Body Mass Index (BMI) Approach to Determining Your Optimum Weight

The Body Mass Index (BMI) value of an individual is a number that is calculated using their weight and height. The BMI value for an individual can be a useful tool in determining whether the individual is underweight, overweight, or within the medically approved weight range for their height.

Although BMI does not measure body fat directly, research has shown that an individual's BMI level correlates to direct measures of body fat. The BMI approach to determining your optimum weight is similar to underwater weighing and dual energy x-ray absorptiometry (DXA).

In many cases, the Body Mass Index approach can be considered a viable alternative to direct measurements of body fat. Calculating one’s BMI level is inexpensive, easy to perform, and a quick way to determine whether an individual’s weight is appropriate for their height. Furthermore, the Body Mass Index technique is used as a screening tool to identify possible weight concerns for adults. There are also BMI tools specifically designed for children and adolescents. However, BMI is not a diagnostic tool.

For example, a person may have a high BMI, but in order to determine if the excess weight represents a health risk, a medical professional would need to perform additional tests. Under this scenario, the medical professional may suggest performing a skinfold caliper test and reviewing the individual's diet, level of physical activity, family history, and possibly a whole host of additional health screenings.

BMI - Metric Approach

BMI = weight (kg) / [height (m)]²

Within the metric system, the formula for BMI is [weight in kilograms] divided by [height in meters squared]. Since height is commonly measured in centimeters, divide the height (in centimeters) by 100 to obtain height in meters.

For example: Weight = 70 kg, Height = 170 cm (1.70 m)

Calculation: BMI = (70 ÷ (1.70)²) = 24.22

BMI (American Standard Approach)

BMI = (weight (lbs) / ((height (in)²) x 703))

Within the American Standard system, calculate BMI by dividing [weight in pounds (lbs)] by [height in inches (in) squared] and multiplying by a conversion factor of 703.

For Example: Weight = 165 lbs, Height = 5'7" (67")

Calculation: BMI = ((165 ÷ (67²)) x 703) = 25.84

BMI and Ideal Weight Range

Once an individual has calculated their BMI value, they can reference Table #1, below, to determine whether their BMI value is within the ideal weight range. If an individual's BMI value is greater than 25.8 (for women) or greater than 26.4 (for men), then they are considered to be at least marginally overweight. Table #1 represents a BMI scale that is specifically designed for adult men and women only. Children and adolescents will utilize an alternative scale, discussed later in this article.

Non-Mathematical Approach

For those who do not enjoy math, an alternative approach is to reference Table #2 below. Table #2 can be used to determine an individual's BMI value without performing any mathematical computations. The Y-axis represents the height of an individual, and the X-axis depicts an individual's weight. The first step is to identify your height on the Y-axis. Thereafter, simply move to the right until you are under your specific weight. The intersection of your height and your weight will represent your BMI value. Table #2 is color-coded so that an individual can quickly determine whether their body weight is within the healthy range. 

Table #2: Body Mass Index (BMI) Chart

Note: Table #1 and Table #2 are to be used only by male and female adults 20 years of age and older. The BMI values in these tables are appropriate for both men and women over the age of 20. However, for children and adolescents, the interpretation of their BMI values are both age and sex-specific.

BMI and % of Body Fat

The correlation between the BMI number and an individual's level of body fat is fairly strong. However the strength of the correlation can vary somewhat by sex, race and age. Below are some of the variations determined by the medical industry:

1. At the same BMI level, women tend to have a higher level of body fat than men.

2. At the same BMI level, older individuals tend to have a higher level of body fat than younger adults.

3. Athletes may have a high BMI value due to their elevated level of lean muscle mass, which weighs more than fat.

BMI and Weight-Related Diseases

It's also important to remember that an individual's BMI value is only one factor related to risk for disease. For assessing someone's likelihood of developing overweight or obesity-related diseases, the National Heart, Lung and Blood Institute guidelines recommend looking at two additional predictors:

1. The individual's waist circumference (abdominal fat is a predictor of an individual's risk level for obesity-related diseases).

