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The Body Mass Index (BMI)

Before starting any weight loss journey, it's important to understand if your current weight is healthy or not.

The body mass index (BMI) is a numeric measure that uses your height and weight to work out if your weight is healthy. For most adults, an ideal BMI is in the 18.5 to 24.9 healthy range.

Categories for BMI weight classes

A BMI calculator can tell you if you are underweight, healthy weight, overweight or obese. These are the four main categories to class how healthy someone's weight is. Get more information on BMI categories.


If your BMI is under 18.5, you're classed as being Underweight. Being underweight could be a sign you're not eating enough or you may be unwell.

If you're underweight, a GP can help. You can also refer to the NHS website for underweight adults for more information.

Healthy weight (or 'Normal')

If your BMI is between 18.5 and 24.9, you're classed as being a Healthy weight or 'Normal'.


If your BMI is between 25 and 29.9, you're classed as being Overweight.


If your BMI is above 30 and 39.9, you're classed as being Obese.

Severely obese

If your BMI is 40 or over, you're classed as Severely obese.

If you're pregnant

If you're pregnant, your BMI calculation will be based on your weight before pregnancy. If you are currently pregnant, it is not appropriate to calculate your BMI on the weight that you are now.

Speak to your midwife or healthcare professional if you're worried about your weight during pregnancy.


Ethnicity also plays a part in how healthy your weight is as well.

For example, if you're black, Asian or other minority group, a BMI of 23 or more comes with an increased risk of weight-related diseases like diabetes Type 2. 

Those with a BMI of more than 27.5 are at a high risk of developing these types of diseases.

Working out your BMI

If you wish to calculate your own BMI you can try our healthy weight quiz.

If your BMI rating suggests you're overweight or obese, you may wish to make steps to start losing weight. Find out more about the benefits of losing weight.

Limitations of the BMI

One of the main limitations of the BMI, is that it doesn't take into account muscle. Muscle is denser and weighs more than fat.

This can mean that muscular people such as boxers or heavyweight athletes fall into the 'obese' class using the BMI criteria. This is even though they are very fit, active and healthy.