Free Body Mass Index (BMI) Calculator – Instant & Accurate
The Body Mass Index (BMI) is an internationally recognized screening tool developed by Belgian polymath Adolphe Quetelet. It quantifies the amount of tissue mass (muscle, fat, and bone) in an individual to categorize them into standard weight brackets defined by the World Health Organization (WHO).
Mathematical BMI Formulas
Our calculator supports both the International Metric System and the Imperial / US Customary Measurement System:
- Metric Formula:
BMI = weight (kg) / [height (m)]² - Imperial Formula:
BMI = [weight (lbs) × 703] / [height (inches)]²
For example, an adult who weighs 70 kg with a height of 1.70 meters has a BMI of: 70 / (1.70 × 1.70) = 24.22 kg/m², placing them in the normal weight category.
WHO Body Mass Index Categories
According to the World Health Organization guidelines for adults aged 20 and older:
- Underweight (< 18.5): May indicate malnutrition, an eating disorder, or underlying health issues.
- Normal Weight (18.5 – 24.9): Associated with the lowest statistical risk of chronic cardiovascular and metabolic diseases.
- Overweight (25.0 – 29.9): Moderately elevated risk for high blood pressure, type 2 diabetes, and heart disease.
- Obese (≥ 30.0): Subdivided into Class I (30.0–34.9), Class II (35.0–39.9), and Class III (≥ 40.0), indicating significantly increased health risks.
Understanding BMI Limitations
While BMI provides an effective, convenient population-level benchmark, it has specific physiological limitations:
- Muscle Mass vs Fat Mass: Muscle is denser than fat. Bodybuilders and highly muscular athletes may register as "overweight" or "obese" despite having very low body fat percentages.
- Age and Bone Density: Older adults naturally lose muscle mass and bone density, meaning an older person may have more body fat than a younger adult with the same BMI.
- Fat Distribution: BMI does not distinguish between visceral fat (stored around internal abdominal organs) and subcutaneous fat.
Frequently Asked Questions (FAQs)
Q: What is a healthy BMI for men and women?
A: For adult men and non-pregnant women aged 20 and older, a healthy BMI standard is between 18.5 and 24.9 kg/m² according to WHO benchmarks.
Q: How do I convert feet and inches to inches for imperial BMI?
A: Multiply the number of feet by 12 and add the remaining inches. For example, 5 feet 7 inches is (5 × 12) + 7 = 67 inches total.
Q: What is BMI Prime?
A: BMI Prime is the ratio of your calculated BMI to the upper limit of normal BMI (25.0). A BMI Prime under 0.74 indicates underweight, 0.74 to 1.00 indicates normal weight, and greater than 1.00 indicates overweight.
Q: Is my biometric data stored or sent to a server?
A: No. All calculations occur strictly in your web browser client-side. No personal health information or measurements are saved or transmitted.