How does BMI work and why do I need to know about it?
The BMI is a measurement used by health professionals to determine if people are a healthy weight in comparison to their height.
What is BMI?
BMI is the shortened abbreviation of Body Mass Index and it is calculated by taking your height and weight. The results are then placed on chart and will tell you whether you are underweight, healthy or overweight. For adults, the result will be placed in a weight category and for children the results will be placed on a centile scale, which will determine whether your child is a healthy weight.
The BMI result is calculated based on the ration of height to weight and based on the principle that the taller you are, the more you should weigh.
The BMI range
The BMI calculation will give you a number, which is placed in a range. The categories include underweight, healthy, overweight and obese.
The ranges are classified as follows:
- Underweight: below 18.5
- Healthy weight: 18.5- 24.9
- Overweight: over 25
- Obese: over 30
How accurate is BMI?
In the majority of cases, the BMI is an accurate representation of whether an individual is a healthy weight; however, there are some issues with the index. Rugby players, for example, are likely to end up in the obese category because they often weight a lot in comparison to their height. This, therefore, is not an accurate result, as they are a healthy weight and live a very healthy lifestyle. The reason they are heavier than average is because they have a lot of muscle and the same problem applies for other sports players, including boxers. For the majority of people, however, BMI measurements are usually very accurate.
How is BMI used?
The BMI is used by health professionals to determine if individuals are a healthy weight and the results can indicate if an individual is at risk of developing health problems as a result of their weight. People who are underweight may be at greater risk of conditions such as osteoporosis and malnutrition, while being overweight can increase the risk of diabetes, heart disease and strokes.
Doctors use the BMI to advise patients and decide upon treatment plans. If somebody is experiencing health problems and is overweight, the doctor will usually advise them to take steps to lose weight, which include following a healthy eating plan and doing more exercise. In extreme cases, where an individual is very overweight, with a BMI of more than 40 (sometimes known as morbidly obese), doctors may recommend more drastic treatments, such as gastric band surgery and gastric bypass surgery, which are both types of weight loss surgery.
How do I calculate my BMI?
You can calculate your BMI by simply entering your height and weight into an online tool. NHS Choices provides an online calculator and if you cannot find one, your GP will be able to calculate your BMI for you. Most GPs measure and weigh their patients when they first register with the practice to determine their BMI.