The body mass index (BMI) is a helpful tool for many people, as is the calorie calculator , for keeping an eye on your weight and achieving your body-related goals. However, what many people don't know is that BMI changes with age. Our bodies are in a state of constant change throughout our lives. As a result, our body shape changes repeatedly over time. The same applies to the composition of fat and muscle mass. In this article, you will learn what this means for BMI and what you need to pay attention to depending on your age.
What is the body mass index?
The BMI is a measure that assesses a person's body weight in relation to their height. However, it only serves as a rough guide, because the formula takes into account neither body shape nor the composition of muscle and fat. In most cases, gender is also not included in the calculation.
The body mass index was invented by the Belgian mathematician Adolphe Quetelet with the aim of developing an equation that maps people in a normal distribution. He succeeded in calculating the ratio between weight and height. Soon after, however, the formula was no longer used to describe the “normal”, as intended by Quetelet. Instead, it was used as a measure to determine obesity.
BMI can be calculated using a simple formula: body weight in kilograms (kg) divided by height in meters squared.
What does the BMI tell you?
Now that you know your BMI thanks to our free BMI calculator, you are probably wondering what the calculated value actually means.
As the table below shows, the B Body Mass Index is divided into different levels by the World Health Organization (WHO) . The categories range from severe underweight to obesity type III.
| BMI value | Category |
| < 16.0 | severe underweight |
| 16.0 – 17.0 | Moderately underweight |
| 17.0 -18.5 | Slightly underweight |
| 18.5 -24.9 | Normal weight |
| 25.0-29.9 | Pre-obesity |
| 30.0-34.9 | Grade I obesity |
| 35.0-39.9 | Grade II obesity |
| ≥ 40 | Grade III obesity |
Even though the result is often used as a benchmark for assessing a person's health, this conclusion is actually incorrect. This approach should be viewed critically, especially in the middle categories. On the other hand, if the calculation produces an extreme value in the upper or lower range of the scale, this can certainly entail a certain risk of secondary diseases. For example, obese people are more likely to develop type 2 diabetes or cardiovascular disease and are more likely to suffer a stroke.
How does BMI change with age?
The body mass index is used as a rough guide to determine whether a person is of a normal weight or is affected by being overweight or underweight, which in some cases requires treatment. However, the values as we know them must not be calculated and interpreted in the same way across all age groups. The following should be noted:
The BMI for children and adolescents
Since the relationship between height and weight is constantly changing at the beginning of our lives, the classic BMI calculator is not suitable for people under the age of 18. In addition to height and gender, the body fat percentage must also be included in the calculation, which is why there is a separate BMI calculator for children.
Since the classic BMI calculator is not suitable for children, a separate BMI calculator has been developed for the youngest.
It is also important to know that the calculation only makes sense for children aged eight and up. The weight of smaller children can fluctuate greatly due to baby fat and growth spurts. If you want to be sure that the weight of your child is within the normal range, it is best to consult your trusted pediatrician.
Ideal weight for adults
As soon as adolescents have stopped growing, their body mass index can be determined using the well-known BMI calculator. The BMI table mentioned above is used for interpretation. The guidelines apply to everyone between the ages of 19 and 65.
According to the German Nutrition Society (DGE), a distinction can also be made according to gender. According to this, the ideal BMI value for men is between 20 and 25. For women, the normal weight is between 19 and 24. Athletes are an exception. Due to their high muscle mass, different rules apply to them.
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The BMI for senior citizens
Not only at a young age is the measurement prone to error, but also with advancing age. Over the course of a lifetime, the body composition changes. The proportion of fat increases, while the muscle mass decreases.
For this reason, the categories have to be adjusted a little from the age of 65. For older people, a higher BMI and a few extra pounds are definitely recommended. If you become ill and your body is weakened, additional fat reserves can actually be useful. You can read more details at BMI for women over 60.
