BMI and Obesity: The Ranges Explained, Including Morbid Obesity

What does a BMI number actually tell you about your health?

BMI is one of the main tools used to classify overweight and obesity in adults. For most adults, a BMI of 30 or above falls within the obesity range. From there, obesity is divided into Class I, Class II and Class III.

You may also come across the term "morbid obesity" when reading about a BMI of 40 or above. This is an older term that is now used less often in UK healthcare. Current guidance generally uses "severe obesity" or "Class III obesity" instead.

These ranges can be useful for understanding how BMI is classified, but they do not tell the whole story. Your waist measurement, ethnic background, existing health conditions, medicines and other personal factors can also affect how your health is assessed. 

Key things to know

• For most adults, a BMI of 30 or above is classified as obesity.
• Obesity is divided into Class I, Class II and Class III.
• Class I is a BMI of 30 to 34.9, Class II is 35 to 39.9, and Class III starts at 40.
• A BMI of 40 or above has historically been called "morbid obesity". In current UK healthcare, "severe obesity" is more commonly used.
• BMI is a useful screening measure, but it cannot show how much of your weight comes from muscle or where body fat is stored.
• Health risk tends to increase as BMI increases, but BMI alone cannot predict an individual's health.
• Your ethnic background can affect the BMI threshold used to assess health risk.
• A clinical assessment can put your BMI result into the context of your overall health.

The full BMI range, from underweight to severe obesity

These are the standard adult BMI ranges used for classification. They are a starting point rather than a complete assessment of an individual's health.

BMI 30 to 34.9: Obesity Class I

A BMI between 30 and 34.9 falls within Class I obesity for most adults.

This does not mean that everyone in this range has the same health concerns. BMI is a population measure, so your individual health can depend on other factors too.

If your BMI falls within this range, a healthcare professional may look at your blood pressure, waist measurement, medical history, medicines and any existing health conditions alongside your BMI.

BMI 35 to 39.9: Obesity Class II

A BMI between 35 and 39.9 falls within Class II obesity.

At this level, the likelihood of weight related health problems can be higher. NICE guidance also uses BMI thresholds alongside other factors when considering more intensive weight management support.

For example, some treatment pathways consider whether a person has a health condition that could improve with weight loss. This is one reason why BMI should not be viewed in isolation when discussing treatment.

BMI 40 and above: Severe obesity, Class III

A BMI of 40 or above falls within Class III obesity, also called severe obesity.

You may still see this range described as "morbid obesity" or "morbidly obese", particularly in older information. However, these terms are used less often in current UK healthcare because they can be stigmatising. "Severe obesity" or "Class III obesity" is generally clearer and more appropriate.

A BMI of 40 or above is associated with a higher average risk of a number of health conditions. However, it does not mean that a person will definitely develop these conditions.

Why does BMI 40 matter?

BMI 40 is an important clinical threshold because it is used in some UK guidance when considering more intensive weight management options, including assessment for bariatric surgery.

For example, NHS guidance says weight loss surgery may be available to adults with a BMI of 40 or more. It may also be considered at a BMI of 35 to 39.9 when a person has a health condition that could improve with weight loss. Eligibility can vary, so meeting a BMI threshold does not automatically mean that surgery or another treatment is appropriate.

The assessment may also take into account your medical history, existing health conditions, previous weight management support and other individual circumstances.

Does a BMI of 40 mean you are unhealthy?

Not necessarily.

A BMI of 40 or above indicates a higher BMI category, but it cannot tell you everything about your health. Two people with the same BMI can have different health profiles.

Your healthcare professional may look at factors such as blood pressure, blood tests, waist measurement, existing conditions, medicines and your general health before discussing what the result means for you.

Why "morbidly obese" is used less often

"Morbid obesity" is an older term for obesity at a very high BMI, usually 40 or above.

You may still see it in older NHS documents, research papers or online searches. However, current UK guidance and health information increasingly use terms such as "severe obesity" and "Class III obesity".

This change is not about changing the BMI threshold. A BMI of 40 or above remains an important clinical classification. It is mainly about using clearer and less stigmatising language when talking about a person's health.

Where possible, it is also more considerate to talk about "a person living with obesity" rather than defining someone by their weight.

Does a higher BMI always mean a higher health risk?

On a population level, higher BMI is associated with a higher risk of several health conditions. However, BMI cannot predict exactly what will happen to one person.

For example, BMI does not show where body fat is stored. Carrying more fat around the waist can be linked with health risks that BMI alone does not show.

This is why healthcare professionals may use other measurements alongside BMI. NICE recommends using waist to height ratio alongside BMI for adults with a BMI below 35.

A simple way to think about waist to height ratio is that your waist measurement should ideally be less than half your height.

Does ethnicity affect the BMI range?

Yes. The standard BMI ranges do not apply in exactly the same way to everyone.

NHS guidance says people from South Asian, Chinese, other Asian, Middle Eastern, Black African and African Caribbean backgrounds can have a higher risk of some health conditions at a lower BMI.

For these groups, a BMI of 23 to 27.4 is classed as overweight and a BMI of 27.5 or above as obese in NHS guidance.

This is important because two people with the same BMI may not have the same level of health risk. A healthcare professional can take your ethnic background into account when assessing your weight and health.

Why BMI is not the whole picture

BMI is useful because it gives healthcare professionals a simple way to classify weight in large groups of adults. But it has some clear limits.

It cannot:

• distinguish muscle from body fat
• show where fat is stored
• measure your overall fitness
• tell you whether you have a health condition
• predict your individual health risk on its own

For example, someone with a lot of muscle may have a high BMI without having a high amount of body fat.

This does not make BMI useless. It means the result is best understood alongside other information rather than treated as a diagnosis by itself.

What do these BMI ranges mean for treatment?

A BMI range can be one part of a discussion about weight management, but it does not automatically determine which treatment you should have.

Depending on your health and circumstances, weight management may involve changes to eating habits and physical activity, structured support, medicines or, for some people, assessment for bariatric surgery.

The options available depend on the treatment, your BMI, your health conditions and the relevant clinical criteria. A higher BMI can be one factor when considering more intensive support, but it is not the only one.

If you are considering treatment, an individual assessment can help you understand which options may be suitable for you.

Starting treatment in the UK

The online consultation process

If you are considering medically supervised weight management through an online service, you will usually need to complete a health questionnaire.

This may ask about your medical history, current medicines, existing health conditions, weight and other relevant information. Your BMI may also be calculated as part of the assessment.

A UK registered prescriber can then review the information and decide whether a particular treatment is clinically appropriate. Treatment is not suitable for everyone, so an assessment is important before starting a prescription medicine.

If you already take medicines or have an existing health condition, make sure you include this information during your assessment.

Pricing

Prices vary depending on the treatment and provider. If you include a treatment price on this page, check it against the relevant live product page before publication and update it when the price changes.

Safety considerations

A BMI result can sometimes raise questions about your health, but it does not need to be something you deal with on your own.

If you are concerned about your weight, speak to a GP, pharmacist or another qualified healthcare professional. They can help you understand your BMI in the context of your wider health.

If thoughts about food, eating, weight or body shape are becoming distressing or difficult to manage, it is also worth speaking to a healthcare professional. Focusing only on a BMI number may not be helpful in this situation.

Medical disclaimer

This content is provided for general information only and does not constitute medical advice. It is not a substitute for a consultation with a qualified healthcare professional.

Treatment suitability depends on an individual clinical assessment. If you are concerned about your weight or general health, speak to a GP, pharmacist or qualified healthcare professional.