Patient Guide
Your BMI for bariatric surgery is the first step to knowing if you qualify.
Body Mass Index (BMI) is the first measure bariatric surgeons worldwide use to assess excess weight and decide whether a patient is a candidate for weight-loss surgery such as sleeve gastrectomy or mini gastric bypass. In this guide we explain how to calculate your BMI for bariatric surgery, what each number means, and when surgery becomes the right option.
💡 What you will learn
- The BMI formula with a worked example.
- Weight categories and obesity classes according to the WHO.
- The minimum BMI for bariatric surgery.
- Other factors the surgeon evaluates beyond BMI.
What Is Body Mass Index?
BMI is a measure that relates your weight to your height to give a single number that roughly reflects the amount of fat in your body. It is simple, quick and needs no equipment, which is why international medical bodies use it as the main standard for diagnosing obesity and grading its severity.
How to Calculate Your BMI
Formula: BMI = weight in kilograms ÷ (height in meters × height in meters)
- Convert your height to metersExample: 170 cm = 1.70 m.
- Multiply the height by itself1.70 × 1.70 = 2.89.
- Divide your weight by the resultIf you weigh 120 kg: 120 ÷ 2.89 = 41.5 — which means class III obesity.
Weight Categories by BMI
| BMI (kg/m²) | Category | Health risk |
|---|---|---|
| Below 18.5 | Underweight | Moderate |
| 18.5 – 24.9 | Normal weight | Low |
| 25 – 29.9 | Overweight | Slightly increased |
| 30 – 34.9 | Class I obesity | High |
| 35 – 39.9 | Class II obesity | Very high |
| 40 and above | Class III (severe) obesity | Serious |
When Are You a Candidate for Bariatric Surgery?
According to the 2022 guidelines of the American Society for Metabolic and Bariatric Surgery (ASMBS) and the International Federation for the Surgery of Obesity (IFSO), bariatric surgery is an appropriate option in the following cases:
BMI 40 or higher
A candidate even without associated diseases.
BMI 35 or higher
A candidate, especially with obesity-related conditions such as hypertension, diabetes and sleep apnea.
BMI 30 – 35
Assessed individually when type 2 diabetes remains uncontrolled despite medication.
BMI alone does not settle the decision. There are groups of people who should not have bariatric surgery even with a high BMI, so the decision must follow a full assessment with a specialist surgeon.
Beyond the Number: What Else Does the Surgeon Assess?
- Waist circumference: fat around the abdomen raises the risk of heart disease and diabetes even with a moderate BMI.
- Associated diseases: hypertension, diabetes, fatty liver, joint pain and polycystic ovaries.
- Previous weight-loss attempts: diets, medications and how your body responded.
- Psychological readiness: the ability to follow the diet and attend follow-up after surgery.
- Age and medical history: to choose the most suitable procedure and reduce complications.
For more trusted information, see the World Health Organization fact sheet on obesity and overweight.
FAQs About BMI for Bariatric Surgery
Is BMI alone enough to decide on bariatric surgery?
No. BMI is only the starting point. The surgeon also evaluates associated diseases, medical history, psychological readiness and your ability to commit to the post-operative diet before making a decision.
What is a normal BMI?
A normal adult BMI ranges from 18.5 to 24.9 kg/m². A person is considered to have obesity when BMI is 30 or higher.
Can I have a sleeve gastrectomy if my BMI is below 35?
Usually it is not recommended, but modern international guidelines allow selected cases with a BMI between 30 and 35 to be considered when type 2 diabetes remains uncontrolled despite medication. The final decision is made by the surgeon after examination.
Is BMI accurate for athletes?
BMI may read high in athletes with a large muscle mass even though their body fat is normal, so the assessment is completed with waist circumference and body fat measurements.
Find Out If You Are a Candidate
Book a consultation with Dr. Ahmed Shinkar to assess your case and choose the best solution for you.
Book Your ConsultationDisclaimer: This article is for educational purposes only and does not replace consultation with a qualified physician.