Understanding your BMI

BMI stands for Body Mass Index, and is supposed to be a measure of general health and wellness. There are copious studies, diets, exercise programs, and statistics that use BMI to determine if a person is “normal” weight. Other options include: too skinny, too fat, and way too fat. Okay, so they’re listed as “underweight”, “overweight”, and “obese”, but the idea holds true. Each category falls into a range of BMI numbers to determine where you fall. On the surface, this can seem like a good measure, because it uses your height and weight to calculate the final number. I know you love math as much as I do, and a little math action (see below) will tell you where you fall – in theory – on the BMI weight spectrum.

For people with normal body types (more on that later), this can be a quick and easy measure of basic health. But not everyone is “normal” enough for the BMI measurement to have any value.

So, who shouldn’t use BMI? The calculation works for most people with normal body types, but anyone falling into an outlier group – very tall, very short, heavily muscled, very lightly muscled, suffering from an eating disorder, or Sumo wrestlers – may find that their BMI calculation doesn’t match up with their actual physical fitness. I’ll show you an example below of how that can look in practice. Before we get to examples, lets learn some math! Don’t worry, no calculus required.

Note: A quick Google search will turn up basically anything. If you go look for “BMI Calculator” you’ll find some nice tools to do the work for you. The down side is, you don’t always know what formula they’re using or if they messed up somewhere typing it in. I’m going to give you the formula and work through it in some examples, so you can either calculate it yourself whenever you like or check the math on your favorite online tool.

1. Find your weight in kilograms

Scales with kilogram output options are awesome, but if you don’t have one, you can convert pounds to kilograms petty quick: 1 lbs = 0.453592 kg

2. Find your height in meters

A tape measure with meters on it would do here, but most people in the U.S. know their height in feet/inches. Since you know that 1 foot = 12 inches, all you need to know now is that 1 inch = 0.0254 meters.

3. Now that we have our numbers, we just plug them into the BMI formula and get our result!

Body Mass Index = {Weight (kg) / [Height (m) * Height (m)]}

Hey look, I’m an example! I weigh 165lbs and stand 5ft 9in tall. Using the conversions above…

165lbs X 0.453592 = 74.84268 kg

5’9″ = 69 inches

69 inches X 0.0254 = 1.7526m

BMI = 74.84268 kg / (1.7526m X 1.7526m) = 74.84268 / 3.07160676 = 24.36597059709557

So if we round to a friendlier number, my BMI is 24.4.

So, what’s a good BMI?

The National Heart, Lung, and Blood Institute (NHLBI) says that a normal BMI is between 18.5 to 24.9, a BMI of 25 to 29.9 is overweight, and a BMI of 30.0+ is obese. On the other hand, a BMI under 18.5 is considered underweight, so lower isn’t always better. Looking at my BMI, we see that I am apparently on the high side of normal! This used to bother me, but it doesn’t anymore. Let me tell you why (it has to do with those outlier groups).

Sometimes BMI just doesn’t make sense!

When you’re really muscular or really tall – basically when you have a non-standard body type – the BMI calculation falls apart. My BMI puts me at the high side of normal, but I am more defined and stronger than I was when I had a BMI of 22 about a year ago. My weight has increased not because of overeating, but because of strength training. Lifting heavy,  leads to muscle growth (called hypertrophy) and thus more body weight. But at the same time, I lost some fat, which means that overall I appear more defined. At this point, I’ve moved from standard body type to non-standard body type, and the BMI formula stops working correctly. If I gain another pound or two of muscle, I will move into the overweight category, even though I’m actually pretty fit!

Here are some examples for people more famous than me (I know, it’s hard to imagine someone more famous):

Ray Rice (Running Back for the Baltimore Ravens) weighs in at 212 lbs and stands a towering 5 feet 8 inches. Is he fat? Well his BMI is 32.2, which means he’s technically not just overweight, but obese. Look at him, though. He packs massive muscle onto his smaller frame, skewing his results a ton. Source: NFL Player Profile

Kareem Abdul Jabbar (NBA: Milwaukee Bucks, Los Angeles Lakers) came to 7 feet 2 inches tall and 274 lbs in 1989. His BMI comes out to 26.0 which makes him overweight too. His huge height makes his BMI classify him as unhealthy, but his body fat of 13% shows otherwise. Source: Sports Illustrated, “An Unhappy Ending”, January 23, 1989

So what can you do with BMI?

Use it sparingly and as an informational tool when you first start training. Don’t stress out about it and trust what you see in the mirror before you trust what some calculation tells you. Use other factors to gauge your health like body fat percentage, number of pull-ups you can do, and 2 mile run time. Everyone is different, but the BMI formula is meant for a “normal” distribution. Once you start really training and exercising smart, you’re no longer normal.

You’re extraordinary.