How to Use This BMI Calculator
Body Mass Index (BMI) is an international anthropometric screening formula adopted by the World Health Organization (WHO) and the Centers for Disease Control and Prevention (CDC) to classify body weight in relation to height:
- Choose Your Unit System: Toggle between Metric (centimeters and kilograms) and Imperial (feet, inches, and pounds).
- Enter Your Height and Weight: Input your current measurements accurately.
- Review Classification and Target Range: Examine your computed BMI score, WHO weight category, and the estimated healthy weight window for your specific height.
The BMI Mathematical Formula and WHO Thresholds
Developed in the 19th century by Belgian mathematician Adolphe Quetelet, BMI evaluates body mass scaling across height:
Imperial Formula: BMI = 703 × Weight (lbs) ÷ [Height (inches)]^2
The World Health Organization classifies adult BMI scores into standardized ranges:
- Underweight: BMI below 18.5
- Normal weight: BMI between 18.5 and 24.9
- Overweight: BMI between 25.0 and 29.9
- Obesity Class I: BMI between 30.0 and 34.9
- Obesity Class II: BMI between 35.0 and 39.9
- Obesity Class III: BMI of 40.0 or greater
Step-by-Step Worked Example
Worked Example: Evaluating BMI for an Adult
Consider an individual who is 5 feet 10 inches tall and weighs 165 pounds:
- Step 1 (Convert height to inches): (5 × 12) + 10 = 70 inches.
- Step 2 (Square height): 70 × 70 = 4,900.
- Step 3 (Apply imperial multiplier): 703 × 165 = 115,995.
- Step 4 (Divide): 115,995 ÷ 4,900 = 23.67.
A BMI of 23.7 falls cleanly within the Normal Weight range (18.5 – 24.9).
Frequently Asked Questions
Why doesn't BMI accurately reflect bodybuilders or athletes?
BMI does not differentiate between lean muscle mass and adipose fat tissue. Because skeletal muscle is significantly denser than fat, muscular athletes and weightlifters often register in the "overweight" or "obese" categories despite having very low body fat percentages and exceptional cardiovascular health.
Does BMI apply identically to men and women?
The standard WHO BMI formula is calculated identically for all adults regardless of sex. However, women naturally carry a higher percentage of essential body fat than men at identical BMI values.
What is the Ponderal Index shown in the results?
The Ponderal Index (also known as the Corpulence Index) divides weight by height cubed ($kg/m^3$) rather than squared. It provides a more balanced proportional metric for unusually tall or short individuals, where standard BMI tends to overestimate or underestimate body mass.
Should this calculator be used for children or teenagers?
No. Children and adolescents undergo rapid growth spurts that alter body composition constantly. Pediatricians evaluate youth using BMI-for-age percentile growth charts rather than fixed adult BMI cutoffs.