Body mass index, better known by colloquial abbreviation BMI, is a value that is used to describe a person’s weight-related health risks. Calculated using a special formula, BMI is based on individual’s height and weight and results in a value that can be placed to one of several defined categories. Those with a BMI belonging to higher categories are considered to be at an increased risk of obesity-related health conditions such as type-2 diabetes and various cardiovascular issues.

What’s on this page?

How to calculate my BMI?

If you are curious about your BMI (as most of us would be after reading about it), you can calculate it fairly easily by yourself using a simple formula.

However, in order to make it easier for readers from different parts of the world, here we will provide you with two formulas – the first of them will help you calculate your BMI if you are used to working with metric units, while the second one is intended for those familiar with imperial units.

Calculating BMI (metric units)

BMI = Mass(Kg)/Height(m) squared

Calculating BMI (imperial units)

BMI = (Mass(Kg)/Height(m) squared) x 703

What are BMI categories?

The most commonly used BMI categories are as follows:

  • Healthy weight (optimal weight)
  • Overweight
  • Obese

While the three of these are enough for most laymen and medical professionals to determine the health risks and make decisions regarding treatments or diets, according to World Health Organisation, there are actually eight distinct BMI categories:

BMI Category BMI Value
Very severely underweight <15
Severely underweight 15-16
Underweight 16-18.5
Healthy weight 18.5-25
Overweight 25-30
Obese 30-35
Obese class 2 35-40
Obese class 3 >40

Please note that the values and categories provided here are applicable for UK only. While WHO is working towards standardisation, medical experts in some countries might interpret the values differently.

What do BMI categories and values mean?

The primary purpose of BMI is to provide a simple method of easily determining the presence or absence of health risks stemming from having larger or lower than ideal weight. However, BMI categories and values are most commonly calculated in order to highlight the potential risks related to obesity and being overweight.

Some conditions that are known to be directly connected to increased weight include:

  • Type-2 diabetes
  • High blood pressure
  • Heart disease
  • Stroke

However, WHO has successfully utilised body mass index to highlight other regional problems such as famine consequences, malnutrition and health risks stemming from these conditions. And it is especially important to note that malnutrition is known to be able to compromise the immune system, making those affected more susceptible to infection.

It is also important to note that people of Black or Asian ethnicity might be at increased risk of conditions usually associated with obesity, even if their BMI is lower. More specifically, people belonging to these groups are considered to be at risk if they have BMI higher than 23. And the risk is considered very severe if the calculated BMI is higher than 27.5.

How useful really is BMI?

Despite being a very handy and simple tool, the usefulness and efficiency of the body mass index formula has been brought into question more times than one. Most drawbacks stem from the fact that BMI doesn’t have to be an accurate indicator of the individual’s fat to body mass ratio.

For example, BMI completely disregards the difference between fat and muscles, relying instead on average values and generalisations. And as muscle weighs more than fat, someone who is rather muscular with a low percentage of body fat (professional athletes for examples) can sometimes be misidentified as overweight or obese according to the basic BMI formula.

At the same time, BMI formula also doesn’t take age into account. So, when the formula is applied to older individuals, it can also give somewhat skewed results – for the reasons exactly opposite from those mentioned in the above example. With age, the muscle mass will decrease and in some cases, it will be compensated by increased fat. So, someone with a “healthy” BMI might actually have highly elevated levels of body fat. The weight will be lower because of the lack of muscle mass.

My BMI is not ideal – what should I do?

First of all, don’t panic. As mentioned, body mass index formula is far from ideal. Small deviations from optimal value are generally not a cause for concern and even larger deviations should be confirmed with a doctor.

If there is any reason to worry, you should always consult with a doctor who will be able to provide you with reliable advice and informed assessment of your condition. If your weight needs some adjustments, your doctor will be able to provide you with advice on how exactly to achieve this. Most commonly, you will be asked to work on the development of the personalised dietary and exercise plan that you should adhere to in the future.

Read more about weight loss here.