How is expected weight loss calculated?

Just curious how to interpet widely cited "stats" such as losing 70% of excess weight.  Is the target weight they used when they did the studies the mid range of BMI?  For example, i am 5'5" and the BMI range is something like 120-150 pounds for a normal range.   So, do the studies reflect an average of 70% of excess weight loss based on the target weight of 120, 150 or something in between.

Step one: average 120-150 for a goal weight. That's 135.

step 2: calculate excess weight  by taking current weight and subtracting 135.
            
Step 3: take the result form step two and multiply by 70%

The number you get from step 3 is the number of pounds you can expect to lose AKA your "expected weight loss."

Does that formula work for me too ?  I did the formula and it puts me at a EWL of 26 lbs ?

Why not? If you want to be technical about calculating your excess weight, then you need to use the highest number than keeps your bmi  below 25. I averaged the OP's healthy weight range to be more practical.

I got 25.9

I bet that's gonna take a **** ton of time to get off huh ? I'm serious.

yehuda thats cool i didn't know that was how they did that.

That is cool to know.  Thanks.  My surgical group estimated my weight loss percentage according to the average statics of percentage of loss after WLS.  The average is 33%.  That means some will loose more and some will loose less.  After they told me my expected weight loss goal I was like - I didn't have my innards rearranged to still be considered obese!  I have 3 pounds to go to reach their goal and 39 to reach my own.  Now with this information I can figure it out on my own.  Thanks again.

Actually, most studies use the weight that corresponds to a BMI 24.9 as the excess weight marker. My surgeon uses a BMI of 24 and I've seen a study where they used a BMI of 27!

That's how people can lose more than 100% of their excess weight -- by getting to a BMI lower than 24.9.