BMI Calculator

What is BMI?

In general terms, body mass index (BMI) is a rough calculation of how much body fat you have.  More specifically, for adults, BMI is essentially a ratio of your weight to height used to determine whether or not you’re at a healthy body weight.  

Here’s how it breaks down:

Underweight: BMI < 18.5

Normal weight: BMI = 18.5 – 24.9

Overweight: BMI = 25 – 29.9

Obese: BMI is ≥ 30

Both men and women can use BMI to assess their body weight, however, there are some potential limitations you should be aware of.  First, the formula is only for adults — there are additional things to consider when measuring a child’s BMI that aren’t taken into account.

Secondly, athletes and individuals with a lot of muscle mass may find that their BMI is above the normal range even though they may, in fact, be at a healthy body weight.  

Finally, because the aging process is often accompanied by muscle loss, BMI can underestimate the amount of body fat in older folks.  So even if you’re in the normal range, your ratio of body fat to lean body mass may still be less than ideal.    

How Is BMI Calculated?

BMI is a standardized formula and can be calculated using either the metric or imperial measurement systems.  Here’s what the formulas look like for each:

Metric Formula: weight (kg) ÷ [height (m)]²

Here’s what it looks like for an 80kg, 1.8 meters tall person:

80 ÷ 1.8²

80 ÷ 3.24

BMI = 24.69

Imperial Formula: 703 x weight (lbs) / [height (in)]²

Here’s what it looks like for a 175lb, 6’ tall person

703 x 175 ÷ 72²

123,025 ÷ 5184

BMI = 23.73

Why Is Knowing Your BMI Important?

Overweight/Obese

If your BMI is over 24.9, and especially if it’s over 30, it’s an indicator that you have too much body fat in relation to your height.  Those who are overweight/obese, on average, have lower life expectancies compared to individuals with less body fat.    

That’s because being overweight/obese has been linked with a host of health-related issues, including type 2 diabetes, heart disease, osteoarthritis, and high blood pressure just to name a few.  It’s also been shown to raise your risk of cancer.  

⫸How to lower your BMI

When it comes to losing weight, only one thing is essential: a hypocaloric diet, which is what happens when you consume fewer calories than your body burns off in a day.  One of the easiest ways to figure out how many calories your body burns per day is by using a Total Daily Energy Expenditure (TDEE) calculator.  

A TDEE calculator will give you an estimate of how many calories it takes to maintain your current weight based on your age, sex, height, weight, and activity level.  From there, you simply apply a 25% – 30% calorie restriction to your diet for healthy weight loss.  

While you can certainly lose body fat simply by restricting your calories, eating a hypocaloric diet can also cause you to lose muscle, which just about no one wants.  However, resistance training can help you to preserve your lean muscle mass and improve your body composition to a better degree than a hypocaloric diet alone.  

Underweight

On the opposite side of the spectrum, having a BMI below 18.5 can also be detrimental to your health.  Being underweight can increase your risk of having micronutrient deficiencies, which amongst other things, can negatively impact your immune system, leaving you more vulnerable to sickness and infection.  

On top of that, having an abnormally low BMI can also increase your risk of bone-related conditions like osteopenia and osteoporosis, which can, in turn, increase your chances of injuring and fracturing your bones.

⫸How to Increase Your BMI

The first step towards gaining weight lies in increasing your calorie intake.  Just like with losing weight, you’ll need to know your TDEE in order to do it with precision. 

Once you’ve figured out how many calories it takes to maintain your current weight, you can begin to add some additional calories into your diet.  For healthy, gradual weight gain, you’ll want to go with a calorie surplus in the vicinity of 500 extra calories per day.

But you don’t want to gain too much fat during your weight gain, you want to focus on building lean muscle mass.  That’s where a high protein intake and resistance training come into the picture.  

In order to maximize your muscle-building potential, you’ll need to be taking in around 1g of protein per pound of body weight each day.  On top of that, you’ll need to maintain a regular weight lifting program in order to make sure you’re gaining lean mass, not body fat.