Body mass index (BMI) is a simple tool used to estimate whether your weight is appropriate for your height.
It is widely used by healthcare professionals around the world because it is easy to calculate, requires only basic information, and provides a useful starting point for identifying potential weight-related health concerns. However, it is not a perfect measure. It does not directly measure body fat or account for differences in body composition and individual health factors.
How is BMI calculated?
To calculate BMI, your height is squared (i.e., multiplied by itself), and then your weight is divided by the resulting number.
BMI = weight (in kilograms) ÷ height² (in meters²)
To get a reliable BMI, it’s important to measure your weight and height properly. Take your weight in the morning before eating, while naked or wearing light clothing, and with no shoes on. Use a reliable scale placed on a flat, hard surface.
It is usually better to have someone else take your height reading. Before measurement, remove your shoes, look straight ahead, and stand straight against a wall, keeping your heels, back, and head touching the wall.
What are the BMI categories?
Once your BMI is calculated, the number falls into a category that helps indicate your potential health risk. According to the World Health Organization (WHO), BMI categories are:
- Underweight: BMI less than 18.5
- Normal weight: BMI 18.5 – 24.9
- Overweight: BMI 25.0 – 29.9
- Obese: BMI 30 or more
Obesity is further classified into:
- Class I: BMI 30.0 – 34.9
- Class II: BMI 35.0 – 39.9
- Class III: BMI 40 or more
Why do the BMI categories matter?
Normal weight is generally associated with lower health risk when combined with healthy lifestyle habits, like regular exercise and eating a healthy diet.
Being overweight or obese is associated with higher risk of chronic conditions like type 2 diabetes, hypertension, stroke, coronary artery disease, and certain cancers. On the other hand, being underweight is associated with malnutrition, weakened immunity, bone loss (osteoporosis), and fertility issues.
What are the limitations of the BMI tool?
While BMI is a useful screening tool, it has certain limitations that you should consider while interpreting your result. They are:
- It considers your total body weight, not it’s composition. Because it doesn’t distinguish between fat and muscle, muscular individuals (such as athletes) may have a high BMI but low body fat while older individuals may have a normal BMI but higher body fat.
- It does not assess fat distribution. Where fat is located matters. Fat around the abdomen (central or visceral fat) carries a higher health risk than fat stored in other areas. Tools that check whether you have more central fat include the waist to height ratio and waist to hip ratio.
- It doesn’t account for other individual differences like genetics, ethnicity, activity level, sex, and pregnancy status.
Does age affect what someone’s BMI means?
Yes, although BMI is calculated the same way for all age groups, its interpretation differs. In children and adolescents under 18 years, BMI is interpreted using age- and sex-specific growth charts, not the standard adult categories. This is because they are still growing and their body composition changes over time.