NASM'S BMI CALCULATOR

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BMI Calculator

BMI, or Body Mass Index, is more than just a number; it's a key indicator of weight-related health used by everyone from fitness experts to doctors. By simply looking at your height and weight, BMI gives an overview of your health status. However, it's important to note that while BMI is an effective tool, it doesn't always differentiate between muscle and fat. But it's still a prime predictor of potential health concerns.

Here, we'll guide you through understanding, calculating, and interpreting BMI for both you and your clients. BMI is used by personal trainers, nutrition, coaches, physicians, and epidemiologists to categorize body weight based on person's height and weight.

Wondering about your own BMI?

Calculate it for a snapshot of your health. And for those keen on mastering their weight journey, don't miss out on our calorie calculator — an invaluable tool for your health journey.

What is BMI?

BMI, or Body Mass Index, is a key metric doctors rely on to gauge the relationship between your weight and overall health. Depending on where you are in the world, you might calculate BMI using metric or imperial units, with the metric system being more widely adopted.

After determining your BMI, it's categorized to provide insights into your weight status. It's important to remember that age factors in too. Those over 20 are in the adult category, while kids between 2 and 20 fall under a different bracket. For the tiny tots under 2, there's a distinct scale altogether.

 

How to calculate bmi

Understanding your Body Mass Index (BMI) is simple when you have your weight and height at hand. Here's how you can work it out:

In metric measurements:

BMI = weight in kilograms ÷ (height in meters squared)

For imperial measurements:

BMI = 703 × weight in pounds ÷ (height in inches squared)

Let's use Mary as an example:

Mary weighs 160 lb (approximately 72.57 kg) and is 5 ft 7 in. (which is 67 inches or 1.70 meters). Plugging these values into either formula, we find that Mary's BMI is 25.1.

bmi chart

For adults, BMI is grouped into specific categories that reflect a range of weight statuses, from "very severely underweight" to "very severely obese". These classifications are rooted in typical data from the general population. The chart provided (Table 1) is tailored to the American population, but it's worth noting that variations exist based on regional standards in different countries.

Table 1
Category BMI  
  from to
Very Severely Underweight 0 15
Underweight 16 18.5
Normal (healthy weight) 18.5 25
Overweight 25 30
Obese Class 1 (moderately obese) 30 35
Obese Class II (severely obese) 35 40
Obese Class III (very severely obese) 40 40+
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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.

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).

Is BMI Accurate?

 

BMI is a topic of much debate, but often, the essence of what BMI truly represents is overlooked.

Essentially, BMI doesn't distinguish between fat and muscle. Instead, it's based on extensive data to predict health outcomes for large groups of people. When looking at broad populations, BMI can give a general idea about obesity levels and can predict risks for conditions like heart disease or diabetes.

However, BMI isn't always spot-on for individuals. For a more complete picture of someone's health, it's best paired with other assessments, like body composition tests. For instance, bodybuilders with lots of muscle might have a high BMI but low body fat. On the other hand, someone who isn't active might have a BMI in the "normal" range, but still carry excess body fat.

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Is BMI calculated differently for males and females?

The Body Mass Index (BMI) is a universal tool used to categorize weight in adults, irrespective of gender. Interestingly, given the inherent physiological differences between men and women, women generally possess a slightly elevated body fat percentage at an identical BMI. This distinction is where body fat scales can offer enhanced insights into the unique body compositions of both genders. Refer to the table for a breakdown of body fat percentages delineated by biological sex. Now, BMI works pretty well if your height is around the average. But if you're notably shorter or taller than most, it might not be as accurate. In simpler terms, shorter folks might find BMI paints them as having more body fat than they do, while taller individuals might find the opposite.

Category Men Women
Essential Fat 2.5% 10-13%
Athletes 6-13% 14-20%
Fitness 14-17% 21-24%
Average 18-24% 25-31%
Obese 25% or higher 32% or higher
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Is there a child bmi calculator?

For children, the BMI classification system takes a distinct path. Unlike adults, where BMI falls into categories, children's BMI isn't categorized solely by the metric. Though BMI calculation remains the same, their classification hinges on percentiles relative to their peers.

Here's the breakdown: A BMI below the 5th percentile is marked as underweight. Meanwhile, children with BMI ranging from the 85th to 95th percentile are recognized as overweight.

In essence, children's BMI evaluation involves percentiles, offering a unique perspective on their weight status.