What Is BMI and Why Does It Matter Before Bariatric Surgery?
Body Mass Index (BMI) is an internationally recognised standard for classifying overweight and obesity in adults. BMI is calculated by dividing a person’s weight in kilograms by the square of their height in metres. For example, a person who weighs 120 kg and is 1.7 metres tall has a BMI of 41.5. This would place them in the “obesity class 3” range, which is one of the criteria for weight loss surgery.
Limitations of BMI
While BMI is a useful screening tool, it has some limitations. It does not distinguish between fat and muscle, nor does it account for age, sex, or body composition. A muscular athlete, for example, may have a high BMI but low body fat. For this reason, BMI should be considered alongside other factors like waist circumference, metabolic health, and overall fitness.
BMI classifications also vary based on ethnicity. People of Middle Eastern, South Asian, Southeast Asian, and Pacific Islander backgrounds are at higher risk of diabetes and heart disease at lower BMI levels. For some groups, a BMI over 23 is considered elevated risk.
Understanding Ideal Weight
The ideal weight is calculated using the standard BMI value of 25, which is the upper limit of the normal weight range. It is determined by multiplying 25 by the square of the person’s height in metres. Ideal weight provides a reference point to calculate how much “excess weight” a person is carrying, which is commonly used to measure success after surgery.
It’s important to know that your personal goal weight might differ. Many people enjoy excellent health well above their “ideal” BMI if their metabolic risk factors have improved. However, using BMI 25 as a benchmark helps keep things consistent across different patients and surgeries.
What Is Excess Weight?
Excess weight is the difference between your current weight and your ideal weight. For example, if you weigh 120 kg and your ideal weight is 75 kg, your excess weight is 45 kg. This figure is important because it helps track your progress after surgery.
How Is Weight Loss Measured After Bariatric Surgery?
After weight loss surgery, your progress is usually reported in two ways:
- Percent of Excess Weight Loss (%EWL): This is the proportion of your excess weight that you’ve lost. If you had 40 kg of excess weight and lost 20 kg, your %EWL would be 50%.
- Percent of Total Body Weight Loss (%TBWL): This is how much of your overall body weight you’ve lost. If you weighed 120 kg and lost 24 kg, that would be 20% TBWL.
Most modern research now prefers %TBWL because it is easier to compare across patients, regardless of their starting BMI. It’s also a better predictor of long-term health benefits.
To learn more about how success is measured after bariatric surgery, including factors that contribute to weight loss success, read our article on How We Measure Success After Bariatric Surgery .
BMI and Excess Weight Calculator
BMI Classification Chart
| BMI Range | Classification |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obesity (Class 1) |
| 35.0 – 39.9 | Obesity (Class 2) |
| 40.0 and above | Obesity (Class 3 – Severe or Morbid) |
What to Do Next
If your BMI is above 35 and you have health problems like diabetes, sleep apnoea, or high blood pressure, you may be eligible for bariatric surgery. If your BMI is over 40, you may qualify even without other conditions.
For personalised advice or to find out if surgery is right for you, contact our team or attend one of our free bariatric information seminars.
