Bariatric surgery6 min read
Obesity classes and BMI: how doctors measure obesity
Obesity is not just a number on the scales. Doctors use body mass index (BMI) to classify excess weight, but they also look at waist size, fat distribution and blood tests to understand the real health risk. This article explains how obesity is classified, how to calculate your BMI, which thresholds define each class, when BMI can be misleading, why waist size matters and what your class means when bariatric surgery is being considered.
The main points in 30 seconds
- BMI is weight in kilograms divided by height in metres squared.
- Obesity classes: 1 is BMI 30–34.9, 2 is 35–39.9, 3 is 40 and above.
- For some ethnic backgrounds, obesity starts at a lower BMI of 27.5.
- BMI cannot tell muscle from fat; waist size and lab tests complete the picture.
- Surgery is usually considered at BMI above 40, or above 35 with related conditions.
Key facts about the procedure
- Back to work2–4 weeks
- Stay in clinic1–5 days
- Final result18–24 months
- Price
$4,800from $3,600
How obesity is classified
Obesity is a long-term condition in which excess body fat affects health. To compare people and choose treatment, doctors need a simple, consistent measure, and the most widely used one is body mass index. BMI relates weight to height and places adults into categories: underweight, healthy weight, overweight and obesity. Obesity itself is divided into three classes, from class 1 to class 3, as the risk to health grows.
BMI is a screening tool rather than a diagnosis on its own. It tells a doctor whether a closer look is needed. The full assessment adds waist measurement, a review of existing conditions such as type 2 diabetes or high blood pressure, and sometimes body composition and laboratory tests.
Calculating your BMI
BMI is calculated by dividing weight in kilograms by height in metres squared:
- Formula: BMI = weight (kg) ÷ height (m) ÷ height (m).
- Example: a person who weighs 110 kg and is 1.70 m tall has a BMI of 110 ÷ 1.70 ÷ 1.70, which is about 38.
- Calculators: health services offer free online BMI calculators that do the arithmetic for you.
Weigh yourself in the morning in light clothing and measure height without shoes for the most accurate result. BMI categories for adults do not apply to children, whose results are compared with growth charts for their age and sex.
BMI thresholds for obesity classes
For adults, the widely used categories are:
- Below 18.5
- Underweight
- 18.5 to 24.9
- Healthy weight
- 25 to 29.9
- Overweight
- 30 to 34.9
- Obesity class 1
- 35 to 39.9
- Obesity class 2
- 40 and above
- Obesity class 3 (severe obesity)
Health services note that people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean background face health risks at a lower BMI. For them, a BMI of 23 to 27.4 is considered overweight and 27.5 or more is considered obesity, and thresholds for treatment are adjusted accordingly. The same classes are used to decide when surgery is worth discussing, as described on the bariatric surgery page.
When BMI can be misleading
BMI cannot tell the difference between muscle and fat. That creates several blind spots:
- Muscular people: athletes and people doing heavy physical work may have a high BMI while carrying little fat.
- Older adults: with age, muscle is replaced by fat, so a normal BMI can hide excess fat.
- Fat distribution: two people with the same BMI may have very different amounts of fat around the organs.
- Pregnancy and fluid: pregnancy or fluid retention raise weight without reflecting body fat.
This is why doctors never rely on BMI alone. It remains useful because it is simple and reproducible, but it is combined with other measurements to judge real health risk.
Waist size and body fat distribution
Where fat is stored matters as much as how much there is. Fat around the abdomen, including fat around the internal organs, is closely linked with type 2 diabetes, heart disease and high blood pressure. Fat on the hips and thighs carries less risk.
A simple way to check is to measure your waist midway between the lowest rib and the top of the hip bone. Health services suggest keeping your waist size to less than half your height: someone 170 cm tall should aim for a waist under 85 cm. A waist-to-height ratio of 0.5 or more signals increased risk even when BMI is only in the overweight range. For people with a BMI of 35 or more, the waist measurement adds less, because the risk is already high.
Body composition and lab markers
When a more detailed picture is needed, doctors can use additional tools:
- Body composition analysis: bioimpedance scales or special scans estimate the share of fat, muscle and water in the body.
- Blood sugar tests: fasting glucose and long-term sugar markers show whether insulin resistance or diabetes is developing.
- Lipid profile: cholesterol and triglycerides help assess heart and blood vessel risk.
- Liver tests: excess fat often affects the liver before any symptoms appear.
- Blood pressure and sleep: high blood pressure and signs of sleep apnoea are part of the assessment.
Together with BMI and waist size, these results show not only how much excess weight there is but how much it is already affecting health, which matters more for choosing treatment.
What your class means for treatment at City Medical Centre
City Medical Centre works with international patients and performs sleeve gastrectomy, Roux-en-Y gastric bypass, mini gastric bypass (MGB) and SADI-S. According to the bariatric surgery page, surgery is usually justified with a BMI above 40 (class 3 obesity), or above 35 (class 2) when there are obesity-related conditions such as type 2 diabetes or high blood pressure; for sleeve gastrectomy the page gives a typical BMI range of 35 to 45.
At lower BMI levels, lifestyle programmes and treatment by your own doctor usually come first. City Medical Centre does not perform gastric balloons or endoscopic procedures and does not prescribe weight-loss injections; whether a gastric band is performed at the clinic is clarified at the consultation. Before surgery, the team reviews your BMI, other conditions and test results, and the surgeon explains whether surgery is suitable and which method fits. A personal coordinator stays in touch from the first message to the end of recovery. The first consultation is free and can be held online; the price is named after assessment and fixed in the contract.
How recovery usually goes
- First hoursYou may sit up, stand and drink fluids
- 1–5 daysObservation at the clinic, depending on the extent of surgery
- 2–4 weeksReturn to office work; physical work after about 1.5 months
- 1–2 monthsStaged diet from liquids to solids to let the stomach adapt
- Every monthOn average 10% of excess weight lost per month
- 1.5–2 yearsFinal result, stable eating habits prevent the weight from returning
Sources
Frequently asked questions
What BMI is considered obese?
For most adults, a BMI of 30 or more is considered obesity. For people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean background, health services use a lower threshold of 27.5, because health risks start at a lower BMI.
What are the classes of obesity?
Class 1 is a BMI of 30 to 34.9, class 2 is 35 to 39.9 and class 3, also called severe obesity, is 40 or above. The higher the class, the greater the risk of obesity-related conditions.
Is waist size more important than BMI?
They answer different questions. BMI shows overall excess weight, while waist size shows fat around the abdomen, which is closely linked with diabetes and heart disease. Doctors use both; keeping your waist under half your height is a useful target.
What BMI qualifies for bariatric surgery?
Surgery is usually considered with a BMI above 40, or above 35 with obesity-related conditions such as type 2 diabetes or high blood pressure. The final decision is made after a full assessment of your health.
Can muscular people have a high BMI?
Yes. BMI cannot distinguish muscle from fat, so athletes and people with heavy physical jobs may have a high BMI with little body fat. Waist measurement and body composition analysis help clarify the picture.
Have a question about your case? Get a free consultation
Message a coordinator on Telegram: we reply right in the chat, and you can send photos and questions there too.
- The first consultation is free, online consultations are available
- The doctor names the cost after the assessment, and it is fixed in the contract
- A personal coordinator guides you from the first message to recovery
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