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 Body Mass Index (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.
Standard vs. Asian BMI Thresholds
Clinical research indicates that the relation between body fat percentage and health risks varies across ethnic groups. Individuals of Asian descent have a higher percentage of body fat at lower BMI values, presenting elevated risks of cardiovascular disease and type 2 diabetes. Below are the standard WHO categories compared to the modified guidelines for Asian populations:
| Weight Category | Standard WHO BMI | Asian Guidelines BMI |
|---|---|---|
| Underweight | < 18.5 | < 18.5 |
| Normal / Healthy Range | 18.5 – 24.9 | 18.5 – 22.9 |
| Overweight | 25.0 – 29.9 | 23.0 – 24.9 |
| Obese | ≥ 30.0 | ≥ 25.0 |
Understanding BMR and TDEE
In addition to your BMI, managing your weight effectively requires an understanding of energy balance. This calculator incorporates two key metrics:
- Basal Metabolic Rate (BMR): Calculated using the clinical Mifflin-St Jeor formula, this is the number of calories your body burns just to stay alive at complete rest.
- Total Daily Energy Expenditure (TDEE): This is your BMR multiplied by your physical activity level. It represents the total daily calories needed to maintain your current weight. Eating fewer calories than your TDEE will result in weight loss, while eating more will result in weight gain.
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.
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.
Benefits of Using the BMI Calculator
Example Calculations
Example 1 — Metric System Calculation
Adult weight: 75 kg · Height: 1.75 meters (175 cm)
Height squared = 1.75 × 1.75 = 3.0625 m²
BMI = 75 ÷ 3.0625 = 24.49
WHO Category = Normal Weight (18.5 – 24.9)
Example 2 — Imperial System Calculation
Adult weight: 180 lbs · Height: 5 feet 10 inches (70 inches)
Height squared = 70 × 70 = 4,900 in²
BMI = (180 ÷ 4,900) × 703 = 25.82
WHO Category = Overweight (25.0 – 29.9)
Frequently Asked Questions
- What is BMI and how is it calculated?
- BMI (Body Mass Index) is a statistical measurement that estimates body fat by dividing your weight in kilograms by your height in meters squared (BMI = kg/m²). For imperial units, the formula is: BMI = (weight in pounds ÷ height in inches squared) × 703. For a 180 lb adult who is 5'10" tall, the calculation is (180 ÷ 4900) × 703 = 25.8.
- What are the official World Health Organization (WHO) BMI categories?
- The WHO defines adult body weight categories as: Underweight (BMI < 18.5), Normal Weight (18.5–24.9), Overweight (25–29.9), and Obese (BMI ≥ 30). Obesity is further split into Class I (30–34.9), Class II (35–39.9), and Class III or Severe Obesity (BMI ≥ 40).
- Why are there different BMI categories for Asian populations?
- Clinical research indicates that individuals of South Asian, East Asian, and Southeast Asian descent face higher risks of type 2 diabetes, hypertension, and cardiovascular disease at lower BMI levels. The modified WHO Asian thresholds are: Underweight (< 18.5), Normal Weight (18.5–22.9), Overweight (23.0–24.9), and Obese (≥ 25.0).
- Why is BMI sometimes inaccurate for muscular athletes?
- BMI only considers weight and height; it cannot distinguish between fat mass and lean muscle mass. Because muscle tissue is much denser than fat tissue, highly muscular individuals (like bodybuilders, weightlifters, and athletes) can have an "overweight" or "obese" BMI despite having extremely low body fat levels.
- What is Basal Metabolic Rate (BMR) and how is it calculated?
- BMR is the baseline number of calories your body needs to maintain basic life-sustaining functions (like breathing, cell production, and temperature regulation) at complete rest. This calculator uses the Mifflin-St Jeor equation: Men: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age) + 5. Women: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age) − 161.
- What is TDEE and how do I use it to manage weight?
- TDEE (Total Daily Energy Expenditure) is the total number of calories you burn per day, calculated by multiplying your BMR by your physical activity factor. To maintain weight, consume calories equal to your TDEE. To lose weight, eat at a deficit (e.g., TDEE − 500 calories/day for about 1 lb of fat loss per week). To build weight, eat at a calorie surplus.
- Can you have a "normal" BMI but still be unhealthy?
- Yes. This condition is known as "normal weight obesity" or colloquially as "skinny fat." An individual may have a normal BMI but carry high levels of visceral fat (fat wrapping around internal organs) combined with low muscle mass, which carries similar metabolic risks to obesity.
- How does age affect BMI interpretation?
- For adults (20 and older), BMI is calculated using the same formula regardless of age. However, older adults (65+) may benefit from a slightly higher BMI (e.g., 23–27) because extra weight can offer protection against osteoporosis, fractures, and muscle wasting during illness.
- Does BMI apply to children and teenagers?
- The basic formula is the same, but the interpretation is completely different. Children and teens (ages 2–19) are evaluated using BMI-for-age growth percentiles because their body compositions change rapidly as they grow. A child is considered obese if their BMI is in the 95th percentile or higher for children of the same age and gender.
- What other health metrics should I track alongside BMI?
- To get a complete view of your health, monitor: (1) Waist circumference (cardiovascular risk increases at > 35 in for women and > 40 in for men), (2) Body fat percentage via calipers or DEXA scans, (3) Blood pressure, and (4) Lipid panel and HbA1c blood sugar levels.