What Is BMI?
Body mass index (BMI) is a number calculated from a person's weight and height, used as a screening measure for weight categories in adults. BMI = weight in kilograms ÷ height in meters squared. The World Health Organization classes a BMI of 18.5 to 24.9 as the normal weight range for adults.
BMI Categories for Adults (WHO)
Below 18.5 is underweight, 18.5–24.9 normal weight, 25–29.9 overweight, and 30 or above obesity.
Limitations
BMI does not distinguish muscle from fat or account for body shape, age, or ethnicity, so it is a population screening tool rather than a diagnosis. Children and teenagers use BMI-for-age percentiles instead.
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Put Body Mass Index (BMI) into practice with these free calculators — no sign-up, nothing stored.
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BMI Calculator
Calculate your Body Mass Index in metric or imperial units.
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Ideal Weight Calculator
Healthy weight range for your height from four formulas and BMI.
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Estimate your body fat using the U.S. Navy circumference method.
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Last updated: September 27, 2026. This definition is for general educational purposes only. It is not financial, investment, tax, legal, or medical advice, and it does not recommend any product or course of action. For decisions about your own situation, consult a qualified professional.