There is no question that weight loss surgery is the most effective treatment for severe obesity. The goals of surgery are:

1. Significant and durable weight loss
2. Resolution or improvement of obesity related health conditions
3. Improvement in quality of life

Successful Weight Loss

Weight loss surgery is considered successful when a patient loses 50% or more of the excess weight. 

Here is how the calculations are done.

Initial Weight  = your weight measured before the start of weight loss journey
Ideal Weight = weight for your height that gives you a BMI of 25 ( upper range of normal BMI). This is calculated from the BMI equation.
Ideal Weight = 25 x (Height in meters)^{^2}
Excess Weight = (Initial weight) - (Ideal Weight)
Excess Weight Loss (%EWL)= [(Intial Weight)-(Postoperative Weight)]/[(Inital Weight)-(Ideal Weight)] x 100

The  Excess Weight Loss (%EWL) is the most common method for measuring weight loss however, its main disadvantage is that it does not reflect successful weight loss in patients with BMI above 55 kg/m^² despite having a very good weight loss.

An alternative measure of weight loss that is more accurate for patients with BMI > 55 kg/m^{^2} is  Total Weight Loss (%TWL). This is widely used by other medical speicalists and is a better reflection of the success of surgery. It is calculated as follows:

%TWL = [ (Initial Weight) - (Postoperative Weight)]/(Initial Weight) x 100

Using this measure, surgery is considered successful when patients lose 20% or more of the their total body weight.

You may want to use the simplified calculator below to help you understand these calculations. 

How Much Weight Will I Lose After Sleeve Gastrecotmy?

The average Excess Weight Loss (%EWL) at 3 years after sleeve gastrectomy is 60% ¹*. (* Results may vay from person to person).
This means that half of the patients will lose less than 60% and the other half will lose more than 60% of their excess weight. 

Most of the weight is lost in the first 6 months then weight loss slows down and stabilizes between 9 and 12 months. There will be a normal weight regain of 1-5 kg after the first year due to the expected adaptation. 

Losing all the excess weight (that is %EWL of 100%) is achieved by only a small percentage of patients and, therefore, is a not realistic goal.

Patient Expectation for Weight Loss After Surgery

Patients considering weight loss surgery often have unrealistic expectation of the amount of weight they are going to lose. About two thirds of patients considering surgery report that they would be disappointed if they lose only 50% of excess weight, although this level is considered successful outcome. It is therefore important to think about your own goals from having surgery and set achievable goals and manage your own expectations.

Weight Loss Surgery Is Not Only About Weight Loss

In considering weight loss surgery it helps to remember these points.

• Weight loss alone (as measured by kilograms lost) should not be the sole reason to undergo weight loss surgery.
• Resolution, improvement or prevention of obesity related health conditions (such as type II diabetes, hypertension, sleep apnoea, metabolic syndrome, etc.) is as important as weight loss.
• Even a modest weight loss of 5% of total body weight can improve metabolic factors and health risks.
• Ask yourself this question "What is lowest amount of weight you hope to lose to not feel disappointed?"
• It is important to pay attention to quality of life after surgery. Frequent vomiting or inability to eat good quality food is not worth looking skinny.
• Patients weight loss result vary widely due to many factors as will discussed in a separate article.

References:
1. Diamantis  T, Apostolou  KG, Alexandrou  A, Griniatsos  J, Felekouras  E, Tsigris  C.  Review of long-term weight loss results after laparoscopic sleeve gastrectomy.  Surg Obes Relat Dis. 2014;10(1):177-183.