What is BMI? Here's how to understanding what the numbers mean and how to calculate them.

What Is BMI?

Last Updated on July 12, 2021


BMI stands for “body mass index”. It’s a technical-sounding term that simply refers to a person’s body size based on their weight and height. Don’t be intimidated by the jargon, the concept is really easy to understand.

Why Do We Calculate Body Mass Index?

You’re probably wondering why would it be necessary to go through the trouble of calculating BMI when you can just use a person’s weight to convey their body size? After all, if someone weighs 600 lbs, you probably have a pretty good sense of their body size. It’s pretty big!

But, wait. Imagine if I told you that someone weighs 200 lbs. Would you really be able to say whether this person was struggling with their weight? I mean 200 lbs sounds kind of big doesn’t it? You probably don’t know many people who weigh more than 200lbs and are “skinny”. Right?

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Well, the answer depends to some degree on the height of the person.

If you’re talking about your tiny 91-year-old grandmother whose less than 5 feet tall, then a weight of 200 lbs almost certainly means she’s struggling with her weight. On the other hand, if you’re talking about an NBA basketball player who is 7ft 6in tall then 200 lbs would actually be considered underweight.

What is BMI?

The Problem With Body Mass Index

Unfortunately, Body Mass Index is not perfect and can sometimes be quite misleading.  

For example, when Arnold Schwarzenegger was in the prime of his bodybuilding career, he was 6 feet tall and weighed 235 lbs. This gave him a BMI of 31, putting him in the obese range. If you look at the photo below, I don’t think anyone would call this man obese.

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Have you always wondered what BMI is? The numbers aren't always what they seem in determining a healthy weight.

In addition, Body Mass Index fails to convey a whole host of other important factors for health: someone who has a BMI of 35 with diabetes, high blood pressure and high cholesterol is quite different from someone with a BMI of 45 without any other medical conditions. Likewise, someone with a BMI of 45 who can’t climb a flight of stairs is different from someone with a BMI of 55 who can. So a lower BMI doesn’t always mean someone is “healthier”. 

While many experts continue to argue over the merits of using Body Mass Index, the term remains the most commonly used shorthand for conveying body size. It’s easy to calculate and requires minimal information.

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Understanding the Numbers

The table below shows you what the various Body Mass Index numbers mean. As you can see, obesity kicks in at a BMI of ≥30. 

Weight categories based on BMI chart What is BMI? Here is a chart of WHO Classification of Weight Status


How do you calculate body mass index?

The formula for BMI is simply:

A chart of how you calculate BMI. If you always wanted to know what BMI is, read this blog post

To be honest, no one calculates BMI longhand. There are tons of BMI calculators online where you can just enter your weight and height and receive your BMI.

Click here to calculate your BMI. 


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