Obesity Classes Explained: Class 1, Class 2 and Class 3 BMI Ranges

• Gives the three ranges immediately, then explains what genuinely differs in practice at each class rather than just listing numbers.
• Includes the adjusted class boundaries used for several family backgrounds, which almost no page publishes in full.
• Explains why the classes are not a staging system and why two people in the same class can be in very different positions.
• Covers the practical differences at higher classes, including measurement and equipment issues rarely mentioned to patients.

Obesity is divided into three classes by body mass index (BMI). Class 1 is a BMI of 30 to 34.9, class 2 is 35 to 39.9, and obesity class 3 means a BMI of 40 or above, or 37.5 and above for some family backgrounds, and is also called severe obesity. Class 3 obesity is a BMI of 40 or above under the standard classification, or 37.5 or above when the lower thresholds for certain ethnic groups apply. It is also called severe obesity. A BMI of 40 or above is one of the thresholds at which referral for specialist assessment for bariatric surgery is generally considered in UK guidance.

Those numbers are easy to find. What is harder to find, and more useful, is an explanation of why the divisions exist at all. The classes are not there because something changes in a person's body at 35 or at 40. They exist because clinical decisions change between them: which measurements are used, which treatment options come into consideration, how closely other conditions are looked for, and which practical challenges and health risks are more likely with severe obesity.

For adults aged 18 and over in the UK who want clear, clinically relevant information about obesity classifications, this page sets out the BMI ranges, including the adjusted thresholds used for different ethnic and family backgrounds, then explains what actually differs between class 1, 2 and 3, when class 3 affects treatment decisions, and what BMI cannot tell you on its own.

Key things to know

• The three classes are: class 1, BMI 30 to 34.9; class 2, BMI 35 to 39.9; class 3, BMI 40 or above.
• For people from South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family backgrounds, the class boundaries are 2.5 lower: class 1 begins at 27.5, class 2 at 32.5 and class 3 at 37.5.
• Class 3 is also described as severe obesity. "Morbid obesity" is an older term that has largely fallen out of use in UK practice.
• The classes are a classification of one measurement, not a staging system and not a progression you are moving along.
• UK guidance encourages men aged 18 and over with a BMI below 35 to measure their own waist to height ratio alongside BMI. Above that, BMI is used on its own.
• Two people in the same BMI class can be in very different clinical situations because BMI does not capture overall health, body-fat distribution or the presence of weight-related conditions.
• Licensed weight management treatment criteria begin below class 1, at a BMI of 27 with a weight related health condition.

What are the three obesity classes?

The classes divide the obesity range of the BMI scale into three bands. They sit above the healthy weight range of 18.5 to 24.9 and the overweight range of 25 to 29.9, and overweight and obesity are linked to multiple comorbidities.

What are the standard class ranges?

Class 1 obesity covers the lowest BMI band within the obesity range.

What are the adjusted class ranges?

UK guidance sets lower BMI thresholds for people from South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family backgrounds. Obesity begins at a BMI of 27.5 for these groups rather than 30, and the class 2 and class 3 boundaries are each reduced by 2.5.

The reason is that these groups, including many Asian populations, tend towards more central adiposity, meaning a greater proportion of fat carried around the abdomen and organs at any given BMI, and health risk appears at a lower number. It is not a stricter standard. It is the same level of risk arriving at a different figure.

What actually changes between the classes?

Several things change, and they are the reason the divisions exist. This is the part most explanations leave out. Across the different obesity classes, a 2022 study found that higher class was associated with higher comorbidity risk.

What is specific to class 1?

Class 1 is the band where BMI is used alongside a second measurement. UK guidance recommends using waist-to-height ratio alongside BMI in adults with a BMI below 35 kg/m². A ratio of 0.5 or above indicates increased central adiposity and health risk.

The practical consequence is that two people in class 1 can be assessed differently depending on their waist. Someone with a BMI of 32 and a waist under half their height is in a different position from someone with the same BMI and a much larger waist.

This is also the band where the most is likely to be achieved by changes that do not involve any procedure. Weight loss of a modest proportion of body weight produces measurable improvements in blood pressure, blood glucose and blood fats.

What is specific to class 2?

Class 2 sits above the point at which guidance stops encouraging waist to height measurement, so BMI is generally used on its own from here upwards.

It is also the band at which assessment for weight loss treatments, including weight loss surgery, starts to be considered, though not on BMI alone. NICE uses lower BMI thresholds for some ethnic groups when assessing overweight and obesity and for certain weight-management medicines. Surgical referral criteria should be assessed according to the specific NICE recommendations and individual clinical circumstances.

One specific situation is worth knowing about. NICE recommends considering referral for bariatric surgery for people with a BMI of 35 kg/m² or above who have recent-onset type 2 diabetes, generally within the last 10 years, because surgery may improve diabetes control and remission is more likely when diabetes duration is shorter.

What is specific to class 3?

Class 3, or severe obesity, is the band where BMI thresholds alone are enough for surgical assessment to be considered, without a separate health condition needing to be present, and it carries higher health risks, including cardiovascular disease and a greater likelihood of developing type 2 diabetes.

It is also the band where practical issues appear more often. These are worth naming plainly, because people rarely hear about them until they encounter one:

• Blood pressure measurement. A cuff that is too small for the arm gives a falsely high reading, and the error can be substantial. Asking for the correct cuff size matters more here than anywhere else.
• Imaging. Scanners have weight and aperture limits, and image quality can be reduced. This occasionally affects which test is used.
• Anaesthetic and procedures. Airway management and drug dosing need additional planning, which is why anaesthetic teams ask about weight in advance.
• Mobility and joints. Load through the knees, hips and lower back is higher, and joint issues, including severe osteoarthritis, are more common; reduced mobility can itself make weight harder to manage.

