The BMI or Body Mass Index score is often referred to as a critical measure of health. It provides an indication of whether an individual is of normal weight or not.

But what exactly is it and how is it measured? Can we trust it o direct our actions in terms of diet and exercise?

In this article we will discover where it came from, how it is calculated and what it tells us about our state of health.

What is Body Mass Index?

Body Mass Index, or BMI, is a numerical value that indicates whether you are underweight, of normal weight or overweight. It is basically a ratio of your height and your weight which provides a standardised measure.

Essentially, a shorter person is expected to have a lower weight while a very tall person would have a higher weight. A value in the normal range on the BMI scale suggests that a person heigh and weight are in proportion.

It is widely used as one of a number of health indicators and studies have should that there is a correlation between a higher BMI and various obesity related diseases.

How is BMI calculated?

The standard BMI calculation is straightforward and is the same for both men and women. The formula is your weight in kilograms divided by your height in meters squared.

Alternatively, you can calculate it as your weight in pounds (lbs) divided by your height in inches squared multiplied by 703.

MetricBMI = Weight (kg) / Height (m)
ImperialBMI = Weight (lbs.) / Height (in.) x 703

For a person who is 1.8 m tall (70 inches) and 70 kg (154 lbs.), the formula gives a BMI value of 21.6.

What is my BMI telling me?

Your BMI value places you in one of a number of categories. This simplifies the task of interpreting the value and can help you understand the steps you need to take if you are not already in the normal range.

The range you should be in is between 18.5 and 24.9, and the ideal value is 22. If you are below 18.5 you are considered underweight and if you are 25 or more you are considered overweight, obese or extremely obese.

In theory, as your BMI increases, your risk of developing a weight related condition, such as diabetes, cancer or hypertension also increases. You need to deal with that at any age, but it is particularly important to take steps if you are in your 40s or 50s to avoid complications or chronic disease later.

BMI to weight category chart

Body Mass Index is an excellent way to quickly and easily assess one aspect of your physical condition and a significant contributor to your state of health.

However, while it is simple to calculate and understand, it can be misleading. Studies suggest that, among other things, gender, age and body composition affect your BMI and should probably be factored in when interpreting the numbers.

For example, muscle weights more than fat, so if you have a very high percentage of muscle and low percentage of fat, your BMI may be high, suggesting that they are overweight when in fact your idea target should be adjusted to reflect your physieightcal condition.

Beyond the age of 30 we loose about 3% to 5% of our muscle mass ever 10 years. This natural change in body composition will also affect how we interpret BMI and what we consider a healthy target.

Your doctor will likely calculate your BMI, but will consider it in combination with other measures and health factors such as body fat percentage. In that respect, it’s a useful tool, but not one to be relied upon in isolation.


Body Mass Index is a convenient measure that provides a good indication of whether someone is overweight, underweight or at their ideal weight.

It’s a ratio of a person’s weight and height and can be quickly calculated at home, making it an ideal way to measure and track progress made through dietary adjustments or a program of exercise. Just be aware of its short comings and remember that everyone is unique and there are many factors that determine overall health.