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 another great weight loss tool, check out our calorie calculator.

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

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.

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.

1. 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).
2. 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).
3. 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).
4. 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).
5. 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). .

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

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.