Health Calculator
BMI Calculator
Calculate your Body Mass Index (BMI) instantly using the WHO standard formula. Enter your height, weight, age, and activity level to get your BMI score, category, ideal weight range, Basal Metabolic Rate (BMR), and Total Daily Energy Expenditure (TDEE) — in both metric and imperial units.
BMI Calculator
Body Mass Index with BMR & TDEE
Results
Enter your height, weight & age, then click Calculate BMI
What Is BMI?
Body Mass Index (BMI) is a simple numerical measure of body size calculated from your height and weight. Developed by Belgian mathematician Adolphe Quetelet in the 1830s, it was designed as a population-level statistical tool — not as a diagnostic measure for individuals.
Despite its limitations, BMI remains the most widely used screening tool for body weight categories globally because it requires only two easily measured values and correlates broadly with body fat levels across large populations. The World Health Organization (WHO) defines the standard classification thresholds used in this calculator.
BMI should always be interpreted in context. A BMI in the "Normal" range does not guarantee good health, and a BMI in the "Overweight" range does not mean someone is unhealthy — particularly for athletes or individuals with high muscle mass. Use BMI as one data point alongside waist circumference, body composition, blood markers, and clinical evaluation.
BMI Formula & BMR Calculation
This calculator uses the standard WHO formula for BMI and the Mifflin–St Jeor equation for BMR, the most accurate general-purpose BMR formula validated in clinical research.
WHO BMI Categories
| Category | BMI Range |
|---|---|
| Severely Underweight | < 16.0 |
| Underweight | 16.0–18.4 |
| Normal Weight | 18.5–24.9 |
| Overweight | 25.0–29.9 |
| Obese (Class I) | 30.0–34.9 |
| Obese (Class II) | 35.0–39.9 |
| Obese (Class III) | ≥ 40.0 |
Health Implications of BMI
Research consistently shows that BMI outside the normal range is associated with increased risk of several chronic conditions:
- Underweight risks: Malnutrition, weakened immune system, osteoporosis, anemia, fertility issues, and increased surgical complications. BMI below 18.5 is associated with higher all-cause mortality.
- Normal weight benefits: Lower risk of cardiovascular disease, type 2 diabetes, hypertension, sleep apnea, and several cancers. Associated with longer life expectancy and better metabolic health markers.
- Overweight & obese risks: Significantly elevated risk of type 2 diabetes, heart disease, stroke, hypertension, sleep apnea, osteoarthritis, and several cancers (breast, colon, endometrial). Each 5-unit increase in BMI above 25 raises all-cause mortality risk by approximately 29%.
Waist Circumference Matters Too
Waist circumference is an independent predictor of metabolic risk. Health risk increases substantially when waist circumference exceeds 88 cm (35 in) for women or 102 cm (40 in) for men — even within a "normal" BMI range. Central (abdominal) fat is more metabolically active than subcutaneous fat.
Frequently Asked Questions
- What is BMI and how is it calculated?
- BMI (Body Mass Index) is a numerical value calculated by dividing your weight in kilograms by the square of your height in meters: BMI = kg / m². It was developed in the 1830s by Adolphe Quetelet as a population-level screening tool. For a 75 kg person who is 1.75 m tall, BMI = 75 / (1.75²) = 24.5.
- What are the BMI categories?
- The WHO defines four primary categories: Underweight (BMI < 18.5), Normal weight (18.5–24.9), Overweight (25–29.9), and Obese (≥ 30). Obesity is further subdivided into Class I (30–34.9), Class II (35–39.9), and Class III (≥ 40). Some classifications also include Severely Underweight (< 16.0).
- Is BMI an accurate measure of body fat?
- BMI is a useful screening tool but does not directly measure body fat. It cannot distinguish between fat and muscle mass — a muscular athlete may have an 'overweight' BMI despite having very low body fat. Similarly, an elderly person with low muscle mass may have a 'normal' BMI despite high fat percentage. BMI should be interpreted alongside waist circumference, body fat percentage, and clinical context.
- What is BMR and how is it calculated?
- BMR (Basal Metabolic Rate) is the number of calories your body burns at complete rest — just to maintain vital functions like breathing and circulation. This calculator uses the Mifflin–St Jeor equation, the most validated formula: Men: (10 × weight_kg) + (6.25 × height_cm) − (5 × age) + 5. Women: (10 × weight_kg) + (6.25 × height_cm) − (5 × age) − 161.
- What is TDEE and how do I use it?
- TDEE (Total Daily Energy Expenditure) is your BMR multiplied by an activity factor. It represents how many calories you burn per day in total. To lose weight, consume fewer calories than your TDEE. To gain weight, consume more. A 500 kcal/day deficit typically leads to approximately 0.5 kg (1 lb) of weight loss per week.
- Does BMI apply equally to all ethnicities?
- No — research shows that certain ethnic groups, particularly South Asian, East Asian, and some Middle Eastern populations, face increased health risks at lower BMI thresholds. The WHO recommends lower cut-off points for these groups (e.g., overweight at BMI ≥ 23 for many Asian populations). Always consult a healthcare professional for interpretation.