BMI Calculator

How the BMI Calculator works

The BMI Calculator computes your Body Mass Index — a screening metric that uses your height and weight to estimate body fat and categorize your weight status. Enter your measurements in either metric (centimeters, kilograms) or imperial (inches, pounds) units, and the calculator returns your BMI along with your category: underweight, normal, overweight, or obese.

The BMI formula

Metric

BMI = Weight (kg) ÷ Height (m)²

Imperial

BMI = ( Weight (lb) ÷ Height (in)² ) × 703

Worked examples

Metric: Height 175 cm (1.75 m), Weight 70 kg. BMI = 70 ÷ (1.75)² = 70 ÷ 3.0625 = 22.9 (Normal).

Imperial: Height 69 inches, Weight 155 lb. BMI = (155 ÷ 69²) × 703 = (155 ÷ 4761) × 703 = 0.0326 × 703 = 22.9 (Normal).

BMI categories (adults 20+)

• Underweight: BMI below 18.5 — may indicate malnutrition or eating disorder
• Normal weight: BMI 18.5-24.9 — associated with lowest chronic disease risk
• Overweight: BMI 25.0-29.9 — increased risk for type 2 diabetes, hypertension
• Obese: BMI 30.0+ — significantly increased risk for cardiovascular disease, diabetes, certain cancers

Limitations of BMI

BMI is a useful screening tool but has well-documented limitations. It does not distinguish between fat and muscle mass, so muscular athletes often classify as "overweight" or "obese" despite having low body fat. Bodybuilders, sprinters, and certain positions in football and rugby frequently have BMIs above 25 with body fat percentages below 12%. Conversely, BMI can underestimate body fat in older adults who have lost muscle mass (a condition called sarcopenia), giving them a "normal" BMI despite unhealthy body composition.

BMI also does not account for fat distribution. Visceral fat (around the abdomen) is far more metabolically dangerous than subcutaneous fat (under the skin, especially in hips and thighs). Two people with identical BMI can have very different disease risks based on where their fat is stored. This is why waist circumference and waist-to-hip ratio are increasingly used alongside BMI for cardiovascular risk assessment.

Using BMI responsibly

Treat BMI as a starting point, not a verdict. If your BMI classifies you as overweight but you are physically active, eat well, and have good blood pressure and cholesterol numbers, your actual health risk may be lower than the BMI suggests. Conversely, a "normal" BMI is not a guarantee of health — sedentary individuals with normal BMI can have elevated visceral fat and metabolic dysfunction (sometimes called "skinny fat").

For a complete picture of your health, combine BMI with other metrics: waist circumference (target: under 40 inches for men, under 35 inches for women), resting heart rate, blood pressure, and basic blood panel results. Always discuss your BMI and overall health with a qualified healthcare provider before making significant changes to diet or exercise. The number on the calculator is one data point, not a complete picture.

Frequently asked questions