BMI calculator

Table of contents

• What is the BMI (Body Mass Index)?
• How is the BMI calculated?
  • BMI formula
• How do I interpret the BMI?
  • WHO classification
  • Other classification
  • BMI chart
• What are the risks of overweight and underweight?
  • Risks of overweight
  • Risks of underweight
• How is the BMI used?
• Are there advantages and limitations of the BMI?
• What alternatives to the BMI exist?
• History
• Conclusion: BMI as a guide
• Sources

What is the BMI (Body Mass Index)?

The Body Mass Index (BMI) is an internationally recognized measurement that indicates the ratio of a person's body weight to their height. It is often used to determine whether a person has a healthy weight in relation to their height. The BMI serves as a rough guide to classify weight into different categories: Underweight, normal weight, overweight and obesity (adiposity). The BMI is calculated by dividing the body weight in kilograms by the height in meters squared. The result of this calculation gives a value that is then classified into one of the following categories:

• Underweight: People in this category have a body weight that is considered too low in relation to their height. This can indicate various causes such as malnutrition or health problems.
• Normal weight: A BMI in this range is considered ideal as the weight is considered to be in the right proportion to height. People with a BMI in the normal range tend to have a lower risk of health problems associated with being overweight or underweight.
• Overweight: People with a higher BMI fall into this category. They have more body mass than is considered healthy, which can increase the risk of health problems such as cardiovascular disease or diabetes.
• Obesity: When the BMI is particularly high, it is called obesity. People in this category have a significantly increased risk of serious health problems, including heart disease, high blood pressure and joint problems.

Although BMI provides a useful initial assessment of body weight, it also has limitations. For example, it does not take into account the proportion of muscle mass, fat or bone mass in the body. This means that people with a lot of muscle mass (such as athletes) can have a higher BMI without being overweight. Similarly, BMI does not provide information about the distribution of body fat, which is also important for health.

BMI is a simple method of estimating body weight in relation to height, but does not take into account all individual health aspects. More modern classifications also account for age and gender to be more specific.

How is the BMI calculated?

The BMI formula is a simple method of calculating the relationship between body weight and height. With the help of our BMI calculator you can instantly and for free calculate your indiviudal BMI value. The idea behind the formula is that weight should increase in proportion to height. To calculate the BMI, the body weight is divided by the height multiplied by itself. This means that you take the height and put it in relation to the body weight by “squaring” the height (i.e. multiplying the height by itself). The square of the height is used because weight is related to the area or volume of the body, not just the height.

Depending on the measured variables, a distinction is made between a metric and a USC formula. They both result in the same BMI value.

BMI formula

Metric system using kg and m

For example, a person who weighs 70 kg and is 1.75 m tall has a BMI of around 22.86. This value is within the normal range.

USC system using lbs and in

For example, a person who weighs 155 lbs and is 70 in tall has a BMI of around 22.2. This value is within the normal range.

How do I interpret the BMI?

The interpretation is based on defined BMI categories that indicate whether the weight is in the healthy range or whether there is a risk of health problems. There are different BMI classifications, which are described in further detail below. These categories give an indication of whether the weight is healthy or whether there is a risk of health problems such as cardiovascular disease, diabetes or joint problems.

WHO classification

The World Health Organization (WHO) defines a general classification that only refers to weight and height and does not differentiate further according to age or gender. This classification should therefore be regarded as rather rough and not tailored to individual cases. However, the WHO approach is considered the most commonly used methodology.

Other classification

Other BMI classifications, such as those from the University of Mannheim in Germany, have developed the classic approach further and also include age and gender in the interpretation. This enables a more specific and accurate evaluation. The classifications are listed below according to gender and age.

BMI for women

BMI for men

BMI chart

The following chart shows the BMI for a range of heights and weights in metric and imperial units (USC). It allows a quick assessment of which BMI category a person falls into without having to calculate the formula manually. By reading the weight on one axis and the height on the other, you can find the BMI value and the corresponding WHO category in the diagram. It is particularly useful for medical staff or lay people who want to quickly determine the BMI without using the formula. This chart applies to adults and is not applicable to children.

