What does your body mass index say about your health?

The body mass index () is a way of categorizing a person’s body weight. It is calculated using their weight and height:

How to calculate the body mass index

The resulting is then used to determine the person's weight category:

Table: categories
Medical weight category
Below 18.5 Underweight
18.5 to 24.9 Normal weight
25 to 29.9 Overweight
30 to 34.9 Obese (class 1)
35 to 39.9 Obese (class 2)
Over 40 Obese (class 3)
Illustration: Body shape for different BMI categories

What can the BMI tell us?

The relationship between your and your health is influenced by many factors. For instance, having a of 25 to 30 is defined as being overweight in medical terms. But research shows that a in this range doesn’t have much impact on life expectancy or various health problems. Being overweight mainly affects a person’s health if they have other conditions too, like diabetes.

It is an accepted fact that obesity ( over 30) can have a negative effect on health and life expectancy. The more severe the obesity is, the higher the risk of other diseases. But, here too, the overall situation is what counts. There are people who are obese but have a good level of fitness and a healthy heart and blood vessels (cardiovascular system). Their weight isn’t necessarily a problem in terms of their health. A lot of obese people do improve their health by losing a little weight though.

Apart from body weight, the risk of cardiovascular disease is particularly dependent on a number of factors, mainly: age, gender, blood pressure, cholesterol level, family medical history, diabetes and smoking. So it makes sense to take these other risk factors into account when considering the health impact of obesity.

How relevant is body fat distribution?

Research has shown that fat around the stomach area is more of a health problem than fat in other parts of the body because it has more of an effect on the metabolism. But it doesn't tell you anything about how your body fat is distributed. So it can be a good idea to measure your waist too. A waist size of more than 102 centimeters for men and over 88 centimeters for women is considered bad for your health. But these measurements don’t take into account that people have different body shapes.

You can measure your waist by placing the end of a tape measure half way between the top of your hips and the bottom of your ribs and wrapping the tape measure around you. Breath out slightly and don’t pull your tummy in. The best time to measure is before breakfast.

Illustration: Measuring your waist – as described in the article

Sometimes other measurements are used, like the “waist-to-hip-ratio.” To calculate it, divide your waist measurement in centimeters by your hip measurement in centimeters. A ratio of more than 0.9 in men or over 0.85 in women is considered bad for your health.

How can you determine your body fat percentage?

A body fat percentage of 8 to 25% for men and 20 to 35% for women is considered normal. Researchers use elaborate methods to determine body fat percentage as accurately as possible. A simpler method is to use bioelectrical impedance analysis (BIA). BIA devices measure the in a person’s body (how much electricity it conducts). They can then estimate the water content and give a rough estimate of the fat content. You can buy scales that estimate your body fat using BIA.

It is important to be aware that doesn’t take a person’s build into account and that it can be unreliable if you are very short or very tall. It doesn't differentiate between muscle and fat, either. That means it overestimates the fat in people with a very muscular build (bodybuilders, for example). In older people, it tends to underestimate it because muscle mass decreases with age.

The categories used in western countries are also not very suitable for other ethnic groups that have a different build. For example, they underestimate the body fat in people from Southeast Asia and overestimate it in people of African descent.

IQWiG health information is written with the aim of helping people understand the advantages and disadvantages of the main treatment options and health care services.

Because IQWiG is a German institute, some of the information provided here is specific to the German health care system. The suitability of any of the described options in an individual case can be determined by talking to a doctor. informedhealth.org can provide support for talks with doctors and other medical professionals, but cannot replace them. We do not offer individual consultations.

Our information is based on the results of good-quality studies. It is written by a team of health care professionals, scientists and editors, and reviewed by external experts. You can find a detailed description of how our health information is produced and updated in our methods.

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Please note that we do not provide individual advice on matters of health. You can read about where to find help and support in Germany in our information “How can I find self-help groups and information centers?

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Created on November 21, 2022

Next planned update: 2025


Institute for Quality and Efficiency in Health Care (IQWiG, Germany)

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