What do the different BMI categories mean?

What BMI categories are

BMI stands for body mass index. It is a simple screening tool that uses your height and weight to estimate whether your weight is in a healthy range.

In the UK, BMI is commonly used by the NHS and other health services to help spot possible health risks. It is not a diagnosis, but it can be a useful starting point.

Underweight

A BMI below 18.5 is usually classed as underweight. This may mean a person has less body weight than is generally considered healthy for their height.

Being underweight can sometimes be linked to illness, stress, eating difficulties, or a very active lifestyle. It may also increase the risk of low energy, weaker immunity, and nutrient deficiencies.

Healthy weight

A BMI between 18.5 and 24.9 is usually considered a healthy weight. For many adults, this range is associated with a lower risk of weight-related health problems.

However, BMI does not tell the full story. Muscle mass, body shape, age, and ethnicity can all affect whether BMI is the best measure for a person.

Overweight

A BMI between 25 and 29.9 is usually classed as overweight. This does not automatically mean someone is unhealthy, but it may suggest extra body fat.

People in this range may have a higher chance of conditions such as high blood pressure, type 2 diabetes, and heart disease. Even a small amount of weight loss can improve health for some adults.

Obesity

A BMI of 30 or above is usually classed as obese. This category is often split further into classes 1, 2, and 3, depending on how high the BMI is.

Higher BMI levels can be associated with greater health risks, especially over time. These may include joint problems, breathing difficulties, and a higher risk of long-term illness.

Why BMI is only one part of the picture

BMI does not measure body fat directly, and it does not show where fat is stored. It can also give misleading results for athletes, older adults, or people with a lot of muscle.

In the UK, health professionals often look at BMI alongside waist size, lifestyle, medical history, and overall health. If you are concerned about your weight, a GP can help you understand what the numbers mean for you.