The colors indicate the BMI categories defined by the World Health Organization. Light blue corresponds to underweight, green corresponds to normal weight and yellow, orange and red show the different levels of overweight up to obesity.

By nagualdesign - Own work, CC BY-SA 4.0, Link

What are the risks of overweight and underweight?

These following risks show how important it is to maintain a healthy weight in order to protect and promote long-term health. Regular calculations of your BMI value with our BMI calculator can help to recognize both overweight and underweight at an early stage.

Risks of overweight

Being overweight can entail a number of health risks, including

• Cardiovascular diseases: Increased risk of high blood pressure, heart attack and stroke due to additional stress on the cardiovascular system.
• Type 2 diabetes: Obesity often leads to insulin resistance, which increases the risk of developing type 2 diabetes.
• Fatty liver disease: Excess fat can lead to a buildup of fat in the liver, which can lead to non-alcoholic fatty liver disease (NAFLD).
• Joint problems: The extra body mass puts stress on the joints, especially the knees and hips, which can lead to arthritis and other joint problems.
• Sleep apnea: Being overweight increases the risk of sleep apnea, a condition in which breathing stops and starts repeatedly during sleep.
• Cancer risks: Higher risk of certain cancers such as breast, colon and prostate cancer due to inflammatory processes and hormonal changes.
• Mental health: Obesity can lead to emotional and psychological problems such as low self-esteem, depression and anxiety.
• Digestive problems: Obesity can increase the risk of gastrointestinal disorders such as reflux disease and gallstones.

Risks of underweight

Being underweight can also pose a number of health risks, including

• Malnutrition: Low body weight can indicate inadequate nutrient intake, which can lead to malnutrition and deficiencies in key vitamins and minerals.
• Weakening of the immune system: Insufficient body weight can reduce the body's ability to fight infection and disease by weakening the immune system.
• Osteoporosis: Being underweight increases the risk of osteoporosis, as inadequate nutrient intake, especially of calcium and vitamin D, can affect bone health.
• Muscle loss and weakness: Low body fat can lead to muscle loss and general physical weakness, which can affect physical performance.
• Cardiovascular problems: Being underweight can stress the cardiovascular system and lead to problems such as cardiac arrhythmias and low blood pressure.
• Fertility problems: In women, being severely underweight can lead to hormonal imbalances that can cause menstrual irregularities and fertility problems.
• Difficulty healing wounds: People who are underweight often have slower wound healing and are more prone to complications after surgery or injury.
• Cognitive impairment: Malnutrition and inadequate nutrient intake can have a negative impact on mental performance and concentration.

How is the BMI used?

The BMI is a simple and quick tool to assess body weight. Doctors and nutritionists often use it to determine possible health risks. The BMI plays a particularly important role in the early detection of overweight and obesity. Here are the most important areas of application of the BMI:

• Health assessment: Physicians use BMI to determine weight status (underweight, normal weight, overweight, obese) and assess risk for diseases such as cardiovascular disease, diabetes or malnutrition.
• Clinical diagnoses: BMI is used to diagnose overweight and obesity and helps plan treatment interventions.
• Epidemiology: Researchers use BMI to analyze obesity trends in populations and develop prevention programs.
• Child development: Specialized BMI charts help monitor children's growth and identify potential weight problems early.
• Fitness and lifestyle: People use BMI to set their health goals and monitor their progress.
• Insurance: Health insurers use BMI to assess health risks and set rates.

However, the BMI should not be viewed in isolation, as it does not take certain factors into account, such as

• Muscle mass: people with a lot of muscle mass, such as athletes, can have a high BMI without being overweight.
• Age and gender: BMI can be interpreted differently for older people and women than for younger men.
• Body fat distribution: The BMI says nothing about where the fat is stored in the body. Abdominal fat, for example, is more risky than fat in other parts of the body.