Severe obesity is associated with higher risks of several health conditions and, at a population level, higher mortality.

None of this means care is unavailable. It means planning ahead is useful, and that it is reasonable to ask in advance whether equipment is suitable.

Is "morbid obesity" the same as class 3?

It refers to the same BMI range, but it is an older term and it has largely fallen out of use in UK clinical practice.

"Class 3 obesity" and "severe obesity" are the terms now generally used. The change is partly about precision and partly about tone. "Morbid" is widely experienced as judgemental, and UK guidance asks clinicians to discuss weight sensitively and without stigma.

If you have seen the older phrase in a letter, an older article or on a form, it is describing a BMI of 40 or above and nothing more than that.

What the classes are not

This section matters because the word "class" carries implications that do not apply here, and those implications cause real distress.

Are the classes a staging system?

No. Cancer staging describes how far a disease has spread. Kidney disease staging describes declining function. Obesity classes do none of that. They divide a single measurement into three bands, and that measurement reflects weight relative to height rather than how a condition has developed.

Nothing about your body changes when you cross from 34.9 to 35.0. Risk rises gradually across the whole scale, not in steps at the boundaries.

Am I progressing through the classes?

Not inherently. The classes are not a sequence you move along, and being in class 1 does not mean class 2 is next. Weight can rise, fall or stay the same, and many people remain in the same band for years or move downwards.

Can two people in the same class be in different situations?

Yes, and often dramatically so. BMI does not measure excess body fat, cannot distinguish muscle from fat, and says nothing about where fat is carried or about anyone's actual health.

Two people with a BMI of 36 can differ completely. One may have normal blood pressure, normal blood tests, good mobility, no symptoms, and no related health problems. The other may have type 2 diabetes, raised blood pressure, sleep apnoea and joint pain. They share a class. They do not share a clinical situation.

This is a recognised limitation, and approaches exist in clinical practice that grade obesity by its actual effect on a person's health and function rather than by BMI alone. Those approaches often place the two people above in very different categories, which is closer to how their care would actually differ.

Does BMI work equally well for everyone?

No. UK guidance says BMI should be interpreted with caution in men aged 18 and over with high muscle mass, because it may be a less accurate measure of central adiposity in that group. It should also be interpreted with caution in people aged 65 and over, taking into account other conditions and the possible protective effect of a slightly higher BMI in later life. BMI is not used to assess weight during pregnancy, and adult classes do not apply to children. A BMI calculator can be useful for initial screening, but it has the same limitations described here and should be interpreted with caution.

What treatment options relate to each class?

Class affects which options are considered, and obesity class 3 may be managed with weight-loss medications as well as other weight management strategies, but it does not decide anything on its own. Medical management often involves a multidisciplinary approach, especially where related health conditions are also being considered. What sits alongside the number matters at least as much.

Where do licensed weight management medicines fit?

Eligibility for weight-management medicines depends on the specific medicine, its licence, NICE recommendations, BMI, weight-related health conditions and the clinical setting. Some medicines have different BMI thresholds and eligibility criteria, so there is no single BMI rule that applies to every weight-management medicine.

Meeting a threshold is not the same as being suitable. A prescriber reviews your medical history, current medicines and anything that makes a particular treatment inappropriate. Treatment suitability depends on an individual clinical assessment, and your prescriber will determine the appropriate treatment. If you want to understand which options might apply to you, an online consultation with UK registered prescribers is a reasonable place to start.

What does not change with class?

The foundations. Sustained changes to eating, physical activity, sleep and alcohol act on the same mechanisms at every class, and the same proportional weight loss produces broadly similar improvements in blood pressure, blood glucose and blood fats wherever you started.

Monitoring does not change either. Blood pressure, blood glucose and cholesterol are checked for the same reasons across all three classes, because the conditions being looked for are silent in every one of them. Regular physical activity should accompany weight loss medications for best long-term management.

What should you do with your class?

Treat it as one piece of information and one risk factor rather than a verdict. The more useful questions are what else is going on, what is being monitored, and how this may affect your overall health.

When should you see a doctor?

Book an appointment with a healthcare provider if you have not had blood pressure, blood glucose and cholesterol checked recently, if you have symptoms of a linked condition, or if you want to understand your options.

Seek prompt medical advice for weight gain that has been rapid over days or weeks, weight loss you did not intend, swelling of the ankles or legs especially with breathlessness, increasing thirst with more frequent urination, or being told you stop breathing during sleep.

It is also reasonable to talk to someone if your BMI class is causing you distress or affecting your mental health. Distress about a number is a common experience and worth raising rather than carrying, and people with complex needs may benefit from specialist support.

Medical Disclaimer

This article is for general information and does not replace personalised medical advice. BMI classification is a screening measure, not a diagnosis, and it cannot tell you whether you are healthy or which treatment is appropriate for you. Prescription only medicines should be used only under the supervision of an appropriately qualified prescriber, following a clinical assessment, and results vary between individuals. If you take a prescribed medicine, follow the patient information leaflet supplied with it, never take extra to make up for a missed dose, and do not stop a prescribed medicine without discussing it with the prescriber. Speak to a healthcare professional before making significant changes to your diet or activity, particularly if you have an existing health condition. Seek urgent medical attention for severe or rapidly worsening symptoms.