Understanding your BMI and looking beyond the number

Calculating your BMI

BMI is calculated by dividing weight (in kilograms) by the square of height (in metres), i.e. height multiplied by itself. In adults, overweight starts at 25 and obesity at 30^{1, 2}.

What BMI does not tell you

BMI does not distinguish muscle from fat and does not show where fat is stored. On its own, it is therefore not enough to describe an individual situation^{1, 3}.

BMI

weight (kg) ÷ height (m)²

A first reference point, easy to calculate. It does not distinguish fat mass from muscle mass.

Body composition

fat masslean mass

At the Centre, bioelectrical impedance estimates fat mass and lean mass, to follow how they change.

A medical assessment completes these two measurements.

The medical assessment also looks at your weight history, your sleep, associated conditions, your mood and your quality of life: see Obesity medicine.

Looking beyond the number: the framework proposed in 2025

In January 2025, an international commission of experts, in a report published in The Lancet Diabetes & Endocrinology, proposed a new way of describing obesity³. This framework distinguishes:

• excess adiposity, meaning excess body fat, to be confirmed by a measurement of body composition or waist circumference, in addition to BMI;
• preclinical obesity, when this excess is present but organ function and daily activities are preserved;
• clinical obesity, when this excess leads to impaired organ function or significant limitations in daily activities.

This is a proposal. It helps to describe each situation more accurately, but it does not replace the diagnostic rules in force or the conditions for insurance cover. This framework is distinct from the WHO BMI classification used by the calculator: the classification places weight in relation to height, whereas the commission describes the state of health linked to excess body fat.

Technical details: worked example, waist circumference and thresholds

Example: for 100 kg and 1.70 m, BMI is 100 ÷ (1.70 × 1.70) = 34.6.

When body composition cannot be measured, the Swiss recommendations suggest waist circumference, especially when BMI is between 25 and 30 (benchmark: 102 cm or more in men, 88 cm or more in women)¹.

Lower BMI thresholds are advised for some ethnic backgrounds, notably South Asian and Chinese¹.

Insurance rules also use BMI, together with other conditions: they are set out on the relevant pages, for example Insurance cover in Switzerland.

Frequently asked questions

Is there a BMI at which you have to have surgery?

No. The decision on surgery is not based on BMI alone: it takes into account your health, your history and your preferences, and it is made with you, after an assessment by several professionals.

When BMI is over 50: see Care when BMI is over 50.

What next?

A first consultation is an opportunity to review your situation and ask your questions. Possible treatments are then discussed with you. Your request is received by the Centre’s appointments team, who will organise the next steps with you.

Medical review of the French version: Dr Pierre Fournier

Scientific sources and references

Sources accessed on 15 October 2026. Reimbursement rules and information on medicines change over time.

1. Gerber PA, Collet TH, Favre L, et al. Swiss obesity clinical practice guidance. Swiss Med Wkly. 2026;156:5415. View source
2. Adult BMI classification (table 2.1: below 18.5 underweight; 18.5 to 24.99 normal range; 25 to 29.99 pre-obese or overweight; 30 to 34.99 obese class I; 35 to 39.99 class II; 40 or more class III). World Health Organization. Obesity: preventing and managing the global epidemic. Report of a WHO consultation. WHO Technical Report Series 894. Geneva: WHO, 2000. View source
3. Diagnostic framework proposed by the Lancet commission (2025). Rubino F, Cummings DE, Eckel RH, et al. Definition and diagnostic criteria of clinical obesity. Lancet Diabetes Endocrinol. 2025;13(3):221-262. View source