What Is BMI? | When It Works And When It Really Doesn’t

What Is BMI? | When It Works And When It Really Doesn’t

“What is BMI?” is an incredibly popular google search term. This tells me two big things. One, a huge number of people have heard of BMI. Two, and most importantly, a huge number of people don’t know what it is. I first came across Body Mass Index (BMI) in school when I was studying my GCSE P.E. It was described as a measure of health based on your height versus your weight. I believe in the value of having simple tests that anyone can do to understand their health. However, even at the age of fifteen, I had serious questions about BMI.


liam in a mask in the lab

Photo Credit: Liam Grimley

What Is BMI

Body Mass Index is a way of working out if you are of a healthy body shape and size. It is essentially a comparison of your weight and height.

BMI = Weight (kg) ÷ Height (m)² 

For me that’s 81kg ÷ (1.87 x 1.87) or 3.4963 = 23.2

Many online BMI calculators exist, including this one from the NHS. Once you have your result, the BMI table categorises you as Underweight, Healthy, Overweight, Obese or Clinically Obese.

bmi scale obesity

Here 18.5 is between Underweight and Healthy, 25 is between Healthy and Overweight and over 30 goes into Obese and Clinically Obese.

When BMI Works

I have always appreciated the value of BMI at the extreme ends of the health spectrum. Athletes such as dancers, gymnasts and endurance sports people and those in the modelling and entertainment industries can be at risk of being seriously underweight. The health risks of being severely underweight are significant. They include hormone dysfunction, infertility, diminished physical development. There are athletic and aesthetic pressures put upon these young people from many angles. These come from their social environment, media coverage of their heroes and often misguided or inappropriate advice from coaches and even parents and team mates. If a person falls into the underweight band of the BMI then this needs to be acknowledged and appropriate steps taken to safeguard their health. At the other end of the spectrum are the obese and clinically obese bracket. Serious health risks are associated with being in this category and the BMI scale sends a clear message. This person needs to address their health and lifestyle with the same seriousness as the underweight individual. Sometimes we need a wake up call to realise something needs to change. Especially if we are surrounded by similarly unhealthy people. It can be hard to see that something is wrong, because our perception of “normal” is skewed. As the population continues to grow more overweight this is an increasing problem.

elevate workout boxing
Photo Credit: Virgin Active – Model: Megan Taylor

When BMI Does’t Work

My earliest memory of BMI is wondering, “what about really muscular people?” It was clear to me that just measuring weight and height did not take into account the person’s body composition. At that time, a young Jonny Wilkinson was making waves in international Rugby Union as one of the fittest and most skilful players in the sport. At five feet ten inches and fifteen stone his BMI declared him Clinically Obese. The same can be said of any number of athletes, so we must to take into account more than just weight and height. The BMI scale fails to take into consideration even the most basic natural body shape or “Somatotype.” We each fall somewhere on the spectrum of somatotypes, with our own mix of the following three extremes. Endomorph (round, fat type) at one end, Mesomorph (muscular type) in the middle and Ectomorph (linear, skinny type) at the other end. For me, the Jonny Wilkinson example explains why a mesomorph may be mis-measured as overweight. When world marathon record holder Paula Radcliffe competed, she would have been classified Underweight. She was obviously in fine shape and exhibits the narrow pelvis, long bones and skinny wrist and ankles that typify the more extreme ectomorph.

Skinny Fat

 So we must be mindful of the sporting interests of the individual when using the BMI scale. There are more important examples of when BMI misses the mark. Compare a twelve stone, six foot muscular person to a twelve stone, six foot inactive couch potato. This is essentially what we have come to know as “skinny fat.” They have the same BMI, but are not equally healthy. So you can get a great mark on the BMI scale and be at risk of type 2 diabetes, heart disease and many other illnesses. In older age, this is also true as people become less active, lose muscle and gain fat. BMI may falsely tell us that this person is doing well when in fact they are becoming less healthy and at increasing risk of illness.

A Better Alternative

What if I told you there was a better and easier alternative? Well, there is and not only is it so simple you can do it without a calculator, it is a far superior indicator of health than BMI. It is your Waist to Height Ratio. Measure your waist circumference at the thickest part anywhere between the top of your pelvis and bottom of your ribs. All you have to do is divide your waist circumference (cm) by your height (cm). That’s it!

Healthy = Waist is Half your Height

For me that’s 84cm ÷ 187cm = 0.45

You want to be as close as possible to a score of 0.5 to be healthy.
<0.4 and lower indicates “Take Care” which relates to being too slim.
0.5 (+/-0.1) is “Okay” or healthy.
0.6 (+/-0.1) suggests “Consider Action”
>0.6 is categorised as “Take Action.”

It is not only much simpler to calculate, your Waist to Height Ratio is many times more accurate than BMI. It avoids the issues mentioned above and it gives you a healthy and reliable goal to aim for in your pursuit of your fit, healthy body.

bmi a better alternative

Photo Credit: Taylor-Grimley

To Round Up

BMI is helpful if we have a very underweight or morbidly obese person who needs to understand that they must urgently address their health. In my opinion, the middle zones cannot be relied upon to determine if a person if truly healthy. If you are using BMI, do it in conjunction with a good understanding of the person’s history of physical activity, somatotype and other measurements where possible. Height to Waist Ratio is the best at-home health check you can do. Abdominal circumference is a more direct indicator of health or health risk, that’s why I recommend it in my health and fitness ebook, Eat and Move. This is backed up by a 2016 review in the British Medical Journal.

Have you used BMI? What are your thoughts on all this? Leave me a comment below.


  1. Susie Morgan
    11/01/2019 / 09:16


  2. Nigel Bridge
    11/01/2019 / 09:57

    Hi Liam,
    Great post and I fully agree with what you say. My BMI is 23.6 and have had this highlighted to me as a possible risk at NHS and company medicals in the past, however, my height to weight ratio is 0.447. Thanks to all our PT sessions I’m in the best shape of my life and turning 50 this year!

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