Is 21 a Good Body Mass Index?

Reviewed on 11/18/2021

A normal body mass index (BMI) in adults is between 18.5 to under 24.9, which means that 21 would be considered a normal BMI. However, other factors such as people with a lot of muscle mass or a significant loss of muscle mass would affect their BMI.
A normal body mass index (BMI) in adults is between 18.5 to under 24.9, which means that 21 would be considered a normal BMI. However, other factors such as people with a lot of muscle mass or a significant loss of muscle mass would affect their BMI.

Body mass index, or BMI, is a tool doctors use to determine if a person is overweight, and to what degree. BMI is calculated based on height and weight and it can provide a better estimate of total body fat than weight alone. 

A normal body mass index in adults is between 18.5 to under 24.9, so 21 would be considered a normal BMI. 

How Do You Calculate BMI?

Body mass index is an expression of the relationship between a person’s height and weight as a single number not dependent on frame size. 

BMI is calculated from the weight and square of the height:

  • BMI = body weight (in kg) ÷ height (in meters) squared
BMI Weight
BMI less than 18.5 Below normal weight
BMI 18.5 to under 24.9 Normal weight
BMI 25 to under 29.9 Overweight
BMI 30 to under 34.9 Class I Obesity
BMI 35 to under 39.9 Class II Obesity
BMI 40 or greater Class III Obesity

BMI is used slightly differently in children, in that it is calculated the same way as it is for adults, but the ranges are then compared to percentiles for children of the same age, sex, height, and weight. 

 

Is Body Mass Index Always Accurate?

Body mass index (BMI) is only one measure healthcare professionals may use to determine if a person is a healthy weight, but other factors should also be taken into account. 

  • For example, in people who are muscular, such as bodybuilders, boxers, and other professional athletes, the body mass index may overestimate their body fat because muscle is denser than fat. This means an athletic person may be categorized as obese based solely on body mass index, but they are actually a healthy weight. 
  • Conversely, older people who have loss of muscle mass due to aging may have their body fat percentage underestimated. 
  • Body mass index should not be used as a measurement for pregnant women. 

 

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Reviewed on 11/18/2021
References
Image Source: iStock Images

https://www.uptodate.com/contents/calculator-body-mass-index-bmi-quetelets-index-in-adults?search=bmi&source=search_result&selectedTitle=1~150&usage_type=default&display_rank=1

https://www.uptodate.com/contents/obesity-in-adults-prevalence-screening-and-evaluation?search=bmi&source=search_result&selectedTitle=2~150&usage_type=default&display_rank=2