Body Mass Index Explained

Body mass index (BMI) is a calculation that compares weight against height and is commonly used as a screening tool for obesity.

The average body mass index is rising in the USA. Compared to the 1970s, a far greater number of people now fall into the obese BMI category of over 30kg/m².

In this article, we will explain how body mass index is calculated, what the score means, and its relevance in relation to your overall health risk.

What Is Body Mass Index?

The Body Mass Index formula offers an approximate measure of whether you are underweight, a healthy weight, overweight or obese.

The calculation is based on the ratio between your height and weight. If your BMI calculation is higher than the normal range of between 18.5 and 25, you might have a higher proportion of body fat than is healthy.

Even though the BMI calculation cannot definitively measure body fat or diagnose a specific health condition, it remains an important screening tool.

This is because being overweight or obese may increase your risk of developing certain weight-related health problems. These include high blood pressure, high cholesterol, type 2 diabetes, sleep apnea, stroke, heart disease, and even an increased risk of premature death ( 1 ).

Although BMI can be used to indicate weight status, further medical assessments are then recommended for those who fall into the overweight or obese categories. Subsequent investigations may include measurements of skinfold thickness, an assessment of your diet and exercise patterns, and an in-depth review of any relevant family history.

Your doctor may investigate whether weight-related co-morbidities, such as diabetes or high cholesterol, are already present so that these can be appropriately managed ( 1 ).

Receiving a diagnosis of obesity may also help you gain access to weight loss therapies or interventions more easily ( 3 ).

Why Is BMI Used to Measure Overweight and Obesity?

The BMI calculator is an easy tool for both clinicians and the general population to use. It requires only a person’s weight and height to provide an indication of body fatness. This makes it a cheap, convenient, and extremely safe screening tool.

If your BMI score indicates that your weight-to-height ratio is increased, more accurate or expensive assessments can be requested to diagnose and manage obesity.

In addition, your doctor can investigate and treat any weight-related complications that are present, helping to improve your overall health earlier than if the screening tool had not been utilized.

The Body Mass Index Formula

The Body Mass Index is calculated by comparing weight and height. The BMI formula gives a good indication of whether the two measurements are in proportion.

A short person who weighs 50kg might be overweight according to their BMI, but a tall person of the same weight might be underweight. BMI, therefore, gives a more accurate indication than weight alone of whether a person has a healthy amount of body fat.

BMI can be calculated using metric or imperial measurements of weight and height. For metric measurements, such as a weight of 70kg and a height of 170cm (1.7 meters), the following calculation will provide the BMI.

• Weight in kilograms ÷ Height in meters² = BMI
• For example: 70 kilograms ÷ (1.70m x 1.70m) = 24.22

For imperial measurements, such as a weight of 160 pounds and a height of 5’5” (65”), the following calculation will give you your BMI.

• (Weight in pounds ÷ Height in inches²) x 703 = BMI
• ((160 ÷ (65 x 65)) x 703 = 26.62

To calculate your BMI, you must take both measurements in either metric or imperial units. The Centers for Disease Control and Prevention (CDC) have an online BMI calculator that will determine your BMI for you after you have input your weight and height. You can calculate your BMI here.

How Is BMI Interpreted for Adults?

When you calculate your BMI, you will be left with a numerical score. In the example calculations above, the BMI scores were 24.22 and 26.62. These numbers fall within a class of weight from underweight to class 3 obesity. The following tables show the weight classes for adults in the USA aged 20 years and older.

BMI	Class
<18.5	Underweight
18.5 to <25	Healthy weight
25.0 to <30	Overweight
30.0 or higher	Obese

A BMI of 30 or higher indicates a diagnosis of obesity. For those in this class, the CDC has defined three further classes of obesity.

BMI	Class
30 to <35	Class 1 obesity
35 to <40	Class 2 obesity
40 or higher	Class 3 (also known as morbid obesity, extreme obesity, or severe obesity)

A healthy or normal BMI is between 18.5 and 24.9. If your BMI falls within the healthy weight range, it is understood that you have a higher chance of living a longer and healthier life than someone who is overweight or obese. A study of over 12,000 people in the US showed that those with class 1 obesity were 2.5 times more likely to become diabetic than those with a healthy weight, and those with class 3 obesity were over 5 times more likely to develop type 2 diabetes ( 3 ).

Is There a Male or Female Body Mass Index Chart?

No, there is no separate BMI chart for men and women. The BMI ranges are the same for both adult men and women. This is because the risk of developing weight-related illnesses gets higher as BMI increases, regardless of sex ( 5 ).

What Is the Main Limitation of BMI as an Indicator of Body Composition?

Calculating BMI is an effective way to screen for increased body fat composition, but it has limitations. BMI can only be used to indicate the possibility of excess fat. Although a high BMI suggests increased body fatness, on further investigation, two people with the same BMI may have very different percentages of body fat.

Body fat tends to be higher in women than men, and in older adults compared to younger adults. Asians are prone to having more body fat than Whites. In general, Blacks have less body fat than Whites and Asians ( 1 ).

Athletes tend to have less body fat than non-athletes. Athletes may also have a much higher muscle mass than non-athletes. Muscle is denser than fat, so a slim, muscular person might have a surprisingly high BMI. This does not mean that they are overweight or obese, as they may have little body fat. In this case, BMI is not an appropriate indicator of health.

These limitations indicate the importance of assessing BMI in conjunction with other assessments of weight and fat proportion.

What Are Other Ways to Assess Excess Body Fatness Besides BMI?

BMI is a good initial assessment for excessive body fat. However, alternative measures of body fat are available and include:

• Waist circumference (sometimes divided by a person’s height)
• Skinfold thickness
• Underwater weighing
• Dual-energy x-ray absorption
• Bioelectrical impedance

Although waist circumference and skinfold thickness can be carried out relatively unobtrusively in a clinical setting, the remaining investigations are expensive and intrusive, and may not be easy to standardize.

Is BMI Interpreted the Same Way for Children and Teens as It Is for Adults?

The categories for childhood and adolescent obesity vary as a child gets older due to changes in body composition during childhood. Although it is possible to determine a child’s BMI using the same calculation used for adults, a child will only be classed as overweight or obese in comparison to the average BMI for a child of the same age and sex. A child whose BMI is between the 85^th and 95^th percentile is considered overweight, and a child with a BMI in the 95^th percentile or greater will be classified as obese.

As with the BMI classes for adults, the percentiles define the level above which a child becomes at greater risk of later being diagnosed with a weight-related complication such as diabetes or osteoarthritis.

Summary

Calculating your BMI is an inexpensive and simple method for identifying whether your weight falls within, or outside, the healthy range. Although BMI does not directly measure body fat percentage, a BMI that falls within the overweight or obese category does offer an indication that your proportion of body fat may be higher than is healthy.

As being overweight or obese increases, the risk of adverse health consequences, including high blood pressure, heart disease, and even premature death; screening for a high BMI can help to highlight those who may require additional assessment of weight and body fat percentage. Investigations can also be requested to diagnose any co-morbidities that may already be present.

If the proportion of body fat is found to be high, lifestyle modification or intervention with treatment to reduce the risk of, or treat existing, complications can be instigated promptly for improved outcomes.