Bmi

TL;DR

• BMI quantifies weight relative to height using weight (kg) divided by height (m) squared.
• Standard BMI categories: <18.5 underweight; 18.5–24.9 normal weight; 25–29.9 overweight; ≥30 obese.
• BMI is a screening tool that can indicate potential health risks but has limitations (does not account for muscle mass, age, or gender) and is not appropriate alone for children and teenagers.

Definition

Body mass index (BMI) is a measure of a person’s weight in relation to their height. To calculate BMI, a person’s weight in kilograms is divided by their height in meters squared.

$$\text{BMI} = \frac{\text{weight (kg)}}{\text{height (m)}^2}$$

A BMI of less than 18.5 is considered underweight, a BMI of 18.5 to 24.9 is considered normal weight, a BMI of 25 to 29.9 is considered overweight, and a BMI of 30 or higher is considered obese.

Explanation

BMI is commonly used to categorize an adult’s weight status (underweight, normal weight, overweight, obese) by applying the weight/height-squared formula. It can help identify potential health risks associated with being underweight or overweight, such as heart disease, diabetes, and high blood pressure. However, BMI does not account for factors such as muscle mass, age, and gender, so it is not always an accurate measure of an individual’s health. Because of these limitations, BMI is not a diagnostic tool and is best used alongside other measures—such as waist circumference and body fat percentage—to give a more comprehensive picture of health.

BMI is not appropriate for children and teenagers because their body composition and weight vary during growth and development. In pediatrics, growth charts are used to determine a child’s BMI percentile, which takes into account age and gender.

Examples

Calculation example

• For a person who weighs 70 kilograms and is 1.75 meters tall:

$$70 / (1.75 * 1.75) = 22.9$$

Interpretation examples

• A person with a BMI of 25 would be considered overweight.
• A person with a BMI of 30 would be considered obese.

Pediatric example

• A 10-year-old girl with a BMI of 25 would not be considered overweight, as she may be in the 75th percentile for her age and gender, indicating that she is growing at a healthy rate.

Use cases

• Evaluating an adult’s weight status and identifying potential health risks associated with being underweight or overweight (for example, heart disease, diabetes, and high blood pressure).

Notes or pitfalls

• BMI does not distinguish between weight from fat and weight from muscle; very muscular individuals may have a high BMI but be healthy.
• BMI does not account for age or gender differences in body composition.
• BMI is not a diagnostic tool and should not be the sole determinant of health.
• BMI is not appropriate for children and teenagers; pediatric assessments use growth charts and BMI percentiles.

Related terms

• Waist circumference
• Body fat percentage
• Growth charts
• BMI percentile