BMI = 55 / (1.60)² = 55 / 2.56 = 21.5

Global and Indian classification: Normal weight

Use the BMI calculator to calculate your BMI and see personalised guidance, and the ideal weight calculator to see your healthy weight range.

Key Things to Know

  • Waist circumference adds critical context: BMI doesn't capture where fat is stored. Abdominal (visceral) fat around internal organs is significantly more harmful than subcutaneous fat (fat under the skin). Indian adults with BMI in the "normal" range (18.5–22.9) can still carry dangerous amounts of visceral fat — the "thin fat" or TOFI (Thin Outside, Fat Inside) phenotype is particularly common among South Asians. Waist circumference targets: Men < 90 cm, Women < 80 cm for Indian adults.
  • BMR and TDEE provide action guidance: While BMI tells you where you are, BMR (Basal Metabolic Rate) and TDEE (Total Daily Energy Expenditure) tell you what to do about it. If your BMI indicates overweight, calculating your TDEE reveals your maintenance calories — creating a 15–20% caloric deficit while maintaining protein intake facilitates healthy fat loss. Use BMI as the diagnostic; TDEE as the intervention framework.
  • BMI and disease risk: Indian research shows the risk inflection for type 2 diabetes occurs at BMI 22–23 (significantly lower than the global 25 threshold). The Indian Council of Medical Research and diabetes specialists recommend Indians treat BMI 23+ as overweight and BMI 25+ as obese. For cardiovascular risk screening, BMI combined with waist circumference and metabolic markers (HbA1c, lipid panel) provides a far more complete risk picture than BMI alone.
  • Athletes and BMI inaccuracy: Muscle tissue is denser than fat — a 100 kg professional athlete with 8% body fat and extensive muscle mass will have a "obese" BMI above 30. For individuals with high muscle mass from resistance training, body fat percentage (measured via DEXA or calipers) or waist-to-height ratio (<0.5 is healthy) are more accurate health indicators than BMI.
  • BMI and children — growth charts matter: Adult BMI categories should never be applied to individuals under 18. Children's BMI must be interpreted against age- and sex-specific growth charts. A BMI of 21 could be healthy for a 15-year-old boy but the same value might be "underweight" for an adult. Use age-specific percentile charts from the Indian Academy of Pediatrics for accurate paediatric assessment.

Related Calculators

BMI Calculator →Calculate your BMI instantly using kg/lbs and cm/ft. Includes BMI category, healthy weight range for Indian adults.
Ideal Weight Calculator →Find your ideal body weight and healthy weight range based on height and gender. Uses three medical formulas — Robinson, Miller, and Devine — for an accurate estimate. Free calculator for India.
Body Fat Calculator →Calculate your body fat percentage using the accurate U.S. Navy method. Know your fat mass, lean mass, fitness category, and how much fat to lose.

Related Terms

BMRBasal Metabolic RateTDEETotal Daily Energy Expenditure
Frequently Asked Questions
What are the BMI categories and normal range?
WHO global BMI categories: Underweight < 18.5; Normal weight 18.5–24.9; Overweight 25–29.9; Obese Class I 30–34.9; Obese Class II 35–39.9; Obese Class III ≥ 40. For South Asians including Indians, the WHO recommends adjusted cut-offs due to higher cardiometabolic risk at lower BMI: Underweight < 18.5; Normal 18.5–22.9; Overweight 23–24.9; Obese ≥ 25. At the same BMI, South Asians have higher body fat percentage and greater risk of diabetes and cardiovascular disease than Caucasians.
Is BMI an accurate measure of body fat?
BMI is a screening tool, not a precise body fat measurement. Its limitations: (1) Cannot distinguish muscle from fat — athletes with high muscle mass are often classified as overweight. (2) Does not measure fat distribution — abdominal (visceral) fat is more harmful than subcutaneous fat. (3) Age bias — older adults lose muscle and gain fat while BMI may stay constant. (4) Gender difference — women naturally have more body fat than men at the same BMI. More accurate fat measurements: DEXA scan, hydrostatic weighing, or waist-to-height ratio for cardiovascular risk.
What is BMI for children?
Adult BMI cut-offs do not apply to children. For individuals aged 2–19, BMI is interpreted using growth charts from the Indian Academy of Pediatrics or US CDC. BMI percentile for age is used: Below 5th percentile = Underweight; 5th to 84th percentile = Healthy weight; 85th to 94th percentile = Overweight; 95th percentile or above = Obese. A child with the same BMI can be healthy or overweight depending on their age and sex.
Should I target a specific BMI or body weight?
Rather than targeting a specific BMI, focus on: (1) Waist circumference — below 80 cm for Indian women, 90 cm for Indian men, reducing abdominal fat risk. (2) Metabolic markers — blood glucose, HbA1c, lipid profile — which reflect metabolic health independent of weight. (3) Physical fitness — cardiovascular fitness and muscle strength are as important as weight. An 'overweight' fit person is often healthier than a 'normal BMI' sedentary person. Use BMI as one data point alongside these other markers.
How often should I track my BMI?
For most adults, tracking BMI monthly is sufficient. Daily weight fluctuations (up to 2–3 kg) due to hydration, meals, and salt intake make daily BMI tracking misleading. Monthly tracking reveals genuine trends. When making lifestyle changes, weekly tracking is acceptable but averaging 4-week windows provides a cleaner signal. BMI calculation should use consistent measurement conditions: same time of day (morning before breakfast), same clothing, same scale.