How to Calculate BMI (Body Mass Index)
Learn how to calculate BMI using the standard formula for both metric and imperial units. Understand what your BMI score means and its limitations as a health indicator.
What Is BMI?
Body Mass Index (BMI) is a numerical value derived from a person's weight and height, used as a screening tool to categorize individuals into weight status groups. It was developed by Belgian mathematician Adolphe Quetelet in the 19th century and has since become the most widely used metric for assessing body weight relative to height. BMI does not directly measure body fat, but it correlates with more direct measures of body fat in most people. It is used by health professionals as a quick, inexpensive initial assessment of weight-related health risk.
The BMI Formula
In metric units, BMI is calculated as: BMI = weight (kg) / height² (m²). For example, a person who is 70 kg and 1.75 m tall has a BMI of 70 / (1.75 × 1.75) = 22.9. In imperial units, the formula is: BMI = (weight in pounds × 703) / height² (in inches²). Both formulas yield the same result when units are correctly applied.
BMI Categories
The World Health Organization (WHO) defines four standard BMI categories: Underweight (below 18.5), Normal weight (18.5–24.9), Overweight (25.0–29.9), and Obese (30.0 and above). The obese category is further subdivided into Class I (30–34.9), Class II (35–39.9), and Class III (40+). These thresholds are widely used in clinical settings and public health research. Some countries, particularly in Asia, use lower cutoff points because populations may carry greater metabolic risk at lower BMI values.
Limitations of BMI
BMI does not distinguish between fat mass and lean mass, so muscular athletes may be classified as overweight despite having low body fat. It also does not account for fat distribution — visceral fat around the abdomen carries higher health risk than subcutaneous fat elsewhere. Age, sex, and ethnicity can all affect the relationship between BMI and actual health risk. BMI should be interpreted alongside other measures such as waist circumference, blood pressure, and lipid panels.
BMI in Children and Teens
For individuals under age 20, BMI is calculated the same way but interpreted differently using age- and sex-specific percentile charts. A child is considered underweight if BMI is below the 5th percentile, healthy weight between the 5th and 85th, overweight between the 85th and 95th, and obese at or above the 95th percentile. These charts account for the normal changes in body fat that occur as children grow. Pediatric BMI assessment should always be conducted by a healthcare professional in the context of overall growth and development.
Practical Steps to Calculate Your BMI
First, measure your height accurately in meters or inches and your weight in kilograms or pounds. Apply the appropriate formula: kg/m² or (lbs × 703) / in². Compare your result against the standard WHO categories to determine your weight status. Use the result as a starting point for a broader conversation with your healthcare provider, not as a definitive health verdict.
When to See a Healthcare Provider
If your BMI falls outside the 18.5–24.9 normal range, it is worth discussing with a doctor even if you feel healthy. Individuals with a BMI over 30 face significantly elevated risks of type 2 diabetes, cardiovascular disease, sleep apnea, and certain cancers. Those with very low BMI may be at risk for malnutrition, osteoporosis, and immune dysfunction. A healthcare provider can order additional tests and provide personalized guidance based on your full clinical picture.
Try These Calculators
Put what you learned into practice with these free calculators.
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