2. Additional risk factors an individual may have in association with obesity are as follows:

   • Hypertension

   • Dyslipidemia (i.e. high LDL cholesterol, low HDL cholesterol, or elevated levels of triglycerides)

   • Type 2 diabetes

   • Coronary heart disease

   • Stroke

   • Gallbladder disease

   • Osteoarthritis

   • Sleep apnea and respiratory problems

   • Various cancers (i.e. endometrial, breast and colon)

Interpreting BMI Results for Children

Although the BMI value is calculated the same way for children, adolescents and adults, the criteria used to interpret the results of the BMI value for children and adolescents are different from those used for adults. For children and adolescents, BMI value is not only age-specific, but sex-specific as well. Hence, percentiles are used to improve the accuracy of the results for two primary reasons:

1. The amount of body fat changes with age.

2. The amount of body fat differs between girls and boys.

The Centers for Disease Control and Prevention (CDC) have compiled BMI charts that take into account these differences and facilitate the translation of the BMI value into a percentile. This percentile takes into account both the child's age and sex.

After the BMI value has been calculated for a child or an adolescent, the BMI value is plotted on the CDC BMI age growth chart (for either girls or boys) to obtain a percentile ranking. Percentiles are the most commonly used indicators to assess the size and growth patterns of individual children in the United States. The percentile value represents the position of the child's BMI value relative to children of the same sex and age. The CDC BMI charts display the weight status categories used with children and teens (i.e. underweight, healthy weight, overweight and obese).

Table #3 represents the CDC BMI weight status categories and the corresponding percentiles used for both children and adolescents.

BMI values are used as screening tools to identify potential weight concerns for children and adolescents. The CDC and the American Academy of Pediatrics (AAP) recommend the use of the BMI approach to screen for overweight and obesity in children beginning at 2 years of age. For children and adolescents, the BMI approach can be used to screen for obese, overweight, healthy weight or underweight conditions. However, please note that the BMI approach is not a diagnostic tool. For instance, a child or adolescent may have a high BMI value for their age and sex, but to determine if excess fat is a problem, a medical professional will need to perform additional tests. These tests typically include skinfold thickness measurements, evaluations of diet, physical activity and family history, and any other appropriate health screenings.

The standard BMI formulas used earlier in this article for adults can be used for children and adolescents of both sexes. Once the BMI value has been calculated for a child or an adolescent, the value can then be applied to either Table #4 or Table #5, depending on whether the child or adolescent is a male or a female. Table #4 is designed to be used for a male child or adolescent, while Table #5 is designed to be used for a female child or adolescent. Tables 4 and 5 can be used for children that are between the ages of 2 and 20.

To determine the appropriate weight range for a given child or adolescent, Table #4 or #5 will need to be used in conjunction with Table #3. For example, for a boy that is 10 years of age and has a BMI value of 23, using Table #4 shows that the individual is above the 95th percentile. It is now necessary to reference Table #3 and choose the weight category that applies to the equal to or greater than the 95th percentile. By doing so, it would be concluded that the boy is in the obese category. There are three additional examples of how an individual, once the BMI value has been determined, would go about determining their appropriate weight category (i.e. underweight, healthy weight, overweight or obese) depicted in Table #4.

Table #4: Male BMI Chart by Age

Table #5: Female BMI Chart by Age

Conclusion

The BMI method of determining an individual's optimum weight has proven to be fairly reliable and accurate throughout the years. As stated earlier in this article, there are a few instances in which the BMI method fails to accurately depict the optimum weight for an individual (e.g. high level of lean muscle mass). However, as long as the conditions applicable to creating the inaccuracies in the BMI results are not present, this method will prove to be a reasonable benchmark for determining an individual's optimum weight level.

It is recommended that each individual determine their optimum weight via the alternative methods provided within this section of our website. Once the optimum weight levels have been determined using the alternative methods, it is recommended that you compare the results to evaluate the similarities and differences. Thereafter, apply the optimum weight loss method that is most applicable, and makes the most sense for your body, to your personal fitness goals.