BMI Calculator
Calculate your Body Mass Index from height and weight. Get your BMI category, healthy weight range, and health risk assessment for adults.
Calculator
Adjust values & calculateBMI Categories (WHO)
Formula
BMI compares body weight with height. In metric units, divide weight in kilograms by height in meters squared. In imperial units, use BMI = [weight in lbs x 703] / [height in inches]^2.
Last reviewed: January 2026
Worked Examples
Example 1: Normal BMI Calculation
Example 2: Obese BMI Calculation
Example 3: Underweight BMI Calculation
Background & Theory
The BMI Calculator - Body Mass Index applies the following established principles and formulas. Health and medicine calculators are grounded in validated physiological measurement methods established through decades of clinical research. Body Mass Index, or BMI, is calculated by dividing weight in kilograms by height in meters squared (kg/m²), a formula originating from Adolphe Quetelet's 19th-century statistical work and later codified by the WHO into standard classifications: underweight below 18.5, normal weight 18.5 to 24.9, overweight 25 to 29.9, and obese at 30 and above. Basal Metabolic Rate quantifies the minimum energy required to sustain life at rest. The Mifflin-St Jeor equation, published in 1990 and widely regarded as the most accurate for most adults, calculates BMR as (10 × weight in kg) + (6.25 × height in cm) − (5 × age) ± sex adjustment. The older Harris-Benedict equations, revised in 1984 by Roza and Shizgal, remain in common use. Total Daily Energy Expenditure is derived by multiplying BMR by a physical activity factor ranging from 1.2 for sedentary individuals to 1.9 for extremely active ones, following the methodology validated by doubly labeled water studies. Body fat percentage can be estimated without laboratory equipment using the U.S. Navy circumference method, which uses neck, waist, and hip measurements, or via BMI-derived equations adjusted for age and sex. The Jackson-Pollock skinfold method offers higher precision with calipers. Blood pressure classification, according to the American College of Cardiology and the 2017 ACC/AHA guidelines, defines normal as below 120/80 mmHg, elevated as 120 to 129 systolic, and hypertension stage 1 as 130 to 139 systolic or 80 to 89 diastolic. Target heart rate zones for aerobic exercise are derived from maximum heart rate estimates, most commonly using the formula 220 minus age in years, with moderate-intensity training typically defined as 50 to 70 percent of maximum heart rate and vigorous intensity at 70 to 85 percent, consistent with CDC and American Heart Association guidelines. These thresholds guide safe and effective cardiovascular conditioning.
History
The history behind the BMI Calculator - Body Mass Index traces back through the following developments. The history of health measurement stretches back to ancient Greece, where Hippocrates around 400 BCE laid the foundation for observational medicine by systematically recording patient symptoms, diet, and environment. His humoral theory, though scientifically superseded, established the principle that the body operates as an interconnected system subject to measurable imbalance. The transformation toward modern medicine accelerated in the 19th century. Louis Pasteur and Robert Koch developed germ theory in the 1860s and 1870s, identifying microorganisms as disease agents and enabling targeted interventions. Florence Nightingale, working during the Crimean War in the 1850s, introduced statistical analysis to nursing practice, demonstrating through data visualization that sanitation reduced mortality. Her work is foundational to evidence-based health measurement. The discovery of vitamins in the early 20th century, beginning with Casimir Funk's coinage of the term in 1912 and culminating in the isolation of vitamins A through K, created the field of nutritional science and gave rise to dietary reference intake frameworks. The World Health Organization, founded in 1948, subsequently established global standards for health metrics, disease classification through the International Classification of Diseases, and recommended daily allowances. The BMI as a clinical screening tool gained traction in the 1970s through Ancel Keys' large-scale epidemiological work, which validated Quetelet's index as a population-level obesity indicator. Through the 1980s and 1990s, the Framingham Heart Study produced landmark data linking cholesterol, blood pressure, and lifestyle factors to cardiovascular disease risk, directly shaping the numeric thresholds still used in health calculators. The evidence-based medicine movement, formalized by Gordon Guyatt and colleagues at McMaster University in the early 1990s, demanded that all health recommendations derive from systematically graded clinical evidence. The digital health era beginning in the 2000s brought these formulas to consumer devices, wearable sensors, and smartphone applications, expanding access to health self-monitoring on a global scale and enabling population-level data collection that continues to refine clinical reference ranges.
Key Features
- Calculate BMI from height and weight with automatic classification into underweight, normal, overweight, and obese ranges, including interpretation of associated health risks for each category.
- Estimate body fat percentage using multiple validated formulas including the U.S. Navy tape-measure method and the Deurenberg equation, allowing comparison across approaches for greater accuracy.
- Compute ideal body weight using several clinical equations (Robinson, Miller, Devine, Hamwi) so users can see the range of targets used in different medical contexts.
- Determine medication dosage by patient weight and age using standard weight-based dosing formulas, useful for verifying pediatric and adult prescription calculations.
- Estimate glomerular filtration rate (GFR) using the CKD-EPI and Cockcroft-Gault equations to help assess kidney function stages from basic lab values.
