Body Mass Index Calculator

The body mass index, or BMIis a measurement system based on computing the height or height and weight of a person and contrasting it with certain parameters to determine whether, based on these calculations, the person is in a «normal», «high» or «low» weight range, from chronic malnutrition to level 3 obesity.

But what is it really good for? How should you calculate it? Is there an ideal BMI? Is the ideal BMI the same for everyone?

All these questions and more will be answered in this article, so read on to find out!

What is BMI used for?

As we have been saying, the BMI consists of computing a person’s height and weight measurements to find out if his or her weight is considered adequate in relation to his or her comparing the result of the calculation with certain parameters or standards previously defined according to the studies of the Belgian statistician Adolphe Quetelet (this is why it is also called the Quetelet index).

Thus, the BMI makes it possible to establish for a given person whether or not the weight at which he/she is at that moment is considered normal for his/her height, whether he/she is underweight or, on the contrary, overweight. In addition, as one moves up the index scale, one can also establish whether one moves from the underweight level to a level of malnutrition, or to the level of obesity.

BMI and ideal weight

One of the main issues for which BMI is used is to establish the «ideal weight» of a person, based on the correlation that can be drawn between his or her height and a certain level of weight that can then be considered ideal.

However, this is a point that has been very controversial, as there are those who claim that this index has become obsolete, since a person’s ideal weight may vary depending on whether that weight is composed of fat, muscle, etc.

For this reason, establishing it in such broad terms may be simplistic, and is even associated with the prevalence of eating disorders or medical malpractice (for example, in overweight patients who present pathologies, even serious ones, which are minimized because they are perceived as a consequence of their «abnormal» BMI).

For this reason, new medical approaches have pointed out that when analyzing a BMI calculation, it should be taken into account that the «ideal» BMI can be very different for men, for women and for children, and it also changes according to age (even in adults), and according to the percentage of fat or waist circumference.

According to this also changes the risk of suffering from comorbidities associated with weight, and the measures to be taken with respect to a BMI considered abnormal, since this will be very different in an overweight BMI than in an underweight BMI, although it should be remembered that both are high-risk situations.

How is the body mass index calculated?

As we have already mentioned, the calculation of the body mass index is not as simple as it is often thought.

Therefore, the section below can function as a body mass calculator, as we will give you all the information so you can learn how to calculate your body mass.

Let’s get down to business. What you need to calculate your body mass index is:

  • Know your weight in kilograms
  • Know your height in meters
  • Calculate the square of your height (i.e. height x height) to get the resulting measurement in square meters.

Once you have these figures, you can calculate the body mass index as follows:

BMI = weight / height squared

This calculation will give you a resulting figure, which is what is actually known as the body mass index. The result of this operation will be your BMI, and now all you have to do is to know how to interpret this figure with respect to the range in which your BMI falls and how this is positioned with respect to normal weight, underweight or overweight.

In general terms, what you should know is that:

  • A body mass index of less than 16 points (<16) indicates severe malnutrition.
  • A body mass index in the range of 16-18.4 points indicates moderate malnutrition.
  • A body mass index between 18.5-22 points indicates underweight or underweight.
  • A body mass index between 22.1-24.9 points is considered a normal weight for your height.
  • A body mass index between 25-29.9 points indicates mild overweight.
  • A body mass index between 30-34.9 points indicates type 1 obesity.
  • A body mass index between 35-39.9 points indicates type 2 obesity.
  • A body mass index of more than 40 points (>40) indicates type 3 obesity.

Differences in BMI between men and women

In addition to this basic interpretation, there are some other aspects with which the interpretation of BMI can be expanded, taking into account other factors that may vary and are more personalized and individual.

The first of the factors to take into account is gender, as it tends to be considered that there is a difference in what is considered an ideal BMI for men and women. In general terms, it has been established that there is a difference of one point (1) in terms of what is considered normal for men and women, with the «normal» range for men being one point above that for women.

The second factor to consider is age. The biggest variation in that regard is the difference in BMI that is considered normal for children and adolescents, compared to the normal ranges for adults.

In the case of children and adolescents, the calculation is somewhat different, since to compute their weight and height, their age must also be taken into account, since the metabolic needs of each age are different, and, being in growth, they are rapidly changing and we must be more alert to ensure that they are not considered as being underweight or overweight without necessarily being so, when this responds only to the normal fluctuations of their growth.

BMI by age

According to WHO guidelines, BMI ranges for children and adolescents should be analyzed as follows:

  • A body mass index below approximately 12 points (<12) implies chronic malnutrition.
  • A body mass index below 13 points (<13) indicates moderate malnutrition.
  • A body mass index below 14 points (<14) indicates mild malnutrition.
  • A body mass index between 14 and 17 points (14-17) indicates a normal weight.
  • A body mass index between 17 and 19 points (17-19) indicates overweight.
  • A body mass index greater than 19 indicates obesity.

On the other hand, in the case of adults, age is also a factor to consider, since as we get older we tend to increase in weight and decrease in height, especially from the third age onwards, which is considered after the age of 60 or 65, when we start to lose height and bone density. And, starting at about 40, you start gaining fat as well.

In women this is especially true (and normal) after menopause, when their metabolism slows down. In the case of men, metabolism also slows down, accompanied by other signs of aging such as alopecia.

Therefore, a BMI of up to 34.4 points for women and 26.7 points for men, which gives an average of around 30 points, can be considered normal or even optimal in middle age (60 years and older).

This is why age is such an important factor to consider, because although these are the figures for middle age, you go through a gradual maturation process, which also impacts the BMI and the ranges of what is considered normal.

BMI and waist measurements

As we have been mentioning, BMI is much more than just black and white. 

Having a BMI that places you in the underweight or overweight range does not necessarily imply that there is an intrinsic risk associated with your health, as this depends mainly on the percentage of fat and/or body mass you have. 

It is not the same to have a certain weight composed of muscle mass as it is to have a weight composed of fat. 

An important indicator that can help you determine this is waist measurement. The waist-to-height ratio, for example, is a measure that can be of some use in determining whether your BMI is healthy, since by contrasting one measure with another you can more adequately establish your health profile with respect to your weight and whether it is ideal or not. 

Remember to consult a professional

By way of conclusion, it is important to keep in mind that, although everything we have said in this article is a useful guide, it is always best to consult a professional when determining issues related to your state of health, and especially when thinking about taking action.

All of these are as real and tangible threats to health as obesity and excess fat, so you should not minimize them or downplay their impact.

BMI (and AQI) are NOT absolute, and should not be taken lightly. Never start a diet or intensive exercise regimen without the advice and assistance of a health professional, such as a nutritionist or sports physician.

In any case, we hope this article will serve as a starting point for you to think about your health and to have a guide about some aspects of your weight in relation to other health conditions such as your age, sex, height, metabolism, fat percentage, among other aspects.

Take into account all the information provided here to calculate your BMI, or anyone else’s, easily and free of charge.

We hope you find it useful for your health and fitness goals.
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