How to calculate your BMI – and what the result actually means

What is Body Mass Index (BMI) and why is it used?

Body Mass Index (BMI) is a standardized measure used internationally to estimate body weight in relation to height. It is used in both research and clinical practice to classify underweight, normal weight, overweight, and obesity.

BMI was originally developed as a population-based tool and has shown a clear correlation with the risk of cardiometabolic diseases, such as type 2 diabetes, hypertension, and cardiovascular disease. A higher BMI is generally associated with an increased risk of these conditions, although individual variations occur and should therefore be interpreted with caution at the individual level.

In healthcare, BMI is primarily used as a screening tool—a first indication of whether further medical assessment or treatment may be justified. The value is calculated by dividing body weight in kilograms by the height in meters squared ($kg/m^2$). It is important to take both weight and height into account when calculating BMI, as the measure shows the relationship between these two factors, expressed in kilograms per square meter ($kg/m^2$).

How do you calculate BMI (Body Mass Index)?

You calculate your BMI by dividing your body weight in kilograms by your height in meters multiplied by itself (squared).

Example of how to calculate BMI with height and weight

If you weigh, for example, 75 kilograms and are 1.70 meters tall, you perform the following calculation:

1. Multiply your height by itself ($1.70 \times 1.70 = 2.89$).
2. Then divide your weight by this sum ($75 / 2.89 = 25.9$).
3. Your BMI will then be approximately 25.9, which according to the WHO classification corresponds to overweight.

BMI – what is considered normal?

According to the World Health Organization (WHO) guidelines, a BMI between 18.5 and 24.9 is defined as normal weight for adults. These threshold values are based on epidemiological data and are used to identify the levels at which the risk of cardiometabolic diseases increases. BMI is therefore primarily developed for the population level and has limitations in individual assessment.

WHO classifies BMI as follows:

• < 18.5: Underweight
• 18.5 – 24.9: Normal weight
• 25.0 – 29.9: Overweight
• 30.0 – 34.9: Obesity (Class I)
• 35.0 – 39.9: Obesity (Class II)
• ≥ 40.0: Obesity (Class III)

How should you interpret your BMI?

You should interpret your BMI as an overall risk indicator and a starting point for understanding your health rather than an exact receipt of how your body is doing. A value over 25 signals that your body may be carrying more fat than is healthy, which increases the risk of diseases such as type 2 diabetes, high blood pressure, and cardiovascular diseases. However, this does not mean you are automatically ill. It happens that people with a BMI over 25 have normal blood values, good metabolic health, and no signs of disease. A complete medical assessment should therefore always include additional parameters, such as blood pressure, blood lipids, blood sugar levels, and body composition.

BMI under 18.5 and BMI over 30 – what does it mean for your health?

A BMI under 18.5 is classified as underweight and may mean that the body is not getting enough energy and nutrition, which in the long run can affect the immune system, energy levels, and muscle mass. A low BMI can also increase the risk of various diseases and should therefore be taken seriously.

A BMI over 30 is defined as obesity and is associated with an increased risk of, among other things, cardiovascular disease, type 2 diabetes, and other metabolic issues. At this level, the clinical picture also changes—obesity is regarded as a chronic disease that is highly influenced by the body's biology and genetics, rather than just lifestyle factors.

At the same time, it is important to remember that BMI does not provide the whole picture of your health, but should be interpreted together with other factors such as lifestyle, physical activity, body composition, and how you feel in your everyday life.

Why BMI is not enough to understand your health

Body Mass Index (BMI) does not take into account individual variations in body composition, as it does not distinguish between fat mass, muscle mass, bone mass, or fluid. This means that people with high muscle mass, such as individuals who strength train regularly, can be classified as overweight according to BMI despite a low percentage of body fat.

Conversely, a person with relatively low muscle mass may have a BMI within the normal range but simultaneously have a high percentage of body fat—particularly visceral fat in the abdominal area. This type of fat is often more metabolically active and is associated with an increased risk of, among other things, insulin resistance, low-grade inflammation, and cardiometabolic diseases.

Against this background, BMI should always be interpreted in a broader clinical context. Complementary measures, such as waist circumference, can provide additional information about fat distribution, where increased abdominal girth is linked to higher health risks independent of BMI.

Common mistakes when evaluating your value

A common mistake when interpreting BMI is to rely solely on the values without considering age. As age increases, natural changes occur in body composition, where muscle mass gradually decreases while the proportion of body fat often increases. Low body weight in the elderly is often linked to an increased risk of frailty, falls, and morbidity. BMI does not capture these changes in body composition, which further limits the tool's usefulness in older populations.

For this reason, assessment in elderly patients should always be supplemented with additional parameters, such as physical function, nutritional status, and the presence of comorbidities, to ensure a medically sound treatment strategy.

How should BMI be followed up?

Once you have calculated your BMI, it may be relevant to find out and measure your waist circumference to get a more nuanced picture of your health and additional information on how fat is distributed in the body. Waist circumference is a simple and clinically useful complement to BMI and provides further information on fat distribution. Abdominal obesity, especially visceral fat, is more strongly linked to cardiometabolic risks than subcutaneous fat.

A waist circumference over 80 cm for women and 94 cm for men is associated with an increased risk of disease. Combining BMI with waist circumference thus provides a more nuanced risk assessment. For further depth and follow-up, an estimation of body fat percentage can also be of value, although such measurements often require more advanced methods.

Today, there are different ways to get help with weight loss, including through medical treatment and support for eating healthily and moving more to reduce risks.

Common questions about BMI (FAQ)

Is BMI a reliable measure for fit/athletic people?

BMI has limited reliability in fit individuals and people with high muscle mass. Since the measure does not distinguish between fat mass and muscle mass, it can overestimate the degree of body fat in these groups.

Muscle tissue has a higher density than fat tissue, which means that a person with significant muscle mass can have a BMI classified as overweight, despite a low percentage of body fat and good metabolic health. For this reason, BMI in these cases should be supplemented with other measures, such as body composition, waist circumference, or metabolic markers, for a more accurate assessment.

When is it relevant to get help losing weight with medical treatment?

Pharmacological obesity treatment is generally medically justified at a BMI over 30. Treatment may also become relevant at a BMI over 27 in combination with at least one weight-related complication such as elevated blood lipids, prediabetes, or sleep apnea.