- Calculate 10-year cardiovascular risk using the Framingham Risk Score based on age, cholesterol, blood pressure, smoking status, and diabetes, with risk category classification.
- Assess waist-to-hip ratio and compare it against sex-specific thresholds to indicate low, moderate, or high risk for metabolic and cardiovascular disease.
- Compute daily calorie needs using both the Harris-Benedict and Mifflin-St Jeor equations adjusted for activity level, providing a reliable baseline for dietary planning.
Frequently Asked Questions
Reviewed by Daniel Agrici, Founder & Lead Developer · Editorial policy
BMI Calculator Formula
BMI = weight (kg) / height (m)^2
BMI compares body weight with height. In metric units, divide weight in kilograms by height in meters squared. In imperial units, use BMI = [weight in lbs x 703] / [height in inches]^2.
BMI Calculator — Worked Examples
Example 1: Normal BMI Calculation
Problem: Calculate BMI for a 35-year-old male who is 5'10\" (178 cm) and weighs 170 lbs (77 kg).
Solution: Height in meters = 178 / 100 = 1.78 m\nWeight in kg = 170 x 0.4536 = 77.1 kg\nBMI = 77.1 / (1.78)^2 = 24.3\nCategory: Normal Weight\nIdeal weight range: about 129 to 173 lbs
Result: BMI: 24.3 - Normal Weight | Health Risk: Low
Example 2: Obese BMI Calculation
Problem: Calculate BMI for a 45-year-old female who is 5'4\" (163 cm) and weighs 200 lbs (91 kg).
Solution: Height in meters = 163 / 100 = 1.63 m\nWeight in kg = 200 x 0.4536 = 90.7 kg\nBMI = 90.7 / (1.63)^2 = 34.1\nCategory: Obese Class I (30-34.9)\nIdeal weight range: about 107 to 144 lbs\nWeight to lose for normal BMI: about 56 lbs
Result: BMI: 34.1 - Obese Class I | Increased risk of type 2 diabetes and cardiovascular disease
Example 3: Underweight BMI Calculation
Problem: Calculate BMI for a 22-year-old female who is 5'7\" (170 cm) and weighs 105 lbs (48 kg).
Solution: Height in meters = 170 / 100 = 1.70 m\nWeight in kg = 105 x 0.4536 = 47.6 kg\nBMI = 47.6 / (1.70)^2 = 16.5\nCategory: Severe Thinness (below 16)\nIdeal weight range: about 121 to 163 lbs\nWeight to gain for normal BMI: about 16 lbs
Result: BMI: 16.5 - Severe Thinness | Health Risk: Very High
BMI Calculator — Frequently Asked Questions
What is BMI and what does it measure?
BMI, or Body Mass Index, is a screening tool that compares weight and height. It is useful for broad health screening, but it does not directly measure body fat.
What is a healthy BMI range?
For most adults, a BMI from 18.5 to 24.9 is considered a healthy range. Lower values indicate underweight status, while higher values move into overweight and obesity ranges.
What are the limitations of BMI?
BMI does not separate fat from muscle, does not show where body fat is stored, and does not fully account for age, sex, ethnicity, or athletic build. It works best alongside other health measures.
Is BMI accurate for children?
Adult BMI ranges are not used for children and teens. Younger users need age- and sex-specific BMI percentile charts, which is why child and teen BMI tools are handled separately.
What is body composition and why is it better than BMI alone?
Body composition describes what your body is actually made of: skeletal muscle, fat mass, bone mineral density, and water. Unlike BMI — which divides weight by height squared and cannot distinguish a pound of muscle from a pound of fat — body composition identifies whether weight is metabolically active tissue or stored energy. Healthy body fat percentages vary by sex and age: for women, 20-32% is generally considered healthy; for men, 8-19%. Measurement methods include DEXA scans (most accurate, ±1-2%), hydrostatic weighing, Bod Pod air displacement, bioelectrical impedance (consumer scales, ±3-5%), and skinfold calipers. A muscular person with a BMI of 27 (overweight) might have excellent body composition, while a sedentary person with a normal BMI could have metabolically risky visceral fat levels.
Is BMI an accurate measure of health?
BMI (Body Mass Index = weight in kg ÷ height in m²) is a quick, free screening tool with clear population-level correlations to health risk — the overweight range (25-29.9) and obese range (30+) are associated with higher rates of type 2 diabetes, cardiovascular disease, and joint problems. However, BMI has well-documented limitations. It cannot distinguish muscle from fat: a 200 lb athlete with 10% body fat and a 200 lb sedentary person with 35% body fat have identical BMIs but vastly different health profiles. It also ignores fat distribution — visceral fat around abdominal organs (measured by waist circumference) is far more metabolically dangerous than subcutaneous fat. Additionally, BMI thresholds were derived from European populations and may be less accurate for Asian, Black, and other ethnic groups. For a fuller picture, combine BMI with waist circumference, body fat percentage, and blood panel results.