How to Calculate BMI – And Why Waist Size Also Matters

  • Usman 

One question that is always relevant for health concerns is: how much should I weigh? There is no fixed answer to it. The ideal body weight changes according based on age, height and several other factors. 

But one thing is certain and science is unanimous about the conclusion that being overweight increases cardiovascular disease risk, metabolic syndrome, hypertension, diabetes and other disease risks. 

One of the screening tools that is commonly used for determining healthy weight is BMI index. Before we understand how to calculate BMI, we need to be briefly aware of what BMI stands for. 

What is BMI

BMI is short for Body Mass Index. This is a quick measure for determining whether someone is overweight or healthy weight. BMI considers just two factors: height and weight. It might be a very rough measure of body fat percentage, but we will see later that why BMI might not be accurate in many cases for determining body fat percentage and even unhealthy weight and obesity. 

Calculating BMI

If you wanted to calculate BMI, then it is straightforward. You need just two measurements. Get your weight in KGs and your height in meters. Then divide your weight in KGs by square of your height in meters. The following BMI formula simplifies the calculation that is involved: 

BMI=Weight in kgs/Height in meters2

Another version of BMI equation can be as under: 

BMI = Weight in kg/Height in meters X height in meters

Let’s assume that you have weight of 70 kg and height of 1.8 meters, then your BMI will be calculated as below

BMI = 70 /1.8 X1.8= 703.24= 21.6

Now that we have calculated BMI, how do we interpret the BMI? There is a simple guidance given by US Department of Health which defines the normal BMI and different BMi ranges for underweight, overweight and obese. 

CategoryBMI Range
Underweightless than 18.5
Normal18.5 to 24.9
Overweight25 to 29.9
Obese30 and higher

If you do not want to calculate BMI manually, then you can use a BMI chart which lists the height in inches and weight in pounds with corresponding BMI. You can check whether you have normal BMI by looking up your height and weight from this BMI chart. 

BMI Calculators

Even easier way to calculate BMI is by using a BMI calculator online. There are many calculators which instantly tell you if you fall in the normal BMI range or not. Listed below are three reliable BMI calculators that you can rely on for calculating your BMI. 

Calculate your BMI using US Department of Health Calculator

BMI Calculator by Australian Heart Foundation

Diabetes Canada BMI Calculator

BMI Calculator by Center for Disease Control and Prevention

BMI Limitations

While BMI is a great screening tool for identifying potential obesity and overweight problems, but it has the following limitations because it is not a direct measure of body fat percentage: 

  • It ignores the difference between muscle weight and fat weight. Therefore, it can not be used for athletes because they might show higher BMI, but they will be healthy because the extra BMI will be coming from muscle mass which does not carry any health risk. 
  • BMI can not be used for children because their muscles are still growing, therefore, it will not give accurate measurements. A child might have a normal BMI but high fat percentage because the muscles are still in development. 
  • BMI is also not reliable in case of elderly people because of muscle loss attributed to old age. 
  • BMI can not be suitable for pregnant women

To overcome these limitations of BMI, we also need to consider waist circumference. 

Waist circumference and BMI

Apart from BMI, it is useful to measure waist circumference. If the waist measurement is not within recommendations then probably a higher BMI would be a clearer indication of being overweight or obesity. 

According to WHO recommendations, the health risks increase if the waist is above 37 inches (ca. 94 cm) in case of men and 31.5 inches (ca. 80 cm) for women. There is a slight variation for South Asian, Chinese and Japanese males and recommended waist circumference in their case is 35 inches (ca. 89 cm). 

The bottom line

It is straightforward to calculate BMI which is an important indicator of disease risk and healthy weight. But BMI measurements need to be correlated with other indicators like waist circumference and waist to hip ratio to further cement the conclusions about obesity. Weight loss can significantly decrease the health risks associated with a higher BMI. 

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Muhammad Usman

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