Are there advantages and limitations of the BMI?

Advantages:

• Simple calculation: BMI can be calculated quickly and easily using a BMI calculator or online tools.
• Wide application: It is recognized worldwide and serves as a standard for assessing weight problems.
• Early warning system: BMI can indicate overweight or underweight, allowing early intervention.
• Economical: Since only basic measurements are required, the BMI is inexpensive and widely used.
• Standardized measurement and comparability: BMI provides a standardized method of assessing body weight and allows for comparison of individuals and populations.
• Population studies: BMI is often used in large studies to identify patterns related to overweight and obesity in populations.

Limitations:

• Misinterpretations: The BMI does not differentiate between muscle mass and fat mass. Very muscular people can have a high BMI even though they are not overweight.
• Individual factors are missing: Age, gender, genetic predisposition and physical activity are not included. BMI does not take into account the different body compositions at different ages or between men and women. However, there are more modern classifications which account for such shortcomings.
• Not ideal for all population groups: The BMI is less meaningful for children, older people or pregnant women.
• No consideration of fat distribution: The BMI does not take into account where fat accumulates in the body. A person with a normal BMI can still have unhealthy fat distribution, such as visceral fat.
• Health complexity: BMI alone does not provide complete information about overall health or specific health risks. Further measurements and tests are often necessary to get a complete picture.

What alternatives to the BMI exist?

There are additional measurement methods for a more detailed analysis of body weight and health:

• Waist-to-hip ratio (WHR): Measures the ratio of waist to hips and provides information on the risk of cardiovascular disease.
• Body fat percentage: Body fat measuring devices can be used to determine the actual percentage of fat in the body.
• Bioelectrical impedance analysis (BIA): This method measures the composition of the body, including muscle mass and body fat.
• BMI prime: The BMI Prime is a clever modification of the BMI. It is a decimal number where 1.0 is the upper limit of the “normal BMI” range. This allows you to easily determine whether you are overweight or not. If your BMI Prime is above 1, you should lose some weight.
• Ponderal index: Rohrer's Ponderal Index (PI) is an indicator of a person's weight in relation to their height and, like the Body Mass Index (BMI), is used as an indicator of obesity. The PI is calculated as weight divided by height to the power of 3.

History

The BMI was developed in 1832 by the Belgian mathematician Adolphe Quetelet and the term Body Mass Index (BMI) comes from an article published by Ancel Keys in 1972. However, Keys only recommended the BMI for the statistical comparison of populations, not for assessing the obesity of individuals. The BMI gained importance through its use by US life insurers, who use this simple classification to calculate premiums for life insurance policies in such a way that additional risks due to obesity are taken into account. Since the early 1980s, the BMI has also been used by the World Health Organization. The current BMI classification of the WHO has essentially existed since 1995.

In anthropometric history and historical anthropology, the mean body mass index of population groups is used as an indicator of the standard of living, similar to height. It is possible to look back into the past on the basis of historical data collected, for example, during recruit mustering. Estimates of BMI carried out on bones from archaeological contexts take us back further in time. They can be used to estimate that the average diet in early medieval Europe was quite good.

Conclusion: BMI as a guide

The Body Mass Index (BMI) provides a good initial guide when it comes to assessing your own body weight. However, it is not a perfect tool and should always be considered in the context of other factors. A combination of BMI, body fat percentage and lifestyle provides the most accurate assessment of health risks. If you want to calculate your BMI, simply use our BMI calculator on this website. Enter your weight and height to get your personal BMI value!

Sources

• A healthy lifestyle - WHO recommendations
• The SuRF Report 2 - World Health Organization
• Body mass index - Wikipedia
• Body Mass Index: Uniklinik Mannheim, Germany
• CDC - BMI information and details
• CDC - Calculating BMI using the English System