BMI Calculator — Know Your Body
Body Mass Index (BMI) is one of the world’s most widely used screening tools for assessing whether a person has a healthy body weight relative to their height. Originally devised by Belgian mathematician Adolphe Quetelet in the 1830s and formally adopted by the World Health Organization in 1995, BMI is now used by healthcare professionals in over 190 countries as a first-line indicator of weight-related health risks.
BMI is calculated by dividing your weight in kilograms by the square of your height in metres: BMI = kg ÷ m². The result places you on a standardised scale from severely underweight to morbidly obese, giving both you and your doctor an instant reference point for potential health concerns.
Research consistently links high BMI with elevated risk of type 2 diabetes, cardiovascular disease, hypertension, sleep apnoea, and certain cancers. Being underweight carries equally serious risks — from immune deficiency to osteoporosis. BMI is valued worldwide precisely because it requires only two measurements and produces a meaningful result in seconds, making it ideal for both population-level tracking and personal health monitoring.
Medical Disclaimer: This BMI calculator is provided for general informational and educational purposes only. BMI is a screening tool, not a diagnostic measure. It does not account for muscle mass, bone density, pregnancy, or ethnic body composition differences. Results must never replace a consultation with a qualified physician, registered dietitian, or licensed healthcare professional. Always seek professional medical advice before making significant changes to your diet or exercise routine. If you have concerns about your weight, please visit your doctor for a comprehensive physical examination and personalised guidance.
Enter Your Details
Underweight
<16 Moderate
Underweight
16–16.9 Mild
Underweight
17–18.4 Normal
Weight
18.5–24.9 Over
weight
25–29.9 Obese
Class I
30–34.9 Obese
Class II
35–39.9 Obese
Class III
≥40
BMI Classification Chart
Understanding the Terms
BMI (Body Mass Index)
A numerical value derived from height and weight (kg ÷ m²). It is the most widely used population-level indicator of underweight, overweight, and obesity as defined by the World Health Organization. It does not directly measure body fat but correlates strongly with direct measures of body fat across most populations.
BMR (Basal Metabolic Rate)
The minimum number of calories your body requires at complete rest to sustain basic physiological functions — breathing, circulation, cell repair, and temperature regulation. Calculated here using the Mifflin-St Jeor equation (1990), widely regarded as the most accurate formula for estimating resting energy expenditure in healthy adults.
Ideal Body Weight (IBW)
The weight range corresponding to a BMI of 18.5–24.9 for your specific height. People within this range are statistically associated with the lowest risk of weight-related chronic diseases, including cardiovascular disease and type 2 diabetes.
Caloric Deficit vs. Surplus
A caloric deficit means consuming fewer calories than you burn — the body uses stored fat for energy, causing weight loss. A caloric surplus means the reverse — weight is gained. One kilogram of body fat stores approximately 7,700 kcal. A safe, sustainable deficit or surplus of 300–500 kcal/day typically yields 0.3–0.5 kg of change per week without harming muscle or metabolism.
Obesity — WHO Classification
Class I (BMI 30–34.9): Moderate health risk. Class II (35–39.9): Severe risk — strongly associated with sleep apnoea, type 2 diabetes, and joint disease. Class III (≥40): Also called morbid obesity — dramatically elevated risk of life-threatening cardiovascular events, organ failure, and reduced life expectancy. Medical intervention is generally recommended for Class II and III.
Underweight Health Risks
A BMI below 18.5 is associated with malnutrition, impaired immune function, osteoporosis (bone fragility), anaemia, hormonal disruption, and infertility. In severe cases (BMI below 16), there is significant risk of cardiac complications and organ failure. Underweight conditions warrant medical investigation to rule out underlying disease.
Limitations of BMI
BMI cannot distinguish between fat mass and lean muscle mass. A muscular athlete may register as “overweight” with very low body fat. Conversely, an older adult with muscle loss (sarcopenia) may appear “normal” while carrying excessive visceral fat. BMI also does not account for fat distribution — abdominal fat carries higher cardiovascular risk than peripheral fat at the same BMI. Always use BMI alongside other health markers.
Sources & References
World Health Organization. (2000). Obesity: Preventing and managing the global epidemic. WHO Technical Report Series 894, Geneva.
Mifflin MD, St Jeor ST, Hill LA, Scott BJ, Daugherty SA, Koh YO. (1990). A new predictive equation for resting energy expenditure in healthy individuals. American Journal of Clinical Nutrition, 51(2), 241–247.
National Heart, Lung, and Blood Institute (NHLBI). Classification of Overweight and Obesity by BMI. Bethesda, MD.
Keys A, Fidanza F, Karvonen MJ, Kimura N, Taylor HL. (1972). Indices of relative weight and obesity. Journal of Chronic Diseases, 25(6–7), 329–343.
Gallagher D, Visser M, Sepulveda D, Pierson RN, Harris T, Heymsfield SB. (1996). How useful is body mass index for comparison of body fatness across age, sex, and ethnic groups? American Journal of Epidemiology, 143(3), 228–239.
Hall KD. (2008). What is the required energy deficit per unit weight loss? International Journal of Obesity, 32(3), 573–576.