NASM's BMI Calculator
Body Mass Index, or BMI for short, is a term that is used in virtually every single field involving
human health. Personal trainers, nutrition coaches, physicians, and
epidemiologists all utilize it to categorize bodyweight based on a person's height and weight.
It is one of the single most used tools in modern medicine. Despite this, it is not well understood and
there are a lot of questions surrounding this metric. For example, BMI often doesn't account for a
difference between fat and lean mass. Regardless, it is a useful risk prediction tool as a whole.
Below, we will answer the questions of what BMI is, how you should interpret it, and what this number
means to you and your clients.
Calculate your BMI below to get a snapshot of your current health and see which range it falls under. For
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What is BMI?
BMI, which is short for body mass index, is a tool that is used clinically to define bodyweight concerning health. Essentially it is a mathematical tool that utilizes two metrics of a person, their height and weight, to estimate and categorize their body weight.
There are two ways to calculate BMI, depending on whether you use the metric system or the imperial system. The most common equation is the metric one, but imperial can be used.
Once BMI is calculated, the number is binned into categories that aim to define the bodyweight of the individual. Additionally, BMI metrics apply differently based on age, wherein individuals over 20 years of age are considered adults and individuals aged 2-20 are considered children. Infants under the age of 2 have a different scale.
The formula for BMI simply utilizes height and weight to create an index measurement, which has no units.
Metric formula: BMI = weight (kg) ÷ [height (m)]2
Imperial formula: BMI = 703 × weight (lb) ÷ [height (in.)]2
For example, you can calculate Mary’s BMI score using either formula if she currently weighs 160 lb (72.57 kg) and stands 5 ft 7 in. or 67 in. (1.70 m).
Answer: BMI = 25.1
BMI Ranges Chart
In adults, BMI is classified into distinct categories. It is important to understand that these
categories are based on “normative” population data. These categories range from "very
severely underweight" to "very severely obese" and are listed in the table below (Table 1). This table
is based on the American population, and different countries have slightly different adaptations.
|Very Severely Underweight
|Normal (healthy weight)
|Obese Class 1 (moderately obese)
|Obese Class II (severely obese)
|Obese Class III (very severely obese)
5 Health Risks Associated with a High BMI
BMI is an incredibly useful tool for risk prediction. When we look at human health data, BMI is one of the most effective risk prediction tools we have in modern medicine. This is especially true when BMI is utilized to predict the risk of chronic diseases and many of the leading causes of death.
Here is a list of 5 health risks associated with a high BMI.
Cardiovascular Disease: As BMI increases the risk of cardiovascular disease also increases. Some studies show that individuals with the highest classification of BMI have a 3-fold higher risk of having a cardiovascular event than those in the normal BMI category. However, it doesn’t require being in the highest category, even those in the “overweight” category carry a 20% higher risk of having a cardiovascular event (Khan et al., 2018).
Diabetes: BMI is one of the most robust predictors of developing diabetes. In fact, in some studies, those who have a BMI in the “overweight” category carry a roughly 6-fold higher risk of developing diabetes than those with a normal BMI (Sanada et al., 2012).
Cancer: Cancer is a complicated disease with many different factors leading to its cause. However, BMI is linked to some forms of cancer. For example, higher BMI is associated with higher rates of liver and colon cancer (Bhaskaran et al., 2014).
Hypertension: Hypertension, or elevated blood pressure, is one of the leading modifiable risk factors for cardiovascular disease. Some evidence shows that for every unit increase in BMI, there is a 1-2 mmHg increase in blood pressure (Linderman et al., 2018).
Stroke: Stroke is the fifth leading cause of disease among adults in the United States. Those with a BMI of 30 or higher have reported a roughly 2-fold increase in the risk of stroke (Kurth et al., 2002). .
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Shortcomings of the BMI
BMI is often debated as not being a useful tool. However, oftentimes these discussions fail to understand exactly what BMI is and how best to utilize it as a tool.
It is important to understand that BMI does not discriminate between fat mass and lean mass and does not directly address adiposity (i.e., body fat percentage). However, it is a tool based on large sample sizes and is primarily used as a risk prediction tool over large numbers of people. As such, on a population level, BMI can approximate levels of adiposity to a useful degree. Furthermore, it is a good tool for estimating risk for specific outcomes (e.g., mortality, cardiovascular disease, diabetes).
However, on an individual level, BMI can be less accurate, and it is best used in conjunction with other data (e.g., body composition tests) in the full assessment of an individual’s weight status. Individuals who carry a substantially high level of lean mass (e.g., bodybuilders or powerlifters) may fall into the overweight or obese category based solely on BMI but may have body fat percentages in the single digits.
Conversely, people who carry a minimal amount of lean mass (e.g., highly sedentary people) may fall into the normal/healthy category based solely on BMI but may have levels of body fat that would be considered overweight or obese
BMI Differences for Men, Women, and Children
Currently, BMI among adults is used to define and classify weight the same across men and women.
However, at a given BMI women tend to carry a slightly higher body fat, which is expected given the
physiological differences between men and women. This is where the body fat scales can provide
additional information when examining the anthropometrics of men and women. See the table below for
categories of adiposity status by body fat percentage by biological sex.
||25% or higher
||32% or higher
While BMI tends to be fairly accurate for people with height that is close to the average or median
height, it tends to become less accurate the further away one’s height is from average or
median. In general, BMI tends to overestimate body fatness among shorter people and underestimate it
among taller people.
In children, the BMI classification system is used much differently. In children, the BMI value is
not “binned” into categories based solely on the BMI metric. While BMI is calculated the
same, the classification is based on a percentile compared to all other children of that age. A BMI
less than the 5th percentile is considered underweight while children with a BMI between the 85th
and 95th percentile are considered overweight.
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Bhaskaran, K., Douglas, I., Forbes, H., dos-Santos-Silva, I., Leon, D. A., & Smeeth, L. (2014). Body-mass
index and risk of 22 specific cancers: a population-based cohort study of 5·24 million UK adults. The
Lancet, 384(9945), 755–765.
Khan, S. S., Ning, H., Wilkins, J. T., Allen, N., Carnethon, M., Berry, J. D., Sweis, R. N., & Lloyd-Jones,
D. M. (2018). Association of Body Mass Index With Lifetime Risk of Cardiovascular Disease and Compression of
Morbidity. JAMA Cardiology, 3(4). https://doi.org/10.1001/jamacardio.2018.0022
Kurth, T., Gaziano, J. M., Berger, K., Kase, C. S., Rexrode, K. M., Cook, N. R., Buring, J. E., & Manson, J.
E. (2002). Body mass index and the risk of stroke in men. Archives of Internal Medicine, 162(22),
Linderman, G. C., Lu, J., Lu, Y., Sun, X., Xu, W., Nasir, K., Schulz, W., Jiang, L., & Krumholz, H. M.
(2018). Association of Body Mass Index With Blood Pressure Among 1.7 Million Chinese Adults. JAMA Network Open,
Sanada, H., Yokokawa, H., Yoneda, M., Yatabe, J., Sasaki, Y. M., Williams, S. M., Felder, R. A., & Jose, P.
A. (2012). High body mass index is an important risk factor for the development of type 2 diabetes. Internal
Medicine , 51(14). https://doi.org/10.2169/internalmedicine.51.7410