How to calculate body mass index (bmi)?

People, nowadays, are becoming highly conscious about their health. Everyone is trying to achieve and be in the best health to lead a satisfactory and happy life. As a part of people’s journey towards a healthy lifestyle, weight management has gained a lot of attention. Body mass index is a tool that can help one monitor their weight and manage a healthy weight without much assistance from a health professional.

Body mass index, commonly known as BMI, is the ratio of the weight of a person to the square height of that person. BMI gives an individual a basic idea about their weight status and body type. BMI can be used as a means to determine possible weight issues in people. Depending on their BMI value, an adult can be categorized into one of the following types:   

Category	BMI value
Underweight	≤18.4
Normal	18.4–24.9
Overweight	25–39.9
Obese	≥40

As can be seen from the above table, a BMI value of ≤18.4 indicates that the person is underweight and should gain weight to reach a normal BMI and lead a healthy life. Similarly, the BMI values of 25–39.9 and ≥40 imply that the individuals are overweight and obese, respectively, and should aim to reach the normal BMI value of 18.4–24.9 to lead a healthy life. 

As is well known, people who fall under the overweight and underweight categories according to their BMI can be susceptible to various health conditions. Regularly checking BMI can help one to manage and maintain their body weight so that they can lead a healthy life without the worry of contracting any condition that may be accompanied by being overweight or underweight. However, as BMI only provides a general idea of health, it can be paired with the waist-to-hip ratio to gain further insights into one’s health status. 

How is BMI different for adults and children?

For both adults and children as well as teenagers, BMI is calculated by following the same technique. However, since children’s and teenagers’ BMI depends on their gender and age, their BMI is expressed as a percentile value, which is called BMI for age. BMI for age can be calculated in two steps. First, by employing the same technique used to calculate an adult’s BMI, a child’s or teenager’s BMI is determined. Then, a graph is plotted, or a percentile calculator is used to obtain the BMI for age. BMI for age expresses a child’s BMI about the BMI of other children belonging to the same gender and age. 

BMI for children and teenagers

As BMI for children and teenagers is expressed as a percentile, the categories corresponding to BMI values for them are spread on a scale of 1–100. Depending on their BMI-for-age values, a child or a teenager can be categorized into one of the following types:

Category	BMI-for-age value
Underweight	0–4.9
Normal	5–84.9
Overweight	85–94.9
Obese	≥95

According to the above table, a BMI-for-age value of 0–4.9 indicates that the child or teenager is underweight. This child or teenager should gain weight so that they can achieve a normal BMI and lead a healthy life. Similarly, the BMI-for-age values of 85–94.9 and ≥95 imply that the child or the teenager is overweight and obese, respectively. In this case, the child or the teenager should aim to lower their BMI-for-age value to 5–84.9 so that they can lead and maintain a healthy lifestyle. 

How to calculate BMI?

Nowadays, various websites and online platforms offer free BMI calculators for both adults as well as children. One only has to enter the valid data for the weight (in kg or pounds) and height (in meters, feet, or inches) in the online calculator, and the calculator computes the BMI within a matter of seconds. 

However, the protocol used to calculate BMI can come in handy and is pretty easy to understand and follow. The following steps should be performed sequentially to calculate the BMI of an adult: 

1. Measure the height (in kg) and weight (in meters) of the individual whose BMI needs to be calculated.
2. Calculate the square of the height.
3. Divide the weight by the square of the height calculated in the second step.

To calculate the BMI of a child or a teenager, the following steps should be used:

1. Measure the height (in kg) and weight (in meters) of the child or the teenager whose BMI needs to be calculated.
2. Calculate the square of the height.
3. Divide the weight by the square of the height calculated in the second step.
4. Plot the BMI value on a CDC BMI for age chart. (This chart is different for boys and girls.)
5. Find the percentile corresponding to the BMI value from the graph.

BMI can enable doctors to address weight management objectively for patients who are either overweight or underweight. This can aid overweight and underweight patients to tackle the conditions that may arise from a lack of weight management. 

Although BMI can be treated as a general tool that offers you critical information regarding your health, it cannot be used as a clinically significant guide. Moreover, as BMI is a ratio of weight and height, it does not offer any substantial information about one’s body fat or muscle percentage. In case of any queries related to weight and weight management, one should always opt for general-physician consultation directly and follow the doctor’s advice irrespective of BMI.