BMI Calculator: Calculate Your Body Mass Index

Body Mass Index is calculated using simple algebraic formulas that compare a person's total weight to their height. Depending on your region, you can compute BMI using either the International System of Units (metric) or the Imperial system (US customary units). Both mathematical approaches yield equivalent results when identical physical measurements are provided.

Metric Formula: In the metric system, height is expressed in metres and weight in kilograms. Divide weight in kilograms by the square of height in metres:

BMI = Weight (kg) ÷ [Height (m)]²

Worked Metric Example: Consider an adult with a height of 175 cm and a weight of 70 kg. First, convert 175 cm to metres by dividing by 100: 175 ÷ 100 = 1.75 m. Next, square the height: 1.75 × 1.75 = 3.0625. Finally, divide weight by squared height: 70 ÷ 3.0625 = 22.857..., which rounds to 22.9 kg/m² (Healthy Weight).

Imperial Formula: In the imperial system, height is measured in total inches and weight in pounds. Because pounds and inches use different fundamental units than kilograms and metres, a conversion factor of 703 is applied:

BMI = 703 × Weight (lbs) ÷ [Height (in)]²

Worked Imperial Example: Consider an adult who is 5 feet 9 inches tall and weighs 154 pounds. First, convert height into total inches: (5 × 12) + 9 = 60 + 9 = 69 inches. Next, square the height: 69 × 69 = 4,761. Finally, multiply weight by 703 and divide: (703 × 154) ÷ 4,761 = 108,262 ÷ 4,761 = 22.739..., which rounds to 22.7 (Healthy Weight).

• Metric units: Height in cm (converted to m) and weight in kg.
• Imperial units: Height in feet and inches (converted to total inches) and weight in pounds.
• Rounding rule: Standard clinical screening rounds the final BMI score to one decimal place.
• Calculation consistency: Both formulas yield virtually identical scores for equivalent measurements (e.g., 70 kg = 154.3 lbs; 175 cm = 68.9 inches).

The World Health Organization (WHO), the U.S. Centers for Disease Control and Prevention (CDC), and the UK National Health Service (NHS) classify adult Body Mass Index into four primary standard categories. These brackets apply equally to men and women aged 18 to 65 across diverse adult populations.

The table below outlines the standard adult BMI categories and general screening implications:

• Below 18.5 — Underweight: Indicates lower body weight relative to height. Individuals in this category may be screened for malnutrition, nutritional deficiencies, or underlying medical factors.
• 18.5 to below 25.0 — Healthy Weight: Statistically associated with the lowest average population risk for cardiovascular disease, type 2 diabetes, and premature mortality.
• 25.0 to below 30.0 — Overweight: Indicates higher body weight relative to height. While not an automatic diagnosis of disease, public health data associates this range with moderate cardiovascular and metabolic risk.
• 30.0 and above — Obesity: Subdivided clinically into Class I (30.0–34.9), Class II (35.0–39.9), and Class III (40.0+). Health authorities recommend consulting a physician for comprehensive medical assessment and lifestyle support.

Body Mass Index is widely used because it is fast, non-invasive, objective, and inexpensive. However, it is essential to recognize its physiological limitations when evaluating individual health.

What BMI Can Tell You: BMI offers a reliable screening tool to identify general weight-related health patterns across large populations. Epidemiological research shows that individuals with a BMI significantly outside the 18.5–24.9 range have higher statistical probabilities of developing certain chronic conditions over a lifetime.

What BMI Cannot Tell You: BMI does not measure body fat directly, nor does it distinguish between bone mass, dense muscle tissue, and adipose fat. Consequently, an individual with substantial muscle mass (such as an athlete or weightlifter) may register a high BMI despite having low body fat and excellent cardiovascular fitness.

Furthermore, BMI cannot measure where body fat is located. Medical research shows that visceral fat (fat stored around internal abdominal organs) poses greater health risks than subcutaneous fat (fat stored beneath the skin on hips or thighs). Two individuals with the identical BMI of 27.0 can have vastly different metabolic health profiles based on body fat distribution.

• Muscular Athletes: High muscle density often produces an elevated BMI score that does not reflect excess body fat.
• Older Adults: Sarcopenia (age-related loss of muscle mass) can cause elderly individuals to have high body-fat percentages despite a 'normal' BMI score.
• Pediatric Differences: Standard adult BMI categories must NOT be used for children or teenagers. Pediatric growth requires age-and-sex-specific growth percentile charts.
• Holistic Metrics: For a complete health picture, clinicians evaluate waist circumference, blood pressure, lipid panels, fasting glucose, and physical endurance alongside BMI.

Our calculator supports both the International System of Units (Metric) and US Customary Units (Imperial). You can choose whichever measurement system you feel most comfortable using.

In Metric mode, you enter height in centimetres and weight in kilograms. In Imperial mode, you enter height in feet and inches, and weight in pounds. When you enter equivalent physical dimensions (for example, 175 cm and 70 kg versus 5 ft 9 in and 154 lbs), the underlying algebraic result is equivalent.

Our component features synchronized controls: you can either type your exact numbers directly into the numeric input boxes or adjust the fluid range sliders. If you clear a field or enter a non-positive value, the tool prompts you to enter valid positive measurements rather than outputting misleading numbers.

• Metric (cm & kg): Standard across Europe, Asia, Latin America, and international scientific research.
• Imperial (ft/in & lbs): Standard in the United States and commonly referenced in the United Kingdom.
• No mathematical discrepancy: The 703 constant in the imperial formula accurately aligns pounds and inches with the metric definition.

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The formulas, category thresholds, and educational guidance on this page are aligned with public health documentation from leading international medical organizations:

• World Health Organization (WHO): Obesity and Overweight Factsheet — https://www.who.int/news-room/fact-sheets/detail/obesity-and-overweight

• U.S. Centers for Disease Control and Prevention (CDC): About Adult BMI — https://www.cdc.gov/bmi/

• UK National Health Service (NHS): Calculate Your Body Mass Index (BMI) — https://www.nhs.uk/live-well/healthy-weight/bmi-calculator/