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Broca Index: How to Calculate Normal Body Weight

The Broca Index is a simple formula for estimating ideal body weight. It is calculated by subtracting 100 from a person´s height in centimeters to give the normal weight in kilograms.

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Things worth knowing about "Broca Index"

The Broca Index is a simple formula for estimating ideal body weight. It is calculated by subtracting 100 from a person´s height in centimeters to give the normal weight in kilograms.

What Is the Broca Index?

The Broca Index is a classic, easy-to-use formula for estimating the normal body weight of an adult. It was named after the French surgeon and anatomist Paul Broca (1824–1880). The basic formula is: Normal weight (kg) = Height (cm) – 100. The Broca Index is one of the oldest anthropometric measurement methods and was widely used in clinical practice during the 19th and early 20th centuries.

Calculation and Variants

The simplest version of the Broca Index calculates the so-called Broca normal weight directly from body height:

Normal weight: Height in cm minus 100 = weight in kg
Ideal weight for men: Normal weight minus 10% of normal weight
Ideal weight for women: Normal weight minus 15% of normal weight

Example: A person who is 175 cm tall has a Broca normal weight of 75 kg. The ideal weight would be 67.5 kg for a man and 63.75 kg for a woman.

Application and Clinical Relevance

The Broca Index is frequently used as a first orientation for assessing body weight. It is simple to calculate, requires no tools, and is suitable for a quick everyday assessment. In medical and dietary counseling, it is used as a rough guideline to determine whether a person is underweight, normal weight, or overweight.

Advantages of the Broca Index

Very simple calculation without any tools
Quick first orientation on body weight
Easy to understand for patients and non-specialists
Distinguishes between normal weight and ideal weight

Limitations and Criticism

Despite its ease of use, the Broca Index has significant limitations. It does not adequately account for sex, age, body type, or muscle mass. It tends to produce inaccurate results for very short or very tall individuals. For these reasons, the Broca Index is increasingly being replaced in modern medicine and nutritional science by the Body Mass Index (BMI), which allows for a more differentiated assessment of body weight in relation to height.

Comparison with the BMI

The Body Mass Index (BMI) is calculated as body weight in kg divided by the square of height in meters. Unlike the Broca Index, the BMI provides a standardized classification (underweight, normal weight, overweight, obesity) and is internationally established in clinical practice and research. However, the BMI also has its limitations, as it does not directly capture body composition (fat mass versus muscle mass).

When to Seek Medical Advice

If the Broca Index or BMI indicates significant underweight or overweight, medical or nutritional counseling is advisable. A substantial deviation from normal weight can be associated with increased risks of cardiovascular disease, diabetes mellitus, joint disorders, and other chronic conditions.

References

World Health Organization (WHO): Obesity and overweight. Fact Sheet. Geneva: WHO, 2024. Available at: https://www.who.int/news-room/fact-sheets/detail/obesity-and-overweight
Wirth, A. & Hauner, H. (eds.): Adipositas – Aetiologie, Folgekrankheiten, Diagnostik, Therapie. 4th edition. Heidelberg: Springer Medizin, 2013.
Lenz, M. et al.: Waist circumference, waist-hip ratio and BMI as indicators of total and abdominal adiposity in the general population. European Journal of Nutrition, 2009.

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