BMI Calculator: What Your Result Means for Bariatric Surgery

The formula is simple: BMI = weight (kg) ÷ height (m)², expressed in kg/m².

Example: a person who is 1.70 m tall and weighs 105 kg. Height squared is 1.70 × 1.70 = 2.89. Dividing the weight: 105 ÷ 2.89 = 36.3 kg/m². This value falls into class II obesity.

• below 18.5 — underweight;
• 18.5–24.9 — normal weight;
• 25–29.9 — overweight;
• 30–34.9 — obesity class I;
• 35–39.9 — obesity class II;
• 40 and above — obesity class III.

These categories are used internationally for adults. For children and adolescents, age-specific growth charts are used.

The guidelines updated in 2022 by ASMBS and IFSO replaced the older 1991 criteria. By BMI range:

• BMI ≥ 40 — bariatric surgery is recommended; the assessment focuses on safety and choice of procedure.
• BMI 35–39.9 — surgery is recommended whether or not other conditions are present.
• BMI 30–34.9 — surgery may be considered in people with metabolic disease (for example type 2 diabetes) or when non-surgical methods have not achieved substantial, lasting weight loss.
• BMI below 30 — non-surgical options are usually discussed first. For patients of Asian origin, thresholds are lower because metabolic risk appears at lower BMI values.

For a gastric balloon, suitability is assessed individually, often at a BMI of around 27–35.

• Muscle mass: in very muscular people, BMI can overestimate body fat.
• Age: muscle mass falls with age, so the same BMI may hide more fat.
• Ethnicity: at the same BMI, the risk of diabetes and heart disease differs between populations; this is why adjusted thresholds exist for Asian patients.
• Fat distribution: BMI does not show where fat is stored. Waist circumference complements the assessment, because abdominal fat is linked to higher metabolic risk.

BMI opens the conversation, but the decision is made after a complete evaluation, which includes:

• related conditions: diabetes, high blood pressure, sleep apnoea, reflux, joint problems, fatty liver;
• current treatments and previous weight-loss attempts;
• blood tests (including HbA1c for patients with diabetes) and usually an upper endoscopy;
• an anaesthesia review and, when needed, a cardiology or lung assessment;
• eating habits, smoking and readiness for the changes that follow surgery.

If your result falls into one of the ranges above, you can request a free, confidential online pre-assessment: send your height, weight, known conditions and medications, and your coordinator will reply on WhatsApp. Read more about the gastric sleeve, gastric bypass and gastric balloon, or contact us directly.