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How BMI is Calculated

BMI stands for body mass index. This is a measure that uses your weight and height to determine if your weight is healthy. 

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Rather than being a direct measurement of your total body fat, it is more of an indicator, which you can use to help you maintain a healthy weight or ensure you are on the right track to achieve your goals.

In this post, we will reveal how BMI is calculated, as well as explaining what constitutes as a healthy BMI, and shedding some light on the effectiveness of this calculation. 

How is BMI calculated?

BMI is calculated by diving your weight by your height. The same approach is used for adults and children.

The exact calculation depends on whether you use the imperial or metric measurements system.

Meters and kilograms

Weight (kg) / [Height (m)]2

For example, if you weigh 68 kilograms, and you’re 165 cm tall, the calculation would be the following: 68 ÷ (1.65)2 = 24.98

You’re basically dividing your weight in kilograms by the height in meters squared.

Inches and pounds

Weight (lb) / [Height (in)]2 x 703

For example, if you weigh 150 pounds, and you’re 5’5” tall, the calculation would be the following: [150 ÷ (65)2] x 703 = 24.96

If you’re wondering where we got the 65 from, 5’5” is 65”

You’re basically dividing your weight in pounds by the height in inches squared, and then you’re multiplying this by 703. 703 is simply the conversion factor. 

Using a BMI calculator

The good news is that you do not have to do these calculations yourself, as there are a lot of BMI calculators online that will do the hard work for you.

What is a healthy BMI score?

Now that you know the calculation for BMI, it is important to determine what counts as a healthy score. You can refer to the table below for this information:

Your BMI score Your weight status
A score below 18.5  You’re currently underweight
A score between 18.5 and 24.9 You currently have a healthy and normal weight – This is the score everyone should be aiming for!
A score between 25.0 and 29.9 You are currently overweight
A score of over 29.9  You are currently obese

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How effective is BMI as an indicator of body fat?

The correlation between body fatness and BMI is fairly strong. However, it is essential to point out that even if you have the same BMI as another person, this does not necessarily mean you’re going to have the same level of body fat.

This is why we earlier stated that BMI is an indicator of body fat, rather than being a precise measurement. 

There are a number of reasons why your body fat level may differ from someone else with the same BMI score, including the following:

  • Athletes will have less body fat than non-athletes with the same BMI
  • Older people, on average, usually have more body fat than younger adults with the same BMI
  • Black people have less body fat than white people with the same BMI
  • Asian people have more body fat than white people with the same BMI
  • Females tend to have more body fat than men with the same BMI

As you can see, differences in gender, ethnicity, age, and activity can play a role. 

Why does your BMI matter?

Your BMI is considered the deciding parameter in terms of determining whether you are a normal weight, obese, overweight, or underweight, as per the National Institute of Health (NIH).

Your health is at risk if you do not fall into the normal weight category. 

Being underweight

For example, should you have a BMI of 17.5, this indicates that you are underweight. There are are several risks associated with being underweight. Your body’s immune system will be weak, which means you are going to be more prone to diseases. Plus, you could even be anaemic or get osteoporosis in later stages of your life. 

If you are underweight, there are a number of things you can do to add the pounds healthily. This includes the following:

  • Exercise – Strength training especially can be beneficial, helping you to gain weight by building up your muscle. You may also find that this stimulates your appetite. 
  • Add more calories to your meals – Top off your dishes with a few more calories, for example, by adding some grated cheese. 
  • Make every bite count – When you’re snacking, choose nutritious foods with healthy calories, such as avocado, dried fruit, cheese, peanut butter, and nuts.
  • Try shakes and smoothies – Rather than filling up on diet soda, coffee, and such like, choose drinks that have nutritional value. This includes healthy shakes and smoothies that are made with milk, fruit, and sprinkle some flaxseed te

Being overweight

Being overweight comes with a number of different risks as well. For example, if you are obese or overweight, you have a heightened chance of developing many different diseases. You will be more prone to stroke, coronary artery diseases, high cholesterol, type 2 diabetes, and high blood pressure, to name but a few. 

If you are overweight, there are a number of things you can do to lose the pounds healthily. This includes the following:

  • Plan ahead – Planning is vital when losing weight. Often, we turn to unhealthy snacks because we feel like there is nothing else in the fridge or cupboards. This is why planning is so important, enabling you to shop sensibly. 
  • Add exercise to your daily schedule – Pencil in time to exercise in your weekly schedule. Try to exercise five times per week for 30 minutes. 
  • Keep a photo diary – We have terrible memories in terms of what we eat. A photo food diary can be a life-saver! Take photos of everything you have eaten. It will help you to assess your eating habits, remember what you have already eaten, and review your nutritious meals in terms of what you enjoyed and didn’t enjoy. 

Body Mass Index (BMI) for children

BMI is calculated in the same manner for children as it is adults. However, unhealthy and healthy ranges are determined in a different way. Fixed values are not in place to determine at-risk and healthy thresholds for kids. 

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Instead, a child’s BMI values will be compared with other children of the same sex and age, with a percentile then calculated. 

If your child has a value that is below the 5th percentile for children, he or she will be considered underweight, and your doctor will advise you on different approaches that can be used to help your child gain weight in a safe manner. 

If your child has a value that is between the 5th and 85th percentile, this is considered a normal and healthy weight.

If your child’s value is between the 85th and 95th percentile, this means that they are overweight.

Finally, should your child’s value be over the 9th percentile, this would mean that they are classified as obese.

Should your child’s value be above the 85 percentile, your doctor will be able to advise on safe and effective measures to help your child lose weight and reduce their BMI.

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What are the limitations of BMI?

While there are a lot of benefits that are associated with measuring your BMI, there are also some limitations, which you need to be aware of. For instance, while your Body Mass Index is able to tell you if you are carrying too much weight, it is not able to tell you if you are carrying too much fat.

This calculation is unable to tell the difference between bone, muscle, or fat. It also does not account for muscle mass, gender, or age. As mentioned earlier, body fat can differ based on these qualities.

What does this mean for you?

Well, let’s say you’re extremely athletic and very muscular, this could result in you being classified as overweight or even obese according to your BMI, despite the fact that your body fat is actually very low. 

On the flip side, an adult may fall into the healthy weight range as they get older and start to lose muscle, despite the fact that they are carrying excess fat. 

Pregnancy and your BMI

A woman’s BMI results can also be impacted if she is pregnant. As your weight increases, your BMI is going to go up. Therefore, when calculating your BMI, you should always use your pre-pregnancy weight.

Aside from the limitations that have been discussed, the BMI is a relatively convenient and straightforward way of evaluating someone’s weight. 

Final words

So there you have it: everything you need to know about measuring BMI effectively. As we have discussed, BMI is an exceptional indicator of body fat, yet it is not a precise measurement, so do keep this in mind. 

Nevertheless, measuring BMI is undoubtedly an important step in maintaining a healthy weight. 



cdc.gov (1)




cdc.gov (2)